USMLE Step 1 hematology & oncology questions

Low ferritin reflects depleted iron stores, which is the hallmark of true iron deficiency. The liver responds to low iron stores by increasing transferrin synthesis, raising TIBC, while the low circulating iron relative to this increased transferrin produces a low transferrin saturation. This pattern (low ferritin, high TIBC, low saturation) is diagnostic of iron deficiency anemia, commonly caused by chronic blood loss such as menorrhagia. Chronic disease anemia instead shows low TIBC and low-normal to high ferritin because hepcidin traps iron in macrophages.

• A. Decreased transferrin synthesis would lower TIBC, the opposite of what is seen here.
• B. Inflammation-mediated iron sequestration causes anemia of chronic disease, which shows low TIBC and normal to high ferritin, not this pattern.
• C. Hepcidin elevation is the mechanism of anemia of chronic disease, which produces low TIBC, not elevated TIBC as seen here.
• E. Sideroblastic anemia involves normal or increased iron stores with high ferritin, not low ferritin.

The Mentzer index (MCV divided by RBC count) here is 62/5.9 = 10.5, which is below 13 and suggests thalassemia trait rather than iron deficiency. A normal RDW with a disproportionately high RBC count relative to the degree of anemia and microcytosis, plus normal iron studies, further supports beta-thalassemia minor, in which ineffective erythropoiesis produces numerous small, relatively uniform red cells. Iron deficiency anemia typically produces an elevated RDW from anisocytosis and a lower RBC count for the same degree of microcytosis.

• A. Sideroblastic anemia shows elevated iron stores and ringed sideroblasts, not normal iron studies with this RBC pattern.
• B. Chronic kidney disease causes normocytic anemia from decreased erythropoietin, not microcytosis.
• C. Hereditary spherocytosis causes normocytic anemia with spherocytes and increased osmotic fragility, not microcytosis.
• E. Iron deficiency would show low ferritin and an elevated RDW with a Mentzer index typically above 13.

Deletion of all four alpha-globin genes prevents any alpha-globin synthesis, so excess gamma chains form tetramers called hemoglobin Barts, which has very high oxygen affinity and cannot deliver oxygen to tissues, causing hydrops fetalis and death in utero or shortly after birth. Three-gene deletion causes hemoglobin H disease, a moderately severe hemolytic anemia with beta-chain tetramers (HbH) but compatible with life. One or two-gene deletions cause silent carrier state or mild microcytic anemia (alpha-thalassemia trait), respectively, without hydrops.

• A. A single gene deletion causes a silent carrier state with no clinical findings.
• B. Two-gene deletion in cis causes alpha-thalassemia trait with mild microcytic anemia, not hydrops fetalis.
• C. Three-gene deletion causes hemoglobin H disease, a moderate hemolytic anemia compatible with survival, not fatal hydrops fetalis.
• E. Two-gene deletion in trans also causes mild alpha-thalassemia trait, not a fatal condition.

ALAS2 (erythroid-specific delta-aminolevulinic acid synthase) catalyzes the first and rate-limiting step of heme synthesis, condensing glycine and succinyl-CoA using vitamin B6 as a cofactor to form delta-aminolevulinic acid in mitochondria. Loss-of-function mutations impair this initial step, reducing heme production and causing iron to accumulate in mitochondria, visualized as ringed sideroblasts on Prussian blue stain. This produces a microcytic anemia with elevated iron stores, distinguishing it from iron deficiency, in which stores are depleted.

• A. ALA dehydratase catalyzes this step; it is inhibited in lead poisoning and ALA dehydratase deficiency porphyria, not by ALAS2 mutation.
• B. Coproporphyrinogen oxidase catalyzes this step; its deficiency causes hereditary coproporphyria.
• D. Ferrochelatase catalyzes iron incorporation into protoporphyrin IX; this is inhibited in lead poisoning, not ALAS2 deficiency.
• E. Porphobilinogen deaminase catalyzes this step; its deficiency causes acute intermittent porphyria.

Vitamin B12 is a cofactor for methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA. B12 deficiency causes methylmalonyl-CoA accumulation, which is thought to disrupt normal fatty acid synthesis and myelin formation in the dorsal columns and lateral corticospinal tracts, producing subacute combined degeneration with combined posterior column and corticospinal signs. B12 is also required to regenerate methionine (and thus S-adenosylmethionine) from homocysteine via methionine synthase, but this pathway is more directly linked to the macrocytic anemia and megaloblastic changes rather than the specific myelin pathology.

• A. This step requires thymidylate synthase and folate, not B12, and explains megaloblastic changes, not the neurologic findings.
• B. Tetrahydrofolate regeneration from dihydrofolate requires dihydrofolate reductase, not B12, and relates to folate metabolism, not this neurologic syndrome.
• C. Cystathionine synthesis requires vitamin B6, not B12, and its impairment is not the mechanism of subacute combined degeneration.
• D. Decreased SAM synthesis contributes to impaired methylation but is more closely linked to the hematologic than the specific demyelinating findings.

Both folate and vitamin B12 deficiency cause megaloblastic macrocytic anemia with hypersegmented neutrophils and elevated homocysteine, because both vitamins are needed for homocysteine remethylation to methionine. However, methylmalonyl-CoA mutase requires only B12, not folate, to convert methylmalonyl-CoA to succinyl-CoA. A normal methylmalonic acid level in the setting of megaloblastic anemia and elevated homocysteine indicates that B12-dependent metabolism is intact, pointing to folate deficiency as the cause rather than B12 deficiency.

• A. Decreased haptoglobin reflects hemolysis, which occurs in both B12 and folate deficiency from intramedullary destruction of defective erythroid precursors.
• B. An elevated methylmalonic acid level would indicate B12 deficiency, not support a diagnosis of isolated folate deficiency.
• D. Elevated indirect bilirubin reflects hemolysis from ineffective erythropoiesis, seen in both B12 and folate deficiency, so it does not distinguish between them.
• E. Elevated LDH reflects increased cell turnover from ineffective erythropoiesis but occurs in both B12 and folate deficiency, so it does not distinguish between them.

Pernicious anemia results from autoimmune destruction of gastric parietal cells and antibodies against intrinsic factor, leading to achlorhydria, loss of intrinsic factor, and impaired B12 absorption in the terminal ileum. The resulting hypochlorhydria removes negative feedback on gastrin-secreting G cells, causing hypergastrinemia, and the body/fundus-predominant atrophic gastritis with antral sparing is characteristic. Patients with pernicious anemia have an increased risk of gastric adenocarcinoma and are often associated with other autoimmune conditions such as vitiligo and autoimmune thyroiditis.

• A. G cells are not the target of autoimmune attack in pernicious anemia; their hyperplasia and increased gastrin secretion are a secondary consequence of achlorhydria.
• B. Terminal ileum resection causes B12 deficiency from loss of the absorption site, but there is no history of surgery here, and it would not explain the atrophic gastritis and hypergastrinemia.
• C. H. pylori-associated gastritis typically involves the antrum and does not classically spare it, and does not directly cause B12 deficiency through parietal cell autoimmunity.
• D. Antibodies against the thyrotropin receptor cause Graves disease, which is a separate autoimmune condition that may coexist but does not cause this patient's anemia.

