USMLE Step 1 musculoskeletal system, skin & connective tissue questions

Achondroplasia results from a gain-of-function point mutation in fibroblast growth factor receptor 3 (FGFR3), most commonly G380R in the transmembrane domain. This mutation causes ligand-independent constitutive activation of the receptor, which paradoxically inhibits chondrocyte proliferation and differentiation at the growth plate via the RAS-MAPK pathway. The result is shortened long bones with a normal axial skeleton, since membranous ossification of the skull and vertebral bodies is unaffected. It is the most common cause of dwarfism and is typically sporadic, arising from new mutations in most cases.

• A. This describes RUNX2 haploinsufficiency, seen in cleidocranial dysplasia, not achondroplasia.
• B. This describes a calcium-sensing receptor abnormality, unrelated to FGFR3-mediated growth plate suppression.
• C. This describes a collagen defect, as seen in osteogenesis imperfecta, not FGFR3 mutations.
• D. This describes a lysosomal storage defect, as in mucopolysaccharidoses, not FGFR3 signaling.

This child's recurrent low-impact fractures, blue sclerae, and hearing loss with normal calcium, phosphate, and alkaline phosphatase point to osteogenesis imperfecta, most commonly caused by autosomal dominant mutations in COL1A1 or COL1A2 that reduce type I collagen synthesis or impair its structure. The sclera appears blue because the abnormally thin, defective collagen allows the underlying choroidal veins to show through. Hearing loss results from abnormal ossicle development, and defective dentin collagen can cause dentinogenesis imperfecta. Normal mineral and alkaline phosphatase levels help distinguish this from metabolic bone disease.

• A. Rickets typically shows low calcium, low phosphate, and elevated alkaline phosphatase, which are all normal here.
• B. Osteopetrosis causes dense, fracture-prone bone but presents with anemia, cranial nerve palsies, and bone-in-bone radiographic appearance, not blue sclerae.
• D. While abuse must always be considered with recurrent fractures, the blue sclerae and hearing loss point to an underlying collagen disorder rather than isolated trauma.
• E. Hypophosphatasia presents with low serum alkaline phosphatase and poor bone mineralization, not blue sclerae or hearing loss.

This presentation of diffusely dense, brittle bone with marrow failure (anemia with extramedullary hematopoiesis causing hepatosplenomegaly) and cranial nerve compression from narrowed foramina is characteristic of osteopetrosis. A common cause, particularly in the autosomal recessive infantile form, is carbonic anhydrase II deficiency, which impairs osteoclasts' ability to generate hydrogen ions needed to acidify the resorption lacuna and dissolve bone mineral. Without effective resorption, bone accumulates without being properly remodeled, obliterating the marrow space and narrowing neural foramina. Bone marrow transplant can be curative since osteoclasts are derived from hematopoietic precursors.

• A. Lysyl hydroxylase defects cause a form of Ehlers-Danlos syndrome affecting collagen cross-linking, not osteoclast function.
• B. 25-hydroxylase deficiency impairs vitamin D activation, causing rickets with undermineralized bone, not dense bone.
• D. Tartrate-resistant acid phosphatase is an osteoclast marker but its deficiency is not the classic mechanism of osteopetrosis.
• E. Alkaline phosphatase deficiency causes hypophosphatasia, characterized by poor mineralization, not increased density.

Marfan syndrome is caused by mutations in FBN1, encoding fibrillin-1, a glycoprotein that forms microfibrils in the extracellular matrix and normally sequesters latent transforming growth factor-beta (TGF-beta). Defective fibrillin-1 fails to properly sequester TGF-beta, leading to excessive TGF-beta signaling, which contributes to elastic fiber fragmentation in the aortic media and overgrowth of long bones. This excess TGF-beta activity underlies the aortic root dilation and dissection risk, lens subluxation (from zonule fiber weakness), and skeletal overgrowth seen in this disorder. Angiotensin receptor blockers such as losartan are used therapeutically because they reduce TGF-beta signaling.

• B. PTHrP regulates chondrocyte proliferation at the growth plate, as in disorders like Jansen metaphyseal chondrodysplasia, not Marfan syndrome.
• C. VEGF drives angiogenesis and is not the signaling molecule implicated in fibrillin-1-related connective tissue disease.
• D. IGF-1 mediates growth hormone's effects on linear growth but is not the key pathway disrupted by fibrillin-1 mutations.
• E. BMP2 signaling is central to osteoblast differentiation and skeletal patterning, but is not the primary dysregulated pathway in Marfan syndrome.

This patient's presentation of spontaneous arterial rupture, thin translucent skin, easy bruising, and a family history of sudden vascular death is characteristic of vascular Ehlers-Danlos syndrome (type IV), caused by autosomal dominant mutations in COL3A1, which encodes type III collagen. Type III collagen is a major structural component of hollow organs and blood vessel walls, and its deficiency leads to fragile vasculature prone to spontaneous dissection and rupture, particularly of medium-sized arteries such as the splenic, renal, or mesenteric arteries. These patients are also at risk for bowel and uterine rupture. Management is largely supportive since surgical repair of fragile vessels is technically difficult.

• A. Elastin mutations cause cutis laxa or supravalvular aortic stenosis (Williams syndrome), not this vascular rupture syndrome.
• B. Type I collagen defects cause osteogenesis imperfecta or classical Ehlers-Danlos syndrome, not the vascular rupture phenotype described.
• C. Fibrillin-1 mutations cause Marfan syndrome, which features aortic root aneurysm rather than spontaneous rupture of medium-sized arteries with this skin phenotype.
• E. Lysyl oxidase deficiency impairs collagen cross-linking in cutis laxa, not the vascular type of Ehlers-Danlos syndrome.

Estrogen normally suppresses osteoclastogenesis by limiting osteoblast and stromal cell production of RANKL and by promoting osteoprotegerin (a decoy receptor that binds RANKL and blocks its action). After menopause, falling estrogen levels increase RANKL expression by osteoblasts and stromal cells while decreasing osteoprotegerin, shifting the RANKL/OPG ratio toward increased osteoclast differentiation and activity via RANK signaling. This accelerates bone resorption relative to formation, producing the net bone loss characteristic of postmenopausal osteoporosis, predominantly affecting trabecular bone. This mechanism underlies the rationale for denosumab, a monoclonal antibody against RANKL.

• A. Estrogen does not act primarily by altering PTH receptor density on osteoblasts; the key effect is on the RANKL/OPG axis.
• B. Growth plate chondrocyte apoptosis relates to longitudinal bone growth in childhood, not postmenopausal trabecular bone loss.
• C. Osteoprotegerin is produced by osteoblasts and stromal cells, not osteoclasts, and estrogen loss decreases rather than increases its production, further promoting resorption, not directly.
• E. Postmenopausal osteoporosis is not primarily driven by altered renal calcitriol synthesis; calcium and PTH levels remain normal as in this patient.

This describes a bisphosphonate such as alendronate, which binds hydroxyapatite in bone matrix and is internalized by osteoclasts during active bone resorption. Nitrogen-containing bisphosphonates inhibit farnesyl pyrophosphate synthase in the mevalonate pathway, which disrupts prenylation of small GTPases needed for osteoclast cytoskeletal function and survival, ultimately inducing osteoclast apoptosis. This reduces bone resorption and increases bone mineral density over time. Dosing instructions (empty stomach, upright position, full glass of water) minimize esophageal irritation, a known adverse effect.

• B. This describes selective estrogen receptor modulators like raloxifene, not bisphosphonates.
• C. This describes denosumab, a biologic injection, not an oral drug requiring these administration precautions.
• D. Carbonic anhydrase inhibition is the defect seen in a subtype of osteopetrosis, not the mechanism of bisphosphonates.
• E. This describes teriparatide, a recombinant PTH analog that stimulates bone formation, not a resorption inhibitor.

Measuring intact PTH is the key first step in evaluating hypercalcemia because it distinguishes PTH-dependent causes (primary hyperparathyroidism, where PTH is elevated or inappropriately normal) from PTH-independent causes such as malignancy-associated hypercalcemia, in which PTH is suppressed (often due to elevated PTHrP or ectopic calcitriol). An elevated or high-normal PTH in the setting of hypercalcemia essentially confirms primary hyperparathyroidism and makes further malignancy workup for the cause of hypercalcemia unnecessary.

• A. Useful only after PTH is found to be low, to confirm humoral hypercalcemia of malignancy as the specific mechanism.
• B. Helps distinguish familial hypocalciuric hypercalcemia from primary hyperparathyroidism, not malignancy from hyperparathyroidism.
• D. Nonspecific marker of bone turnover that does not differentiate the underlying cause of hypercalcemia.
• E. Elevated only in granulomatous disease or lymphoma, not the first discriminating test between PTH-mediated and malignancy-related hypercalcemia.

Subperiosteal bone resorption, best seen along the radial sides of the middle phalanges on hand x-rays, is a characteristic finding of osteitis fibrosa cystica caused by chronic PTH excess. Excess PTH stimulates osteoclastic bone resorption, replacing bone with fibrous tissue and cystic brown tumors containing hemosiderin. This finding, combined with hypercalcemia, hypophosphatemia, and elevated PTH, confirms hyperparathyroidism as the cause of the skeletal disease.

• A. Sunburst pattern reflects aggressive periosteal new bone formation seen in osteosarcoma.
• C. Diffuse cortical thickening with bone expansion describes Paget disease, not hyperparathyroidism.
• D. Onion-skin periosteal reaction is characteristic of Ewing sarcoma, not hyperparathyroid bone disease.
• E. Soap-bubble epiphyseal lesions are typical of giant cell tumor of bone.

Paget disease of bone results from excessive, disorganized osteoclastic bone resorption followed by compensatory osteoblastic new bone formation, producing a chaotic mosaic pattern of lamellar bone and markedly elevated alkaline phosphatase, while serum calcium, phosphate, and PTH remain normal because the process is localized rather than a systemic mineral disorder. The disease classically affects the skull, pelvis, and long bones and can present with bone pain, skull enlargement, and hearing loss. Elevated alkaline phosphatase reflects increased osteoblastic activity, and normal calcium/phosphate/PTH help exclude hyperparathyroidism or osteomalacia as the cause.

• A. Osteomalacia from vitamin D deficiency produces undermineralized bone and is usually accompanied by low calcium and phosphate with elevated PTH.
• B. Primary or secondary hyperparathyroidism would be expected to show elevated PTH and abnormal calcium, which are normal here.
• C. This describes osteosarcoma, which is a malignant tumor rather than a benign remodeling disorder confined to a mosaic pattern.
• D. Amyloidosis does not produce this radiographic mosaic pattern or isolated alkaline phosphatase elevation.

In chronic kidney disease, failing kidneys lose the ability to hydroxylate 25-hydroxyvitamin D to active calcitriol (1,25-dihydroxyvitamin D) and also fail to excrete phosphate, causing hyperphosphatemia that further binds and lowers ionized calcium. The resulting chronic hypocalcemia and reduced calcitriol-mediated feedback drive sustained parathyroid gland stimulation, producing secondary hyperparathyroidism. Over time this can progress to renal osteodystrophy, with bone pain and radiographic changes resembling primary hyperparathyroid bone disease.

• A. An autonomous adenoma describes primary, not secondary, hyperparathyroidism and would not be a direct consequence of CKD.
• B. PTHrP-secreting tumors cause humoral hypercalcemia of malignancy, which presents with high calcium rather than this patient's hypocalcemia.
• C. Amyloid infiltration of the parathyroid glands is not a recognized cause of secondary hyperparathyroidism in CKD.
• E. CKD does not increase renal calcium reabsorption, and calcitonin suppression is not the mechanism driving PTH elevation.