Warm autoimmune hemolytic anemia is caused by IgG autoantibodies that bind red cell surface antigens optimally at 37°C. Splenic macrophages recognize the Fc portion of IgG and phagocytose part or all of the red cell membrane, producing spherocytes and extravascular hemolysis predominantly in the spleen. This process is reflected by a positive direct antiglobulin (Coombs) test for IgG, elevated indirect bilirubin and LDH, and low haptoglobin. Warm AIHA is associated with autoimmune diseases such as SLE and lymphoproliferative disorders such as CLL.

• A. IgM-mediated complement lysis describes cold agglutinin disease, not warm AIHA with IgG and spherocytes.
• B. G6PD deficiency causes episodic hemolysis with Heinz bodies and bite cells after oxidant stress, not a Coombs-positive process.
• D. A primary membrane protein defect describes hereditary spherocytosis, which would not have a positive Coombs test.
• E. Fibrin strand shearing causes schistocytes in microangiopathic hemolytic anemia, not the spherocytes seen here.

Cold agglutinin disease is caused by IgM autoantibodies, frequently triggered by Mycoplasma pneumoniae infection (or lymphoproliferative disorders such as CLL in older patients), that bind red cell antigens at low temperatures in the peripheral circulation. IgM strongly activates complement, which deposits C3 on the red cell surface; the direct antiglobulin test is therefore positive for complement but negative for IgG, since IgM itself dissociates at warmer temperatures before testing. Acrocyanosis in extremities exposed to cold reflects agglutination-induced sludging in peripheral capillaries. Warming the blood causes the agglutinated cells to disperse, confirming the cold-dependent nature of the antibody.

• A. ADAMTS13 deficiency causes TTP with schistocytes, not a complement-positive, IgG-negative agglutination picture.
• B. Mechanical valve hemolysis produces schistocytes from shear stress, not temperature-dependent agglutination with acrocyanosis.
• C. Shiga toxin causes HUS with schistocytes and thrombocytopenia, not IgM-mediated agglutination.
• D. A lupus-associated IgG mechanism would produce a warm, Coombs-IgG-positive hemolysis with spherocytes, not cold-induced agglutination with acrocyanosis.

Schistocytes (helmet cells, fragmented red cells) arise when red cells are forced through small vessels narrowed or occluded by platelet-rich microthrombi, as occurs in thrombotic thrombocytopenic purpura, hemolytic uremic syndrome, and disseminated intravascular coagulation. The intravascular shear stress fragments the red cell membrane, producing the characteristic jagged fragments seen on smear, together with thrombocytopenia from platelet consumption and elevated LDH from cell lysis. A normal PT/PTT in this patient argues against DIC and supports TTP, but the smear mechanism (mechanical shear from microthrombi) is shared by all microangiopathic processes. Haptoglobin is consumed because it binds free hemoglobin released by intravascular hemolysis.

• A. Ankyrin deficiency underlies hereditary spherocytosis, which produces spherocytes rather than fragmented cells.
• B. Cold IgM-mediated lysis causes agglutination and acrocyanosis, not schistocyte formation.
• D. Unstable hemoglobin causing oxidative damage produces Heinz bodies and bite cells, not helmet cells.
• E. Splenic phagocytosis of antibody-coated cells produces spherocytes, not fragmented schistocytes, and is not consistent with this smear.

ADAMTS13 is a plasma metalloprotease that normally cleaves large von Willebrand factor (vWF) multimers released from endothelial cells into smaller, less adhesive forms. Severe deficiency of ADAMTS13 (due to autoantibody inhibition or, less commonly, a genetic defect) allows unusually large vWF multimers to persist in the circulation, where they spontaneously bind platelets and promote widespread platelet microthrombus formation in small vessels. This causes the classic findings of TTP: thrombocytopenia from platelet consumption, microangiopathic hemolytic anemia from shear-induced red cell fragmentation, and organ ischemia (notably neurologic and renal), typically with a normal PT/PTT/fibrinogen since the coagulation cascade is not primarily activated.

• A. Extrinsic pathway consumption of coagulation factors describes DIC, which classically shows abnormal PT/PTT and low fibrinogen.
• C. Loss of protein C function describes a hypercoagulable state unrelated to ADAMTS13 or vWF multimers.
• D. Impaired fibrin degradation is not the mechanism of ADAMTS13 deficiency, which acts on vWF rather than fibrin.
• E. Uncontrolled complement activation on endothelium underlies atypical HUS, not ADAMTS13 deficiency.

Enterohemorrhagic E. coli O157:H7 produces Shiga-like toxin, which inhibits the 60S ribosomal subunit and halts protein synthesis in target cells, particularly the glomerular endothelium, which is rich in the toxin's receptor (globotriaosylceramide, Gb3). Toxin-induced endothelial injury exposes subendothelial collagen and promotes local platelet activation and microthrombus formation, producing the triad of microangiopathic hemolytic anemia (schistocytes), thrombocytopenia, and acute kidney injury that defines HUS. This typically follows a prodrome of bloody diarrhea after ingestion of undercooked beef or contaminated produce, and antibiotics are generally avoided because bacterial killing can increase toxin release.

• A. ADP-ribosylation of EF-2 describes diphtheria toxin and Pseudomonas exotoxin A, not Shiga toxin.
• B. Adenylate cyclase activation describes cholera toxin and E. coli heat-labile toxin, which cause watery diarrhea, not HUS.
• D. HUS results from a circulating toxin's endothelial effects, not direct bacterial invasion of the kidney.
• E. Superantigen-mediated T-cell activation describes toxic shock syndrome toxins, not Shiga toxin.

Aplastic anemia results from immune-mediated, typically T-cell-mediated, destruction of hematopoietic stem cells, leading to pancytopenia with a hypocellular, fatty marrow and low reticulocyte count reflecting the inability to produce new blood cells. Known triggers include viral infections (parvovirus B19, particularly in patients with an underlying hemolytic condition), ionizing radiation, and drugs such as chloramphenicol and certain chemotherapeutics, although many cases are idiopathic and autoimmune in nature. The absence of blasts, abnormal infiltrate, or organomegaly distinguishes this from a leukemic or infiltrative process, and the low reticulocyte count with marrow fatty replacement is the key diagnostic finding.

• A. Malignant lymphocytic infiltration describes lymphoma or lymphocytic leukemia involving the marrow, which would show cellular infiltration, not fatty hypocellularity.
• B. Clonal blast proliferation describes acute leukemia, which would show a hypercellular marrow with blasts, not hypocellularity.
• C. Megaloblastic maturation arrest occurs in vitamin B12 or folate deficiency, with a hypercellular marrow showing giant precursors, not marrow aplasia.
• E. Antibody-mediated destruction of mature cells describes peripheral cytopenias such as ITP or AIHA, not a hypocellular marrow failure state.