Osteochondroma is the most common benign bone tumor, typically arising from the metaphysis of long bones near the growth plate in adolescents and young adults. It consists of a bony stalk continuous with the underlying cortex and medullary cavity, capped by hyaline cartilage, and grows in proportion to the patient's overall skeletal growth before becoming quiescent at skeletal maturity. Its painless, slow-growing nature and characteristic imaging distinguish it from malignant bone tumors.

• A. Chondrosarcoma is a malignant cartilage tumor typically arising centrally within bone in older adults, not as a pedunculated cortical outgrowth in an adolescent.
• B. Osteosarcoma is an aggressive malignant tumor presenting with pain, rapid growth, and a destructive lytic-sclerotic lesion, not a slow-growing painless outgrowth.
• C. Giant cell tumor arises within the epiphysis as a lytic lesion, not as an exophytic cartilage-capped bony projection.
• D. Enchondroma is a benign cartilage tumor located within the medullary cavity of small bones of the hands and feet, not a surface outgrowth from the metaphysis.

Giant cell tumor of bone classically arises in the epiphysis of long bones (most often around the knee) in young adults after growth plate closure, producing a well-defined, eccentric, lytic lesion with a soap-bubble appearance on x-ray. Histologically, it is characterized by numerous osteoclast-like multinucleated giant cells interspersed among mononuclear stromal cells that drive tumor growth and bone resorption through RANKL expression. This epiphyseal location and histology distinguish it from other bone tumors, which typically spare the epiphysis until growth plate closure.

• A. Ewing sarcoma arises in the diaphysis of long bones in children and shows small round blue cells, not multinucleated giant cells.
• B. Aneurysmal bone cyst shows blood-filled cystic spaces separated by septa rather than a solid mass of giant cells and typically lacks the characteristic epiphyseal soap-bubble lesion of this presentation.
• D. Osteosarcoma arises in the metaphysis, not the epiphysis, and shows malignant osteoblasts producing osteoid rather than giant cells on histology.
• E. Osteochondroma is an exophytic cartilage-capped outgrowth from the metaphyseal surface, not a lytic epiphyseal lesion.

Rhomboid-shaped crystals that are positively birefringent (blue when parallel to the slow axis of the compensator) are calcium pyrophosphate dihydrate (CPPD) crystals, diagnostic of pseudogout. CPPD arthropathy classically affects the knee and is associated with hereditary hemochromatosis, hyperparathyroidism, and hypothyroidism; the elevated ferritin and transferrin saturation here point to iron overload as the predisposing condition. Iron is thought to inhibit pyrophosphatase, promoting pyrophosphate accumulation and crystal formation in cartilage. Diagnosis requires synovial fluid analysis showing these weakly positively birefringent crystals.

• A. Rheumatoid arthritis synovial fluid is inflammatory but does not contain crystals; it is also typically symmetric and polyarticular.
• B. Septic arthritis would show a Gram stain/culture-positive organism and no crystals; crystals are present here.
• C. Urate crystals are needle-shaped and negatively birefringent (yellow when parallel to the compensator axis), not rhomboid and positively birefringent.
• E. Osteoarthritis does not cause acute inflammatory effusions with 90% neutrophils or crystal deposition.

Loop diuretics such as furosemide cause volume contraction, which increases proximal tubular sodium and water reabsorption; this is coupled with enhanced reabsorption of urate via the URAT1 transporter, reducing uric acid excretion and raising serum urate levels. The resulting hyperuricemia can precipitate monosodium urate crystal deposition in the first MTP joint (podagra), producing an acute gout flare. This is one of the most common drug-induced causes of secondary hyperuricemia, alongside low-dose aspirin and cyclosporine.

• B. Uric acid is not metabolized by the liver in humans; it is excreted renally, and this is not the diuretic mechanism.
• C. Calcium pyrophosphate deposition causes pseudogout, not the acute monoarticular gout flare described with elevated serum uric acid.
• D. Xanthine oxidase overactivity is not induced by loop diuretics; this enzyme is the target of allopurinol, not altered by diuretic use.
• E. Loop diuretics do not increase purine synthesis; this mechanism underlies gout from myeloproliferative disorders or tumor lysis.

This patient has reactive arthritis, a seronegative spondyloarthropathy characterized by the triad of asymmetric oligoarthritis, conjunctivitis/uveitis, and urethritis, often occurring weeks after a gastrointestinal or genitourinary infection. Campylobacter jejuni, a common cause of foodborne (often seafood or poultry-associated) bacterial diarrhea, is one of the classic triggering GI organisms, along with Shigella, Salmonella, and Yersinia; Chlamydia trachomatis is the classic GU trigger. The condition is strongly associated with HLA-B27 and is thought to arise from an autoimmune cross-reactive response rather than direct joint infection, so synovial and urine cultures are typically negative.

• B. Streptococcus pyogenes is associated with rheumatic fever and migratory polyarthritis following pharyngitis, not this triad after diarrhea.
• C. Neisseria gonorrhoeae causes septic (gonococcal) arthritis or disseminated gonococcemia, not a post-diarrheal reactive arthritis.
• D. Borrelia burgdorferi causes Lyme arthritis, typically following a tick bite and erythema migrans, not a diarrheal illness.
• E. Staphylococcus aureus causes septic arthritis with positive joint fluid cultures, not the sterile reactive arthritis triad.

Psoriatic arthritis presents with dactylitis (diffuse "sausage digit" swelling from combined tenosynovitis and arthritis), nail changes (pitting, onycholysis), and a history of psoriasis, as seen in this patient. The characteristic radiographic finding is the "pencil-in-cup" deformity, caused by bone resorption and tapering of one phalanx (the pencil) combined with erosive widening of the adjacent joint surface (the cup). Unlike rheumatoid arthritis, psoriatic arthritis commonly involves the distal interphalangeal joints and is typically seronegative for rheumatoid factor and anti-CCP antibodies.

• A. Rheumatoid arthritis spares the DIP joints, causes symmetric MCP/PIP involvement, and shows marginal erosions rather than pencil-in-cup deformities.
• B. Systemic sclerosis causes acro-osteolysis and skin tightening but not dactylitis with pencil-in-cup joint deformity or nail pitting.
• D. Osteoarthritis causes Heberden/Bouchard nodes at DIP/PIP joints but does not cause dactylitis or pencil-in-cup erosions.
• E. Gouty arthritis can cause chronic tophaceous joint damage but is not associated with dactylitis, nail pitting, or the pencil-in-cup deformity.

Anti-CCP antibodies are highly specific (>95%) for rheumatoid arthritis and target citrullinated proteins (such as vimentin and fibrinogen) that result from peptidylarginine deiminase activity, often triggered by smoking or periodontal disease in genetically susceptible individuals (HLA-DR4). These antibodies and associated immune complexes activate CD4+ T cells (Th17) and macrophages, which release pro-inflammatory cytokines including TNF-alpha, IL-1, and IL-6. These cytokines stimulate synovial fibroblast proliferation and osteoclast activation (via RANKL), forming the invasive pannus that erodes cartilage and juxta-articular bone, producing the characteristic marginal erosions of RA.

• B. Mast cell degranulation is an IgE-mediated hypersensitivity mechanism, not relevant to RA pathogenesis.
• C. RA is an autoimmune, not infectious, process; bacterial invasion characterizes septic arthritis.
• D. Amyloid deposition can complicate chronic RA (AA amyloidosis) but is not the mechanism of pannus formation itself.
• E. Urate crystal deposition causes gout, not pannus formation in RA.

Low-dose weekly methotrexate is a first-line DMARD for rheumatoid arthritis that inhibits dihydrofolate reductase (DHFR), reducing conversion of dihydrofolate to tetrahydrofolate and impairing synthesis of purines and thymidylate, which affects rapidly dividing cells such as oral mucosa and bone marrow, producing mucositis and myelosuppression. Leucovorin (folinic acid) is the reduced folate rescue agent; it bypasses the DHFR block by directly supplying tetrahydrofolate derivatives to cells, rescuing normal tissues from toxicity while methotrexate's anti-inflammatory/immunosuppressive effect (thought to involve adenosine release and effects on immune cell function) is preserved.

• A. Leflunomide, not methotrexate, inhibits dihydroorotate dehydrogenase; uridine is used to reverse leflunomide toxicity, not methotrexate toxicity.
• B. Allopurinol inhibits xanthine oxidase and is used for gout/tumor lysis syndrome, unrelated to methotrexate's mechanism or rescue.
• C. Folic acid cannot be enzymatically reduced to tetrahydrofolate when DHFR is inhibited, so it does not effectively rescue acute toxicity the way leucovorin does.
• E. Methotrexate inhibits DHFR, not thymidylate synthase directly, and vitamin B12 is not the rescue agent used for methotrexate toxicity.

In SLE, antibodies against dsDNA and other nuclear antigens form circulating immune complexes that deposit in the glomerular basement membrane and mesangium. These complexes activate complement (lowering serum C3/C4), recruiting neutrophils and causing the capillary wall thickening and granular immunofluorescence pattern ("full-house" staining) seen in lupus nephritis. This is a type III hypersensitivity mechanism, distinct from direct cellular cytotoxicity or anti-GBM antibody disease.

• B. The complexes activate complement and cause inflammation; they do not directly lyse basement membrane collagen.
• C. Anti-GBM antibody disease (Goodpasture syndrome) is a distinct type II hypersensitivity process, not the mechanism in SLE.
• D. IgA nephropathy, not lupus nephritis, follows mucosal infections with mesangial IgA deposits.
• E. Lupus nephritis is immune complex-mediated, not primarily a direct T-cell cytotoxic process against podocytes.

Hydralazine and procainamide are associated with drug-induced lupus, which characteristically produces antihistone antibodies. Slow acetylators are at higher risk because they accumulate the parent drug or reactive metabolites that alter histone proteins, triggering an immune response. Drug-induced lupus typically spares the kidneys and CNS, and symptoms resolve after stopping the offending drug. Anti-dsDNA and anti-Smith antibodies are more specific for idiopathic SLE.

• A. Anti-centromere antibodies are associated with limited systemic sclerosis (CREST syndrome), not drug-induced lupus.
• B. Anti-topoisomerase I (anti-Scl-70) antibodies are associated with diffuse systemic sclerosis.
• D. Anti-dsDNA antibodies are characteristic of idiopathic SLE and are usually absent or low-titer in drug-induced lupus.
• E. Anti-U1 RNP antibodies are associated with mixed connective tissue disease.

In antiphospholipid syndrome, the lupus anticoagulant is an antibody against phospholipid-protein complexes (e.g., beta-2 glycoprotein I) that interferes with the phospholipid-dependent PTT assay in vitro, falsely prolonging it. In vivo, however, these antibodies promote a hypercoagulable state by activating endothelial cells and platelets, leading to arterial and venous thrombosis and recurrent pregnancy loss. The failure of the PTT to correct with mixing study confirms an inhibitor (antibody) rather than a factor deficiency.

• B. Factor V Leiden causes activated protein C resistance and thrombophilia but does not prolong the PTT or correspond to a positive mixing study.
• C. DIC would show low fibrinogen, elevated D-dimer, and thrombocytopenia from factor consumption, not an isolated antibody-mediated assay artifact.
• D. Protein C deficiency causes a hypercoagulable state but does not prolong the PTT or explain a positive mixing study showing an inhibitor.
• E. Lupus anticoagulant does not act by neutralizing antithrombin III; its target is phospholipid-protein complexes in the clotting assay.

Sjogren syndrome is characterized by autoimmune destruction of exocrine glands, with CD4+ T lymphocyte infiltration of the lacrimal and salivary glands leading to xerophthalmia and xerostomia. The diagnostic gold standard is a lip (minor salivary gland) biopsy showing focal lymphocytic sialadenitis. Anti-Ro (SSA) and anti-La (SSB) antibodies support the diagnosis but the histologic finding is the confirmatory criterion. Parotid enlargement results from lymphocytic infiltration, not granulomatous or neutrophilic inflammation.