In anemia of chronic disease, chronic inflammation (as in rheumatoid arthritis) increases hepatic production of hepcidin, which degrades ferroportin on macrophages and enterocytes. This traps iron within macrophages and reduces dietary iron absorption, producing low serum iron despite normal or increased total body iron stores. Because iron is sequestered rather than depleted, ferritin (an acute phase reactant and marker of stored iron) is normal to high, and TIBC (transferrin) is low because transferrin synthesis is also downregulated by inflammation, distinguishing this pattern from iron deficiency anemia, where ferritin is low and TIBC is high.

• B. Chronic GI blood loss would cause iron deficiency anemia with low ferritin and high TIBC, not the pattern seen here.
• C. B12 deficiency causes megaloblastic anemia with macrocytosis, not the microcytic/normocytic picture with low TIBC seen here.
• D. Autoantibody-mediated marrow destruction describes pure red cell aplasia, which does not explain this iron profile.
• E. A beta-globin point mutation describes beta-thalassemia, which presents with microcytosis and a different iron profile from early childhood.

Leukocyte alkaline phosphatase (LAP) score distinguishes a benign leukemoid reaction from chronic myeloid leukemia (CML). In a leukemoid reaction, driven by infection or inflammation, mature neutrophils express high levels of alkaline phosphatase, giving a high LAP score. In CML, the neoplastic granulocytes derived from the BCR-ABL1 fusion clone have low LAP activity, so the LAP score is characteristically low despite the markedly elevated white cell count. This distinction, together with basophilia and massive splenomegaly favoring CML, helps separate a reactive process from a myeloproliferative neoplasm, which can be confirmed with cytogenetic or molecular testing for the Philadelphia chromosome.

• A. Lactate reflects tissue hypoperfusion or sepsis severity, not marrow clonality.
• C. ESR is a nonspecific marker of inflammation elevated in infection but not helpful in distinguishing CML from a leukemoid reaction.
• D. LDH is elevated in many high-turnover states, including both infection and malignancy, and does not reliably distinguish the two.
• E. Uric acid rises with any high cell turnover state and does not differentiate a reactive from a neoplastic leukocytosis.

This child's presentation with pancytopenia-related symptoms, marked leukocytosis with blasts, and a background of Down syndrome is classic for acute lymphoblastic leukemia (ALL), the most common childhood malignancy and the most common cancer overall in children, with increased incidence in trisomy 21. ALL blasts are lymphoid precursors that express terminal deoxynucleotidyl transferase (TdT), a DNA polymerase found in pre-B and pre-T lymphoblasts but not in myeloid cells or mature lymphocytes, making it a key immunohistochemical marker distinguishing ALL from AML. TdT positivity, combined with small blasts having scant cytoplasm and condensed chromatin, supports a lymphoblastic rather than myeloblastic origin.

• A. Myeloperoxidase is a marker of myeloid differentiation, positive in AML, not in lymphoblastic leukemia.
• B. Tartrate-resistant acid phosphatase is characteristic of hairy cell leukemia, not ALL.
• D. CD15 is expressed on myeloid cells and granulocytes, not lymphoblasts.
• E. Alpha-naphthyl butyrate esterase is a marker for monocytic differentiation, as seen in acute monocytic leukemia.

This patient's presentation with cytopenias, gingival infiltration, and peripheral blasts containing needle-like cytoplasmic inclusions (Auer rods) that stain strongly for myeloperoxidase is diagnostic of acute myeloid leukemia (AML). Auer rods are crystalline aggregates of azurophilic granule material, and their presence confirms myeloid lineage. Myeloperoxidase positivity further supports granulocytic differentiation, distinguishing AML from lymphoblastic leukemias, which lack Auer rods and are myeloperoxidase-negative. Gum infiltration is particularly associated with the monocytic subtype of AML.

• A. Blast crisis of CML can show myeloid or lymphoid blasts but occurs in a patient with a known antecedent chronic phase and typically massive splenomegaly, not described here.
• C. Myelodysplastic syndrome shows dysplastic changes with cytopenias but blast counts below 20% in the bone marrow, not an acute leukemic blast population with Auer rods.
• D. CLL presents with mature small lymphocytes and smudge cells in older adults, not blasts with Auer rods.
• E. ALL blasts lack Auer rods and are myeloperoxidase-negative; they instead stain positive for TdT.

Acute promyelocytic leukemia (APL) is caused by t(15;17), producing the PML-RARA fusion protein, which blocks myeloid differentiation at the promyelocyte stage and recruits corepressors that silence genes needed for maturation. The abnormal promyelocytes release procoagulant granule contents, causing disseminated intravascular coagulation, reflected here by prolonged PT, hypofibrinogenemia, thrombocytopenia, and elevated D-dimer. All-trans retinoic acid (ATRA) binds the retinoic acid receptor portion of the PML-RARA fusion protein, reversing the differentiation block and inducing the promyelocytes to mature into terminally differentiated neutrophils, which then undergo apoptosis. This differentiation mechanism distinguishes ATRA from cytotoxic chemotherapy.

• A. Dihydrofolate reductase inhibition describes methotrexate's mechanism, which is not the targeted treatment for APL.
• B. Alkylation is the mechanism of agents like cyclophosphamide, not ATRA.
• C. Tyrosine kinase inhibition describes imatinib's mechanism in BCR-ABL-driven CML, not the mechanism of ATRA in APL.
• D. Topoisomerase II inhibition describes drugs such as etoposide, not the mechanism used to treat APL specifically.

This patient has tumor lysis syndrome, characterized by hyperkalemia, hyperphosphatemia, hypocalcemia (from calcium-phosphate precipitation), and markedly elevated uric acid leading to acute kidney injury, a known complication following initiation of chemotherapy in high-grade lymphomas such as Burkitt lymphoma with rapid cell turnover. Rasburicase is a recombinant urate oxidase that catalyzes the enzymatic breakdown of uric acid into allantoin, a much more water-soluble compound that is readily excreted, making it the most effective agent for rapidly lowering uric acid and preventing urate nephropathy in high-risk patients prior to chemotherapy. Allopurinol works by blocking new uric acid formation but does not reduce existing uric acid levels as rapidly or effectively as rasburicase in high-risk settings.

• B. Sodium bicarbonate was previously used to alkalinize urine but is no longer routinely recommended because it can promote calcium-phosphate crystallization.
• C. Allopurinol inhibits xanthine oxidase to prevent new uric acid formation but acts more slowly and is less effective than rasburicase in high-risk tumor lysis prevention.
• D. Calcium gluconate treats symptomatic hypocalcemia or hyperkalemia-induced arrhythmia but does not prevent the uric acid-mediated kidney injury.
• E. Furosemide promotes diuresis but does not address the underlying hyperuricemia driving the kidney injury.