• A. IgG4-related disease causes a distinct storiform fibrosis pattern, separate from the lymphocytic infiltrate of Sjogren syndrome.
• C. Neutrophilic infiltration with abscess suggests an acute bacterial sialadenitis, not the chronic autoimmune process of Sjogren syndrome.
• D. Noncaseating granulomas are characteristic of sarcoidosis, which can mimic but is distinct from Sjogren syndrome.
• E. Granulomatous inflammation with giant cells is not characteristic of Sjogren syndrome; this suggests a granulomatous disease process.

Diffuse systemic sclerosis involves widespread skin thickening (beyond the elbows and knees onto the trunk) and early visceral involvement, particularly pulmonary fibrosis, as seen in this patient. It is associated with anti-topoisomerase I (anti-Scl-70) antibodies, which correlate with the diffuse cutaneous subtype and increased risk of interstitial lung disease. Excess collagen deposition by activated fibroblasts, driven by cytokines such as TGF-beta, underlies the fibrosis in skin and viscera. Anti-centromere antibodies are instead associated with the limited cutaneous form.

• A. Anti-centromere antibodies are associated with limited cutaneous systemic sclerosis (CREST), which spares early visceral/pulmonary fibrosis.
• C. Anti-mitochondrial antibodies are characteristic of primary biliary cholangitis, unrelated to systemic sclerosis.
• D. Anti-U1 RNP antibodies are characteristic of mixed connective tissue disease, not systemic sclerosis.
• E. Anti-Jo-1 antibodies are seen in the antisynthetase syndrome associated with polymyositis/dermatomyositis.

This patient has findings of limited cutaneous systemic sclerosis, classically remembered by the CREST mnemonic: Calcinosis, Raynaud phenomenon, Esophageal dysmotility, Sclerodactyly, and Telangiectasia. This limited form is strongly associated with anti-centromere antibodies and generally has a better prognosis than the diffuse form, with skin involvement confined to the face, hands, and feet and later onset of pulmonary hypertension rather than early interstitial fibrosis. Anti-topoisomerase I and anti-RNA polymerase III antibodies are instead linked to diffuse systemic sclerosis.

• B. Anti-Smith antibodies are specific for SLE, not systemic sclerosis.
• C. Anti-RNA polymerase III antibodies are associated with diffuse systemic sclerosis and risk of scleroderma renal crisis.
• D. Anti-topoisomerase I (anti-Scl-70) antibodies are associated with diffuse systemic sclerosis and early pulmonary fibrosis.
• E. Anti-dsDNA antibodies are specific for SLE and correlate with lupus nephritis, not scleroderma.

Mixed connective tissue disease presents with overlapping features of SLE (arthritis), systemic sclerosis (Raynaud phenomenon, puffy fingers, esophageal dysmotility), and polymyositis (proximal muscle weakness), and is defined serologically by high-titer anti-U1 ribonucleoprotein (anti-U1 RNP) antibodies. This antibody target is a core component of the spliceosome involved in pre-mRNA processing. Unlike anti-Smith antibodies, which are specific for SLE, anti-U1 RNP antibodies are the hallmark of this overlap syndrome.

• A. Anti-Smith antibodies are specific for SLE and do not define the overlap syndrome described here.
• B. Anti-histone antibodies are characteristic of drug-induced lupus, not mixed connective tissue disease.
• D. Anti-centromere antibodies are associated with limited cutaneous systemic sclerosis, not this overlap syndrome.
• E. Anti-Jo-1 antibodies are associated with antisynthetase syndrome in polymyositis/dermatomyositis, not this overlap presentation.

Giant cell (temporal) arteritis is a granulomatous vasculitis of medium and large arteries, most often branches of the carotid artery, including the ophthalmic artery and its posterior ciliary branches that supply the optic nerve head. Granulomatous inflammation with giant cells causes intimal thickening and luminal narrowing, leading to ischemic optic neuropathy and sudden painless monocular vision loss, a vision-threatening emergency requiring immediate high-dose glucocorticoids even before biopsy confirmation. The association with polymyalgia rheumatica (proximal shoulder and hip stiffness) and markedly elevated ESR is classic.

• A. Immune complex deposition is not the mechanism of giant cell arteritis, which is a cell-mediated granulomatous vasculitis.
• B. Acute angle-closure glaucoma causes painful, not painless, vision loss and is unrelated to this vasculitic presentation.
• C. Embolic occlusion from carotid atherosclerosis causes amaurosis fugax or retinal artery occlusion but is not the mechanism in giant cell arteritis.
• D. Vasospasm from cold exposure describes Raynaud phenomenon physiology, not the vision loss of giant cell arteritis.

Giant cell (temporal) arteritis carries a risk of sudden, irreversible vision loss from ophthalmic artery branch occlusion. Because biopsy results can take days and the granulomatous inflammation is segmental (skip lesions do not reduce diagnostic yield if biopsy is done within about 1-2 weeks of starting steroids), treatment should never be delayed for biopsy confirmation. High-dose glucocorticoids (e.g., prednisone 40-60 mg/day, or IV methylprednisolone if visual symptoms are present) are started immediately on clinical suspicion, and biopsy is performed afterward to confirm the diagnosis. This approach preserves vision while still allowing histologic confirmation.

• A. Delaying treatment risks irreversible blindness; biopsy can still show diagnostic changes even after glucocorticoids are started.
• B. Aspirin alone does not treat the underlying granulomatous inflammation and does not prevent vision loss.
• C. Methotrexate is a steroid-sparing agent used for relapse or steroid toxicity, not first-line therapy, and delaying glucocorticoids risks blindness.
• D. Imaging is not required before treatment and does not replace biopsy for diagnosis; treatment should not be delayed for imaging.

Polyarteritis nodosa is a necrotizing vasculitis of medium-sized muscular arteries that classically spares the lungs and is strongly associated with chronic hepatitis B infection (immune complex-mediated injury). It presents with constitutional symptoms, new-onset hypertension from renal artery involvement, mononeuritis multiplex, livedo reticularis, and abdominal pain from mesenteric ischemia. Angiography shows the characteristic "string of beads" pattern from alternating aneurysms and stenoses. ANCA serologies are typically negative, distinguishing it from the ANCA-associated small-vessel vasculitides.

• B. Microscopic polyangiitis is a p-ANCA/MPO-associated small-vessel vasculitis without aneurysm formation on angiography.
• C. Granulomatosis with polyangiitis is a small-vessel, c-ANCA/PR3-associated vasculitis with prominent sinus and pulmonary involvement, not a medium-vessel disease with angiographic aneurysms.
• D. Kawasaki disease occurs in children and classically affects the coronary arteries with mucocutaneous findings, not adults with hepatitis B.
• E. Takayasu arteritis affects the aorta and its large branches in young women, causing pulselessness and limb claudication, not mesenteric microaneurysms.

Granulomatosis with polyangiitis is a small-vessel vasculitis classically presenting with the triad of upper respiratory tract involvement (sinusitis, saddle-nose deformity from septal perforation), lower respiratory tract involvement (cavitary nodules, hemoptysis), and rapidly progressive glomerulonephritis. It is associated with c-ANCA directed against proteinase 3 (PR3), a serine protease in neutrophil azurophilic granules. Renal biopsy shows pauci-immune (little to no immune complex deposition) necrotizing crescentic glomerulonephritis, often with granulomatous vasculitis.

• A. Microscopic polyangiitis causes pulmonary-renal syndrome but lacks granulomatous inflammation and upper airway destruction such as saddle-nose deformity.
• B. Eosinophilic granulomatosis with polyangiitis is associated with asthma and marked peripheral eosinophilia, which are not described here.
• C. Goodpasture syndrome is caused by anti-glomerular basement membrane antibodies, not ANCA, and does not cause sinus or cavitary lung nodules.
• E. IgA vasculitis typically occurs in children with palpable purpura, arthralgia, and abdominal pain, not cavitary lung lesions or ANCA positivity.

Both microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis (Churg-Strauss) are classically associated with p-ANCA directed against myeloperoxidase (MPO), a neutrophil granule enzyme. They share the pulmonary-renal pattern without upper airway granulomatous destruction (distinguishing them from granulomatosis with polyangiitis). Eosinophilic granulomatosis with polyangiitis is distinguished from microscopic polyangiitis by its association with late-onset asthma, allergic rhinitis, and marked peripheral eosinophilia, along with eosinophilic and granulomatous tissue infiltration.

• A. Elastase is a neutrophil granule protease but is not the characteristic ANCA target in these vasculitides.
• B. Proteinase 3 is the target antigen of c-ANCA, associated with granulomatosis with polyangiitis, not these two conditions.
• D. Cathepsin G is a neutrophil serine protease but is not the classic ANCA antigen associated with these diseases.
• E. Lysozyme is a neutrophil granule enzyme with antibacterial activity but is not the ANCA target implicated in these vasculitides.

Dermatomyositis is a humorally mediated (CD4+ T-cell and B-cell driven) microangiopathy in which complement activation forms the membrane attack complex (C5b-9), which deposits in and damages endomysial capillaries located in the perimysium. This capillary injury leads to perifascicular ischemia and atrophy, the classic histologic finding. Clinically it presents with proximal muscle weakness plus skin findings such as heliotrope rash and Gottron papules, and anti-Jo-1 antibody positivity along with an increased risk of occult malignancy (most commonly ovarian, lung, gastric, or lymphoma) in adults.

• A. Antibody-mediated acetylcholine receptor blockade describes myasthenia gravis, a disorder of the neuromuscular junction, not dermatomyositis.
• B. Granulomatous neuromuscular junction infiltration is not a feature of dermatomyositis or any common autoimmune myopathy.
• D. Direct CD8+ T-cell-mediated myofiber injury describes polymyositis, which involves endomysial rather than perimysial inflammation.
• E. IgA immune complex deposition describes IgA vasculitis (Henoch-Schönlein purpura), not dermatomyositis.

Polymyositis is mediated by CD8+ cytotoxic T lymphocytes that recognize myofiber antigens presented via aberrantly upregulated MHC class I molecules on the muscle fiber surface. These T cells directly invade and destroy individual, non-necrotic myofibers within the endomysium, explaining the endomysial inflammatory pattern without the perifascicular pattern seen in dermatomyositis. Clinically, it presents with symmetric proximal muscle weakness and elevated creatine kinase but, unlike dermatomyositis, has no characteristic skin findings.

• A. Natural killer cells are not the principal mediators of endomysial myofiber injury in polymyositis.
• B. B lymphocytes are not the primary effector cells causing direct myofiber injury in polymyositis.
• D. Eosinophilic muscle infiltration is characteristic of eosinophilic myositis, not polymyositis.
• E. CD4+ T cells predominantly drive the humoral, complement-mediated injury of dermatomyositis rather than the direct cytotoxic injury of polymyositis.

Fibromyalgia is a disorder of central pain processing characterized by central sensitization, in which amplified neuronal signaling in the spinal cord and brain lowers the pain threshold, causing widespread allodynia and hyperalgesia without identifiable peripheral tissue damage or inflammation. It is a diagnosis of exclusion made in a patient with chronic widespread pain, fatigue, and sleep disturbance after normal inflammatory and autoimmune markers rule out rheumatologic or muscle disease. There is no elevation of creatine kinase or inflammatory markers because there is no primary muscle or joint pathology.

• A. Endomysial T-cell inflammation describes polymyositis, which would elevate creatine kinase, unlike this patient's normal value.
• C. Synovial inflammation with pannus formation describes rheumatoid arthritis, which would show elevated inflammatory markers and joint swelling.
• D. Monosodium urate crystal deposition describes gout, which presents with acute monoarticular joint inflammation, not diffuse chronic pain.
• E. Peripheral demyelination describes neuropathies such as Guillain-Barré syndrome, not fibromyalgia.