This elderly patient's presentation of asymptomatic lymphocytosis with small, mature-appearing lymphocytes, smudge cells on peripheral smear, and B cells co-expressing CD5 (normally a T-cell marker) along with CD23 is characteristic of chronic lymphocytic leukemia (CLL), the most common leukemia in older adults in the United States. Smudge cells result from the mechanical fragility of the malignant lymphocytes during slide preparation. CLL is typically an indolent disease often discovered incidentally, in contrast to more aggressive lymphoid malignancies, and the combination of CD5+/CD23+ B cells is a key immunophenotypic clue for diagnosis.

• A. Infectious mononucleosis occurs in young patients with atypical reactive T lymphocytes, not smudge cells or a CD5+/CD23+ B-cell population in an elderly patient.
• C. Hairy cell leukemia shows cells with fine cytoplasmic projections and is tartrate-resistant acid phosphatase positive, not CD5+/CD23+ smudge cells.
• D. Follicular lymphoma is CD10-positive and associated with t(14;18), not the CD5+/CD23+ phenotype seen here.
• E. Mantle cell lymphoma is also CD5+ but is typically CD23-negative and presents with lymphadenopathy and t(11;14), not smudge cells on peripheral smear.

This patient's markedly elevated white cell count with full granulocytic maturation, splenomegaly, and basophilia along with the t(9;22) translocation is classic for chronic myeloid leukemia (CML). This translocation fuses the BCR gene on chromosome 22 with the ABL1 gene on chromosome 9, creating the BCR-ABL fusion protein with constitutively active tyrosine kinase activity that drives uncontrolled myeloid proliferation. Imatinib is a tyrosine kinase inhibitor that works by competitively binding the ATP-binding site within the kinase domain of BCR-ABL, blocking its ability to phosphorylate downstream substrates and thereby halting the proliferative signaling cascade. This targeted mechanism distinguishes imatinib from cytotoxic chemotherapy and has transformed CML into a chronic, manageable disease for most patients.

• B. Topoisomerase II inhibition describes agents such as etoposide, unrelated to BCR-ABL tyrosine kinase inhibition.
• C. Dihydrofolate reductase inhibition describes methotrexate's mechanism, unrelated to tyrosine kinase inhibition in CML.
• D. Monoclonal antibody mechanisms apply to agents like rituximab, not to imatinib, which is a small-molecule kinase inhibitor.
• E. Differentiation induction describes the mechanism of all-trans retinoic acid in acute promyelocytic leukemia, not imatinib in CML.

Hairy cell leukemia is a B-cell neoplasm characterized by TRAP-positive cells with fine cytoplasmic projections, marrow fibrosis causing a dry tap, splenomegaly without lymphadenopathy, and pancytopenia. Nearly all cases harbor an activating BRAF V600E point mutation, which constitutively activates the RAF-MEK-ERK signaling pathway and drives proliferation; this finding is used diagnostically and as a target for BRAF inhibitor therapy (e.g., vemurafenib). The other mutations are characteristic of different hematologic malignancies and are not associated with this clinical and cytochemical picture.

• A. BCR-ABL1 is the hallmark of chronic myeloid leukemia, which presents with marked leukocytosis and basophilia, not TRAP-positive hairy cells.
• B. JAK2 V617F is seen in myeloproliferative neoplasms such as polycythemia vera, not hairy cell leukemia.
• C. FLT3-ITD is an acute myeloid leukemia mutation associated with poor prognosis, unrelated to this indolent B-cell leukemia.
• D. PML-RARA defines acute promyelocytic leukemia, which presents with blasts containing azurophilic granules and DIC risk, not hairy cells.

Hodgkin lymphoma is characterized by Reed-Sternberg cells, large binucleate ("owl's eye") cells that are CD15+ and CD30+ but typically CD20-negative, set within a reactive background of lymphocytes, eosinophils, plasma cells, and histiocytes. It shows a bimodal age distribution with peaks in young adults and older adults, and the classic/mixed cellularity subtype has a strong association with EBV infection. The disease classically presents with painless lymphadenopathy, often cervical, with contiguous spread and systemic B symptoms such as fever and weight loss.

• A. Peripheral T-cell lymphoma lacks the characteristic CD15+/CD30+ Reed-Sternberg cells and reactive mixed infiltrate.
• C. Diffuse large B-cell lymphoma shows sheets of large atypical B cells that are CD20-positive, not scattered Reed-Sternberg cells in a reactive background.
• D. Anaplastic large cell lymphoma is CD30+ but is a T-cell lineage neoplasm that is ALK-positive in many cases and lacks the classic reactive background and CD15 positivity of Hodgkin lymphoma.
• E. Burkitt lymphoma shows a monotonous population of medium-sized blasts with a starry sky pattern, not binucleate giant cells.

Burkitt lymphoma is driven by a t(8;14) translocation that relocates the MYC proto-oncogene from chromosome 8 to a position adjacent to the immunoglobulin heavy chain enhancer on chromosome 14, causing constitutive overexpression of MYC and unchecked cellular proliferation. This produces the characteristic starry sky histology, where high proliferation and apoptosis create a monotonous sheet of lymphoid cells interspersed with tingible-body macrophages. The endemic (African) form is strongly associated with EBV infection and classically presents as a jaw or facial bone mass in children.

• A. PAX5 rearrangements are not the defining molecular lesion of Burkitt lymphoma; the hallmark abnormality is MYC dysregulation.
• B. This describes the t(11;14) translocation of mantle cell lymphoma, which overexpresses cyclin D1, not MYC.
• D. BCR-ABL1 is the product of the t(9;22) Philadelphia chromosome seen in chronic myeloid leukemia, not Burkitt lymphoma.
• E. This describes the t(14;18) translocation of follicular lymphoma, which overexpresses the anti-apoptotic BCL2 protein rather than MYC.

Follicular lymphoma is characterized by a t(14;18) translocation that juxtaposes the BCL2 gene with the immunoglobulin heavy chain locus, resulting in BCL2 overexpression. BCL2 is an anti-apoptotic protein that resides in the mitochondrial outer membrane and inhibits pro-apoptotic proteins such as BAX and BAK, preventing mitochondrial outer membrane permeabilization and the release of cytochrome c. This blocks the intrinsic apoptotic pathway, allowing abnormal lymphoid cells to accumulate and form the characteristic neoplastic follicles seen on biopsy.

• A. Constitutive tyrosine kinase activation describes BCR-ABL1 in chronic myeloid leukemia, not the BCL2-driven mechanism of follicular lymphoma.
• B. Follicular lymphoma results from a gain-of-function translocation causing BCL2 overexpression, not from loss of a tumor suppressor.
• C. MYC-driven unchecked cell cycle entry describes the mechanism of Burkitt lymphoma, not follicular lymphoma.
• E. Overexpression of cyclin D1 driving G1-S progression describes the mechanism of mantle cell lymphoma, not follicular lymphoma.