Duchenne muscular dystrophy results from a frameshift deletion in the dystrophin gene on Xp21, leading to absent dystrophin protein and X-linked recessive inheritance affecting boys. Dystrophin normally anchors the intracellular actin cytoskeleton to the transmembrane dystrophin-glycoprotein complex, which connects to the extracellular matrix, stabilizing the sarcolemma during repeated contraction. Without dystrophin, the membrane becomes fragile and tears during contraction, causing calcium influx, myofiber necrosis, and progressive replacement by fibrofatty tissue (pseudohypertrophy of the calves). This produces progressive proximal weakness and the Gowers sign, in which children use their arms to push off the floor and climb up their own body to stand.

• A. This describes the actin-myosin cross-bridge cycle itself, regulated by troponin and tropomyosin, not dystrophin.
• B. This describes the ryanodine receptor, mutations of which cause malignant hyperthermia and central core disease, not Duchenne muscular dystrophy.
• D. This describes the nicotinic acetylcholine receptor, whose autoantibody-mediated loss causes myasthenia gravis.
• E. This describes creatine kinase enzymatic activity, which is elevated as a marker of muscle injury in Duchenne muscular dystrophy but is not the protein that is genetically absent.

Becker muscular dystrophy results from in-frame deletions in the DMD gene that preserve the reading frame, allowing synthesis of a shortened but partially functional dystrophin protein, producing a milder, later-onset phenotype with preserved ambulation into adolescence or adulthood. In contrast, Duchenne muscular dystrophy typically results from frameshift or nonsense mutations that disrupt the reading frame, leading to little or no functional dystrophin and a severe phenotype presenting by age 5 with loss of ambulation by age 12. Dystrophin links the actin cytoskeleton to the extracellular matrix via the dystrophin-glycoprotein complex, and its partial presence stabilizes the sarcolemma against contraction-induced damage. Western blot or immunohistochemistry showing reduced-intensity dystrophin staining supports Becker rather than Duchenne, where staining is essentially absent.

• A. This describes the mechanism of Duchenne muscular dystrophy, which produces a severe, not mild, phenotype.
• B. Sarcoglycanopathies cause limb-girdle muscular dystrophy, a distinct entity with normal dystrophin staining.
• C. Ryanodine receptor mutations cause malignant hyperthermia and central core disease, not dystrophinopathies.
• D. Anticipation is a feature of myotonic dystrophy, not Becker or Duchenne muscular dystrophy.

Myotonic dystrophy type 1 is caused by a CTG trinucleotide repeat expansion in the 3' untranslated region of the DMPK gene on chromosome 19. The expanded CUG-containing RNA forms hairpin structures that sequester muscleblind-like (MBNL) splicing proteins, leading to aberrant alternative splicing of multiple transcripts, including the muscle chloride channel CLCN1, which produces the myotonia (delayed muscle relaxation), along with insulin receptor and cardiac ion channel transcripts contributing to other systemic features. This is a gain-of-function RNA toxicity mechanism rather than a loss of a specific protein. The disease shows anticipation, with longer repeats and earlier, more severe onset in successive generations, consistent with the worsening severity compared with this patient's father.

• A. Polyglutamine aggregation causing apoptosis describes CAG repeat disorders like Huntington disease, not myotonic dystrophy.
• B. The repeat lies in an untranslated region and does not shift the reading frame of the DMPK protein.
• C. Myotonic dystrophy involves a toxic RNA gain-of-function effect, not promoter silencing by hypermethylation.
• E. Repeat expansion in myotonic dystrophy is not mediated by somatic gene duplication.

Myasthenia gravis is caused by autoantibodies directed against postsynaptic nicotinic acetylcholine receptors at the neuromuscular junction, often in association with thymoma as seen in this patient's mediastinal mass. These antibodies reduce the number of functional receptors and promote their internalization and complement-mediated destruction, so progressively fewer receptors are available to respond as acetylcholine is normally depleted with repetitive firing, producing a decremental compound muscle action potential on low-frequency (2-3 Hz) repetitive nerve stimulation. This explains the fatigable weakness that worsens with sustained activity and improves with rest. Anti-MuSK antibody myasthenia produces a similar decremental pattern but through impaired receptor clustering rather than direct receptor blockade.

• A. This describes Lambert-Eaton myasthenic syndrome, which produces an incremental, not decremental, response at high-frequency stimulation.
• B. P/Q-type calcium channel antibodies and high-frequency stimulation describe Lambert-Eaton syndrome, which shows an incremental response, not decremental.
• C. While MuSK antibodies can cause myasthenia, the classic decremental mechanism at the neuromuscular junction in this antibody-mediated postsynaptic disease is receptor loss, and MuSK disease is less associated with thymoma.
• E. Ryanodine receptor mutations cause malignant hyperthermia, unrelated to this clinical and electrodiagnostic picture.

Lambert-Eaton myasthenic syndrome is a paraneoplastic disorder most commonly associated with small cell lung carcinoma, caused by autoantibodies against presynaptic voltage-gated calcium channels at the motor nerve terminal. Reduced calcium influx impairs acetylcholine vesicle release, causing proximal muscle weakness, autonomic symptoms such as dry mouth, and diminished reflexes. With repeated stimulation, calcium accumulates presynaptically, transiently overcoming the blockade and producing an incremental response on high-frequency (>10 Hz) repetitive nerve stimulation, the opposite pattern from myasthenia gravis. This mechanism also explains why strength and reflexes improve briefly after sustained voluntary contraction.

• B. Ectopic ACTH causes Cushing syndrome-related myopathy, which does not produce an incremental electrodiagnostic response or improve with exercise.
• C. Anti-MuSK antibodies cause a form of myasthenia gravis with decremental, not incremental, repetitive stimulation findings.
• D. Direct tumor invasion would produce axonal or compressive findings, not the facilitation pattern seen on high-frequency stimulation.
• E. Postsynaptic acetylcholine receptor antibodies cause myasthenia gravis, which produces a decremental, not incremental, response on repetitive stimulation.