Mantle cell lymphoma is caused by a t(11;14) translocation that places the CCND1 gene, encoding cyclin D1, next to the immunoglobulin heavy chain locus, resulting in cyclin D1 overexpression. Cyclin D1 binds CDK4/6 and promotes phosphorylation of the retinoblastoma protein, driving cells through the G1-S checkpoint in an unregulated manner. This produces a monotonous proliferation of small lymphocytes that expand the mantle zone, as seen in this biopsy, and the disease can present with multiple lymphomatous polyposis of the gastrointestinal tract.

• B. Cyclin E is not the product implicated in the characteristic translocation of mantle cell lymphoma; cyclin D1 is.
• C. BCL6 dysregulation is associated with some diffuse large B-cell lymphomas, not the t(11;14) translocation described here.
• D. MYC overexpression from t(8;14) is the mechanism of Burkitt lymphoma, not mantle cell lymphoma.
• E. BCL2 overexpression from t(14;18) is the mechanism of follicular lymphoma, not mantle cell lymphoma.

Diffuse large B-cell lymphoma is the most common non-Hodgkin lymphoma in adults and presents as a rapidly growing, often extranodal or nodal mass with constitutional symptoms. Histology shows diffuse (non-follicular) effacement of the normal architecture by large atypical B cells expressing CD20, with a high proliferation rate accounting for the aggressive clinical course. Unlike Hodgkin lymphoma, the neoplastic cells are CD20-positive and CD15-negative, and unlike follicular or mantle cell lymphoma, the growth pattern is diffuse rather than nodular.

• A. Hodgkin lymphoma shows CD15+/CD30+ Reed-Sternberg cells in a reactive background, not diffuse CD20+ large cell sheets.
• C. Mantle cell lymphoma shows small to medium lymphocytes expanding the mantle zone and is associated with t(11;14), not large diffuse sheets of atypical cells.
• D. Follicular lymphoma shows a nodular (follicular) growth pattern of small cleaved lymphocytes, an indolent course, and is associated with t(14;18), not a diffuse sheet of large cells.
• E. Chronic lymphocytic leukemia/small lymphocytic lymphoma shows small, mature-appearing lymphocytes with an indolent course, not large atypical cells with an aggressive presentation.

In multiple myeloma, clonal plasma cells in the bone marrow stimulate stromal cells and osteoblasts to increase expression of RANKL while decreasing osteoprotegerin (a RANKL decoy receptor), and plasma cells also secrete IL-6, which further promotes osteoclast differentiation and activity. The resulting increase in osteoclastic bone resorption relative to osteoblastic bone formation produces the characteristic punched-out lytic lesions seen on imaging and contributes to hypercalcemia. This mechanism, along with anemia from marrow crowding, renal injury, and the monoclonal (M) spike on SPEP, forms the basis of the CRAB criteria used to diagnose symptomatic myeloma.

• B. Myeloma bone lesions are lytic, not sclerotic; osteoblastic activity is suppressed, not stimulated, in this disease.
• C. Secondary hyperparathyroidism from vitamin D deficiency is not the mechanism of myeloma bone disease, which is driven by local cytokine-mediated osteoclast activation.
• D. Lytic lesions in myeloma result from cytokine-driven osteoclast activation, not complement-mediated lysis of bone cells.
• E. Amyloid deposition can occur in myeloma-associated AL amyloidosis but does not cause the classic punched-out lytic bone lesions; the mechanism is osteoclast activation via RANKL.

Myeloma cast nephropathy results from excess monoclonal free light chains (Bence Jones proteins) filtered by the glomerulus that bind to Tamm-Horsfall protein in the distal tubule and collecting duct, forming insoluble, eosinophilic, fractured casts that obstruct the tubule lumen and incite a surrounding giant cell reaction. The light chains are also directly toxic to tubular epithelial cells, causing apoptosis and further impairing renal function. Because the excreted protein is primarily light chains rather than albumin, a standard urine dipstick (which detects mainly albumin) can be negative or only mildly positive despite a markedly positive result on urine protein electrophoresis.

• B. Tumor lysis syndrome with urate nephropathy is not characteristic of myeloma and does not explain the light chain casts seen on biopsy.
• C. Hypercalcemia can contribute to renal dysfunction in myeloma via vasoconstriction and volume depletion, but it does not explain the fractured casts and giant cell reaction seen on this biopsy.
• D. Immune complex-mediated glomerulonephritis is not the mechanism of myeloma kidney; the injury is tubular and related to light chain cast formation, not glomerular immune complex deposition.
• E. AL amyloidosis causes glomerular deposition and nephrotic syndrome with heavy proteinuria, not the obstructive tubular cast pattern with fractured casts and giant cells described here.

This patient has a monoclonal IgM gammopathy with hyperviscosity symptoms (visual changes, headache, retinal vein dilation) and peripheral neuropathy but no lytic bone lesions, consistent with Waldenstrom macroglobulinemia, a lymphoplasmacytic lymphoma that secretes large pentameric IgM molecules. The large size and pentameric structure of IgM markedly raise serum viscosity, producing the hyperviscosity syndrome. Lytic bone lesions are characteristic of multiple myeloma (IgG or IgA) but are absent in Waldenstrom because the malignant cells rarely cause significant osteoclast activation. Bone marrow shows lymphoplasmacytic infiltration rather than the sheets of plasma cells seen in myeloma.

• A. CLL presents with lymphocytosis and lymphadenopathy, not a monoclonal IgM spike causing hyperviscosity.
• B. MGUS is asymptomatic by definition and does not cause hyperviscosity symptoms or organomegaly.
• C. Multiple myeloma classically causes lytic bone lesions, hypercalcemia, and renal failure, which are absent here.
• E. Amyloidosis presents with organ infiltration by light chains, not a hyperviscosity syndrome from a macroglobulin.

Monoclonal gammopathy of undetermined significance (MGUS) is defined by serum M protein less than 3 g/dL, bone marrow clonal plasma cells less than 10%, and absence of CRAB features (hypercalcemia, renal insufficiency, anemia, bone lesions) attributable to the plasma cell disorder. This patient's M spike, plasma cell percentage, and normal calcium, creatinine, and lack of lytic lesions all fall below myeloma thresholds. Multiple myeloma requires clonal plasma cells of 10% or greater or a biopsy-proven plasmacytoma plus at least one myeloma-defining event. MGUS carries a roughly 1% per year risk of progression to myeloma and requires monitoring rather than treatment.

• B. Age is not a diagnostic criterion for distinguishing MGUS from multiple myeloma.
• C. A monoclonal protein is present here; its presence does not distinguish MGUS from myeloma, only its magnitude and associated findings do.
• D. The plasma cells in MGUS are clonal, not polyclonal; monoclonality is shared with myeloma.
• E. Free light chain ratio can be abnormal in MGUS as well and alone does not define the distinction from myeloma.

This patient has erythromelalgia, aquagenic pruritus, splenomegaly, thrombosis, and markedly elevated hemoglobin/hematocrit with a low erythropoietin level, classic for polycythemia vera. PV is caused in most cases by a point mutation (V617F) in JAK2, a cytoplasmic tyrosine kinase that normally transmits signaling from the erythropoietin receptor; the mutation causes constitutive, erythropoietin-independent activation of downstream JAK-STAT signaling, producing autonomous erythrocytosis. Because red cell production is independent of erythropoietin, the appropriately low EPO level (suppressed by negative feedback from the elevated hematocrit) distinguishes PV from secondary polycythemia, in which EPO is elevated. This also explains the frequent accompanying leukocytosis and thrombocytosis, since JAK2 signaling affects multiple myeloid lineages.

• A. Polycythemia vera is not caused by an autoantibody against the erythropoietin receptor.
• B. Chronic hypoxia with compensatory EPO elevation describes secondary polycythemia, which would show a high, not low, EPO level.
• C. A fusion tyrosine kinase from a translocation describes chronic myeloid leukemia (BCR-ABL1), not polycythemia vera.
• E. Ectopic EPO secretion also causes secondary polycythemia with elevated EPO, not the low EPO seen in this patient.

This patient has marked, isolated thrombocytosis with megakaryocytic hyperplasia on bone marrow biopsy and digital erythromelalgia from microvascular platelet aggregation, findings consistent with essential thrombocythemia. About half of ET cases carry the JAK2 V617F mutation, which activates the JAK-STAT pathway downstream of the thrombopoietin receptor (MPL) and drives thrombopoietin-independent megakaryocyte proliferation; a smaller subset instead carries CALR or MPL mutations. The massively elevated, dysfunctional platelet mass paradoxically causes both thrombotic complications (erythromelalgia, stroke, DVT) and, at very high counts, acquired bleeding from consumption of von Willebrand factor.

• B. NPM1 mutations are characteristic of a subtype of acute myeloid leukemia with a relatively favorable prognosis, not a myeloproliferative disorder.
• C. FLT3 mutations are associated with acute myeloid leukemia and confer a worse prognosis, not essential thrombocythemia.
• D. RUNX1 mutations are seen in acute myeloid leukemia and myelodysplastic syndromes, not essential thrombocythemia.
• E. BCR-ABL1 is the fusion kinase of chronic myeloid leukemia, which presents with marked leukocytosis and basophilia, not isolated thrombocytosis.

This patient has massive splenomegaly, constitutional symptoms, a dry bone marrow tap, and biopsy showing extensive marrow fibrosis with atypical megakaryocyte clusters, diagnostic of primary myelofibrosis. The leukoerythroblastic peripheral smear, with teardrop cells (dacryocytes), nucleated red cells, and immature myeloid forms, reflects extramedullary hematopoiesis in the spleen and liver as the fibrotic marrow fails to support normal hematopoiesis. Most cases are driven by JAK2, CALR, or MPL mutations causing megakaryocyte hyperplasia, which release fibrogenic cytokines such as platelet-derived growth factor and transforming growth factor-beta that stimulate fibroblast proliferation and marrow collagen deposition.

• B. Acute myeloid leukemia presents with circulating blasts exceeding 20% of marrow cellularity, not a leukoerythroblastic smear with teardrop cells.
• C. CML typically shows marked leukocytosis with a full spectrum of granulocytic maturation and basophilia, not a fibrotic dry tap with teardrop cells.
• D. Myelodysplastic syndrome shows dysplastic changes and cytopenias but not the dense marrow fibrosis and dry tap characteristic of myelofibrosis.
• E. Hairy cell leukemia causes splenomegaly and pancytopenia with cells showing hairy cytoplasmic projections, not marrow fibrosis with teardrop cells.

This child has isolated thrombocytopenia with petechiae and purpura following a viral illness, normal coagulation studies, and no schistocytes or organomegaly, consistent with immune thrombocytopenic purpura (ITP). In ITP, IgG autoantibodies, most commonly directed against platelet surface glycoprotein IIb/IIIa, bind circulating platelets, which are then opsonized and phagocytosed by splenic macrophages via Fc receptors, shortening platelet lifespan. Megakaryocytes in the bone marrow are typically increased, not decreased, as a compensatory response. Coagulation times remain normal because the clotting cascade is intact; only platelet number is affected.

• B. Heparin-induced thrombocytopenia requires prior heparin exposure, which this patient does not have.
• C. Bone marrow megakaryocytes are increased, not decreased, in ITP as a compensatory response to peripheral platelet destruction.
• D. ADAMTS13 deficiency causes TTP, which presents with schistocytes, renal dysfunction, and neurologic symptoms, not seen here.
• E. Microvascular thrombi with platelet consumption describes thrombotic microangiopathies such as TTP or HUS, which would show schistocytes and often renal or neurologic findings.

This child has thrombocytopenia with giant platelets, a prolonged bleeding time, and ristocetin-induced aggregation that fails to correct with mixing normal plasma, consistent with Bernard-Soulier syndrome, an autosomal recessive disorder (consistent with the consanguineous parents) caused by deficiency of glycoprotein Ib. GPIb is the platelet receptor for von Willebrand factor and is essential for platelet adhesion to exposed subendothelial collagen at sites of vascular injury. Because the defect is intrinsic to the platelet receptor rather than a deficiency of plasma von Willebrand factor, adding normal plasma does not correct the ristocetin aggregation defect, distinguishing it from von Willebrand disease. Aggregation to ADP, epinephrine, and collagen remains normal because those pathways act through other receptors.

• A. GPVI is a collagen receptor; its deficiency would impair collagen-induced aggregation but not primarily cause the ristocetin defect described.
• C. P2Y12 is the ADP receptor targeted by clopidogrel; its defect impairs ADP-induced aggregation specifically, not ristocetin response.
• D. Thromboxane A2 receptor defects impair platelet activation amplification but do not cause the giant platelets or ristocetin defect seen here.
• E. GPIIb/IIIa deficiency (Glanzmann thrombasthenia) causes defective platelet-platelet aggregation to ADP, epinephrine, and collagen, but ristocetin-induced agglutination is typically preserved.

This child has a normal platelet count with a prolonged bleeding time, normal ristocetin-induced agglutination, but absent aggregation to ADP, epinephrine, and collagen, consistent with Glanzmann thrombasthenia, an autosomal recessive disorder caused by deficiency of glycoprotein IIb/IIIa. This integrin receptor on activated platelets binds fibrinogen, which cross-links adjacent platelets to form platelet-platelet aggregates during primary hemostasis. Because the defect is in platelet-platelet linkage rather than platelet-vessel wall adhesion, ristocetin-induced agglutination (which depends on GPIb-von Willebrand factor interaction) remains normal, distinguishing this condition from Bernard-Soulier syndrome. Platelet count and coagulation times are normal because platelet number and the secondary coagulation cascade are unaffected.

• A. Von Willebrand factor deficiency impairs ristocetin-induced aggregation, which is normal in this patient, making this mechanism inconsistent.
• B. Deficiency of the GPIb receptor for von Willebrand factor causes Bernard-Soulier syndrome, which would show giant platelets and absent ristocetin aggregation, not the pattern described here.
• C. A plasma coagulation factor deficiency would prolong PT or PTT, both of which are normal here, pointing to a primary platelet defect instead.
• E. Autoantibody-mediated platelet destruction describes ITP, which causes thrombocytopenia, whereas this patient has a normal platelet count.