Malignant hyperthermia is caused by an autosomal dominant mutation, most often in the RYR1 gene encoding the skeletal muscle ryanodine receptor, which normally mediates calcium release from the sarcoplasmic reticulum in response to depolarization. Exposure to volatile inhaled anesthetics or depolarizing neuromuscular blockers like succinylcholine triggers excessive, uncontrolled calcium release, causing sustained muscle contraction, rigidity, massive heat production, hypermetabolism, hyperkalemia, and rhabdomyolysis, as reflected by the elevated creatine kinase, hyperkalemia, and lactic acidosis in this patient. Treatment is immediate discontinuation of the triggering agent and administration of dantrolene, which blocks the ryanodine receptor to halt calcium release. This condition is distinct from pseudocholinesterase deficiency, which causes prolonged paralysis rather than hypermetabolic rigidity.

• A. This describes myasthenia gravis pathophysiology, which is unrelated to the acute hypermetabolic reaction seen here.
• B. Dihydropyridine receptor dysfunction is not the mechanism of malignant hyperthermia; the defect is in the ryanodine receptor itself.
• C. Pseudocholinesterase deficiency causes prolonged neuromuscular blockade and apnea, not hyperthermia and rigidity.
• D. Malignant hyperthermia is a pharmacogenetic hypermetabolic reaction, not an IgE-mediated allergic response.

Statins inhibit HMG-CoA reductase, reducing synthesis of cholesterol and downstream isoprenoids needed for normal muscle cell membrane and mitochondrial function, which can cause myopathy and, in severe cases, rhabdomyolysis with myoglobinuria as seen in this patient's dark urine with heme-positive, RBC-negative dipstick. Gemfibrozil increases statin myotoxicity risk primarily by inhibiting hepatic glucuronidation (via UGT enzymes), the pathway responsible for clearing many statins, leading to elevated statin plasma levels and greater risk of muscle toxicity. This interaction is less pronounced with fenofibrate, which is why fenofibrate is generally preferred over gemfibrozil when combination therapy with a statin is needed. The combination substantially raises rhabdomyolysis risk compared with statin monotherapy.

• B. Gemfibrozil is not a CYP3A4 inducer; the clinically significant interaction with statins is through glucuronidation inhibition.
• C. Albumin displacement is not the established mechanism of the statin-fibrate interaction increasing myopathy risk.
• D. The interaction is not primarily due to altered renal tubular secretion of statins.
• E. Coenzyme Q10 depletion is a proposed general contributor to statin myopathy but does not explain the specific pharmacokinetic interaction with gemfibrozil.

This patient has clinical findings concerning for acute compartment syndrome: pain out of proportion to injury, pain with passive stretch, and sensory deficit in the deep peroneal nerve distribution (first web space), following a closed tibial fracture, a classic cause. A palpable distal pulse does not exclude compartment syndrome because arterial flow is typically preserved until pressures become extremely high, late in the process. The most appropriate next step is direct measurement of intracompartmental pressure using a needle manometer; a pressure within 30 mmHg of diastolic blood pressure, or an absolute pressure above 30 mmHg, supports the diagnosis and the need for emergent fasciotomy. Delaying intervention for other studies risks irreversible muscle and nerve ischemia within hours.

• A. Venous Doppler evaluates for deep vein thrombosis, which is not suggested by this presentation and would waste critical time.
• B. Empiric heparin is inappropriate here and could worsen bleeding into a compartment undergoing ischemic injury.
• C. D-dimer is used to evaluate venous thromboembolism, not compartment syndrome, and would delay urgent diagnosis.
• E. Repeat radiographs would not confirm or exclude compartment syndrome, which is a soft tissue and pressure-based diagnosis.

Crush injury causes extensive skeletal muscle breakdown, releasing myoglobin, potassium, and creatine kinase into the circulation, producing the markedly elevated creatine kinase and hyperkalemia seen here. Myoglobin is filtered by the glomerulus and can precipitate within the renal tubules, particularly in an acidic, low-flow environment, forming obstructive casts and causing direct heme-mediated oxidative tubular epithelial injury, resulting in acute tubular necrosis and the rise in creatinine. This is reflected by heme-positive urine dipstick without red blood cells on microscopy, distinguishing myoglobinuria from hematuria. The accompanying severe hyperkalemia from released intracellular potassium poses an immediate risk of cardiac arrhythmia and requires urgent treatment.

• B. Hyperkalemia affects cardiac and neuromuscular membrane potentials but is not the direct cause of tubular injury in this setting.
• C. Disseminated intravascular coagulation is not indicated by the findings given and is not the mechanism of rhabdomyolysis-associated kidney injury.
• D. There is no evidence of immune complex-mediated glomerulonephritis; the heme-positive, RBC-negative urinalysis points to myoglobinuria and tubular, not glomerular, injury.
• E. Diffuse cortical necrosis is a rare complication of severe obstetric hemorrhage or sepsis, not the typical mechanism of crush-related kidney injury.

The single most important prognostic factor for cutaneous melanoma is the depth of invasion, measured histologically as the Breslow thickness (distance from the epidermal granular layer to the deepest invasive tumor cell). Greater Breslow depth correlates with increased risk of lymphatic and hematogenous metastasis and worse overall survival, and it guides staging and the width of surgical margins. The ABCDE criteria (Asymmetry, Border irregularity, Color variation, Diameter >6 mm, Evolving) are useful for clinical detection but do not predict prognosis once melanoma is diagnosed. Many melanomas harbor activating BRAF V600E mutations, which promote constitutive MAPK pathway signaling and are targetable with BRAF/MEK inhibitors, but mutation status is not the primary prognostic determinant.

• B. Diameter aids clinical detection but is not the key histologic prognostic factor once melanoma is diagnosed.
• C. Asymmetry is a clinical screening criterion, not a prognostic measure.
• D. Color variegation helps identify suspicious lesions clinically but does not predict outcome.
• E. Age can influence overall risk but is far less predictive of outcome than tumor depth.