Heparin-induced thrombocytopenia (HIT) occurs when heparin binds platelet factor 4 (PF4), creating a neoepitope that provokes IgG antibody formation. The resulting immune complexes bind platelet Fc receptors, causing widespread platelet activation, aggregation, and consumption (thrombocytopenia) along with release of procoagulant platelet microparticles. This paradoxically causes thrombosis rather than bleeding despite the low platelet count, producing arterial or venous occlusions such as the ischemic limb seen here. Heparin must be stopped immediately and a non-heparin anticoagulant (e.g., argatroban) started.

• B. HIT is not primarily complement-mediated lysis; it is Fc-receptor-mediated platelet activation.
• C. HIT involves PF4-heparin antibodies, not anti-GPIb antibodies or splenic sequestration as the primary mechanism.
• D. Heparin does not directly cross-link GPIIb/IIIa; this is not the mechanism of HIT.
• E. Heparin potentiates antithrombin III rather than depleting it; this is heparin's therapeutic mechanism, not HIT.

An isolated elevated PTT with normal PT and normal platelet count in a male child with hemarthrosis suggests a deficiency of an intrinsic pathway factor (factor VIII in hemophilia A or factor IX in hemophilia B). The mixing study corrects the PTT because normal plasma supplies the missing factor, confirming a factor deficiency rather than an inhibitor. If an inhibitor (such as a factor VIII autoantibody) were present, the PTT would fail to correct because the inhibitor would also neutralize the factor in the added normal plasma. Specific factor assays are needed to distinguish factor VIII deficiency (hemophilia A) from factor IX deficiency (hemophilia B).

• B. An inhibitor would prevent correction of the PTT on mixing study; here the PTT corrected.
• C. Von Willebrand disease typically also prolongs bleeding time and is not X-linked; this boy's isolated PTT correction pattern fits a factor deficiency.
• D. Platelet function defects affect bleeding time, not PTT, and platelet count here is normal.
• E. Vitamin K-dependent factor deficiency would also prolong the PT, which is normal here.

The combination of mucocutaneous bleeding, prolonged bleeding time, and mildly prolonged PTT (due to reduced factor VIII, which von Willebrand factor normally stabilizes) suggests von Willebrand disease (vWD). Ristocetin induces vWF binding to platelet GPIb, and in vWD this agglutination is reduced or absent, directly testing vWF function and confirming the diagnosis. vWF also mediates platelet adhesion to subendothelial collagen, so its deficiency impairs primary hemostasis, explaining the prolonged bleeding time. Desmopressin, which releases stored vWF from endothelial cells, is a mainstay of treatment for mild type 1 disease.

• A. PT and factor VII are unaffected in vWD; PT is typically normal.
• B. Fibrinogen and thrombin time assess the common pathway and fibrinogen, not vWF function.
• C. This nonspecific platelet aggregation test does not directly assess vWF-GPIb interaction as ristocetin does.
• D. Megakaryocyte number is normal in vWD; this is not a platelet production disorder.

Sepsis from endometritis triggers widespread release of tissue factor, which activates the extrinsic coagulation pathway and causes systemic thrombin generation with diffuse fibrin deposition in the microvasculature. This consumes platelets and clotting factors (causing thrombocytopenia, prolonged PT/PTT, and low fibrinogen) while plasmin simultaneously degrades the fibrin, producing markedly elevated D-dimer. The combination of low platelets, low fibrinogen, prolonged PT/PTT, and elevated D-dimer in a septic patient is characteristic of disseminated intravascular coagulation (DIC). Treatment focuses on addressing the underlying trigger and supportive transfusion of blood products as needed.

• A. Isolated vWF deficiency does not explain the low fibrinogen, prolonged PT, or elevated D-dimer seen here.
• B. Primary fibrinolysis is rare and would not typically cause the concurrent thrombocytopenia and factor consumption seen with an infectious trigger.
• D. Isolated immune thrombocytopenia would not explain the concurrent coagulation factor consumption, low fibrinogen, and elevated D-dimer.
• E. Liver dysfunction alone would not cause this degree of platelet consumption or the markedly elevated D-dimer reflecting active fibrin turnover.

Factor V Leiden results from a point mutation in the factor V gene (Arg506Gln) that eliminates the cleavage site normally recognized by activated protein C. This renders factor Va resistant to inactivation, allowing it to persist in an active procoagulant form and sustain thrombin generation, leading to a hypercoagulable state. It is the most common inherited thrombophilia in people of European descent and should be suspected in patients with a strong family history of unprovoked venous thromboembolism. Testing can be done via activated protein C resistance assay or genetic analysis.

• A. This describes the prothrombin G20210A mutation, a separate inherited thrombophilia, not factor V Leiden.
• B. Antithrombin III levels are not increased in factor V Leiden; this describes a different mechanism unrelated to the mutation.
• C. Factor V Leiden does not affect protein S synthesis; protein S deficiency is a distinct, separate thrombophilia.
• E. Factor V Leiden's defect is resistance to protein C cleavage, not altered binding to tissue factor pathway inhibitor.

Antiphospholipid syndrome involves antibodies (lupus anticoagulant, anticardiolipin, anti-beta2-glycoprotein I) directed against phospholipid-binding proteins. In vitro, these antibodies interfere with phospholipid-dependent clotting assays, prolonging the PTT, and this fails to correct with normal plasma (mixing study) because the antibody, not a factor deficiency, is responsible; adding excess phospholipid overwhelms the antibody and corrects the PTT, confirming a lupus anticoagulant. Paradoxically, in vivo these antibodies promote platelet activation, endothelial injury, and thrombin generation, causing a hypercoagulable state with recurrent arterial/venous thrombosis and placental infarction leading to late pregnancy loss. This dissociation between prolonged in vitro clotting time and in vivo thrombosis is the classic diagnostic paradox of the syndrome.

• A. Factor XII deficiency prolongs PTT but is not associated with thrombosis or pregnancy loss; it is typically asymptomatic.
• C. Antithrombin III neutralization is not the mechanism; lupus anticoagulant interferes with phospholipid-dependent assays directly, and antithrombin deficiency would not explain the mixing study correction pattern.
• D. DIC-type fibrinogen consumption would also show low fibrinogen and elevated D-dimer, and would not selectively correct with excess phospholipid.
• E. The prothrombin G20210A mutation increases thrombosis risk but does not prolong PTT via phospholipid interference or explain this mixing study pattern.

Protein C has a short plasma half-life (about 8 hours), similar to factor VII, so warfarin rapidly lowers protein C levels within the first days of therapy before the longer half-life procoagulant factors (II and X) are sufficiently depleted. In a patient with an underlying inherited protein C (or protein S) deficiency, this creates a transient relative hypercoagulable state, leading to microvascular thrombosis in the skin and warfarin-induced skin necrosis. This is why heparin bridging is used when initiating warfarin in patients at risk, to maintain anticoagulation while protein C levels fall. The family history of early unprovoked fatal pulmonary embolism suggests an underlying inherited thrombophilia such as protein C deficiency.

• A. This describes heparin-induced thrombocytopenia, which is unrelated to warfarin-induced skin necrosis and involves a different antibody target.
• B. Warfarin does not cause vasospasm through fat accumulation; the mechanism is procoagulant-anticoagulant imbalance.
• D. Fibrinogen is not a vitamin K-dependent factor and is not preferentially depleted by warfarin.
• E. Warfarin's skin necrosis is mediated through disrupted coagulation factor balance, not direct endothelial cytotoxicity.

Unfractionated heparin is a glycosaminoglycan that binds antithrombin III through a unique pentasaccharide sequence, inducing a conformational change that accelerates antithrombin's inhibition of thrombin (factor IIa) and factor Xa by roughly 1000-fold. Because unfractionated heparin has variable bioavailability and a short half-life, its anticoagulant effect is monitored using the aPTT, which reflects the intrinsic pathway. This contrasts with low-molecular-weight heparins, which preferentially inhibit factor Xa and generally do not require routine monitoring. Protamine sulfate can reverse unfractionated heparin by binding it directly and neutralizing its negative charge.

• A. This describes warfarin's mechanism, which is monitored by PT/INR, not aPTT.
• C. Direct thrombin inhibitors like argatroban or bivalirudin act this way, without requiring antithrombin as a cofactor.
• D. This describes thrombolytics such as alteplase, which activate the fibrinolytic system rather than inhibiting coagulation factors.
• E. This describes direct Xa inhibitors such as rivaroxaban or apixaban, not heparin.

Protamine sulfate is a highly positively charged (cationic) protein, originally derived from salmon sperm, that binds to the strongly negatively charged heparin molecule through ionic interactions, forming a stable inactive complex that is cleared from circulation. This neutralizes heparin's anticoagulant activity within minutes and rapidly normalizes the aPTT, making it the treatment of choice for heparin-induced bleeding or heparin overdose, including after cardiopulmonary bypass. Protamine has a lesser and less predictable effect on low-molecular-weight heparins since smaller fragments bind less completely. Rapid protamine administration can itself cause hypotension or anaphylactoid reactions, particularly in patients previously exposed to protamine-containing insulin (e.g., NPH insulin).

• A. This describes idarucizumab, the reversal agent for dabigatran.
• B. This describes andexanet alfa, used to reverse direct factor Xa inhibitors.
• C. Heparin does not act through antithrombin displacement, and no such vitamin K antagonist mechanism applies here.
• D. Vitamin K reverses warfarin, not heparin, and acts over hours to days, not minutes.

Aspirin irreversibly acetylates a serine residue in the active site of cyclooxygenase-1 (COX-1), permanently blocking the enzyme's ability to convert arachidonic acid to prostaglandin H2, the precursor of thromboxane A2. Because mature platelets are anucleate and cannot synthesize new COX-1 protein, this inhibition persists for the entire 7-10 day lifespan of the affected platelets, even though aspirin itself has a plasma half-life of only about 20 minutes. This is why aspirin's antiplatelet effect lasts far longer than its presence in the circulation and why a single low dose produces a sustained reduction in thromboxane A2-mediated platelet aggregation. New, unaffected platelets released from the bone marrow gradually restore overall thromboxane production over several days.

• A. Reversible inhibition would wear off as drug is cleared and would not explain a 7-10 day effect.
• B. This describes clopidogrel's mechanism, not aspirin's.
• C. Aspirin targets cyclooxygenase-1, not thromboxane synthase, and platelets lack mitochondria-based protein synthesis relevant here.
• D. This describes GPIIb/IIIa inhibitors such as abciximab, not aspirin.

Clopidogrel is a thienopyridine prodrug that is converted by hepatic cytochrome P450 enzymes (primarily CYP2C19) to an active metabolite, which irreversibly binds the platelet P2Y12 receptor. This blocks ADP-induced activation of the glycoprotein IIb/IIIa receptor complex, preventing fibrinogen binding and platelet aggregation. Because the inhibition is irreversible, the antiplatelet effect lasts for the lifespan of the affected platelets, similar to aspirin. Genetic polymorphisms in CYP2C19 can reduce conversion to the active metabolite, leading to clopidogrel resistance and reduced antiplatelet efficacy in some patients.

• A. This describes direct thrombin inhibitors, not P2Y12 inhibitors.
• B. This describes aspirin's mechanism, not clopidogrel's.
• C. This describes drugs such as eptifibatide or tirofiban, not clopidogrel.
• D. This describes cilostazol or dipyridamole, not clopidogrel.

Febrile non-hemolytic transfusion reactions are the most common transfusion reaction and result from recipient antibodies against donor human leukocyte antigens (HLA) or other leukocyte antigens, which react with residual white blood cells and platelets in the blood product, or from accumulated cytokines (such as IL-1, IL-6, and TNF-alpha) released by leukocytes during storage of the blood product. This triggers fever and chills without hemolysis, so hemoglobinuria, decreased haptoglobin, and a positive direct antiglobulin test are absent. It is more common in patients with prior transfusions or pregnancies due to prior sensitization to leukocyte antigens. Leukoreduction of blood products reduces the incidence of this reaction.

• A. This describes an acute hemolytic transfusion reaction, which would show hemolysis markers and hypotension.
• B. This describes an anaphylactic reaction, which presents with hypotension, urticaria, or bronchospasm.
• C. Bacterial contamination typically causes high fever with hypotension and signs of sepsis, and would show positive blood cultures from the unit.
• E. This describes transfusion-related acute lung injury, which causes respiratory distress, not isolated fever.

Transfusion-related acute lung injury (TRALI) occurs when donor plasma contains antibodies against recipient human leukocyte antigens (HLA) or neutrophil-specific antigens. These antibodies bind recipient neutrophils, activating and sequestering them in the pulmonary microvasculature, where they degranulate and damage the capillary endothelium, causing noncardiogenic pulmonary edema. This presents within 6 hours of transfusion with acute hypoxemia and bilateral infiltrates, mimicking acute respiratory distress syndrome, but with normal cardiac filling pressures and normal B-type natriuretic peptide, distinguishing it from transfusion-associated circulatory overload. TRALI is a leading cause of transfusion-related mortality, and management is supportive with oxygen and ventilatory support as needed, since it is typically self-limited.

• A. This describes an anaphylactic transfusion reaction, not TRALI.
• B. This describes transfusion-associated circulatory overload, which would show elevated BNP and signs of volume overload.
• D. This describes an acute hemolytic transfusion reaction, which causes hemolysis, not lung injury.
• E. This describes a febrile non-hemolytic transfusion reaction, which does not cause hypoxemia or pulmonary infiltrates.