USMLE Step 1 renal & urinary system questions

Creatinine clearance approximates GFR because creatinine is freely filtered and not significantly reabsorbed or secreted in healthy kidneys. Clearance = (Urine concentration × Urine flow rate) / Plasma concentration. Urine flow rate = 1440 mL/1440 min = 1 mL/min. Clearance = (100 mg/dL × 1 mL/min) / 1.0 mg/dL = 100 mL/min, which is a normal GFR.

• A. This overestimates GFR; it does not follow from the clearance formula with these values.
• B. This would require either a lower urine creatinine or a higher plasma creatinine than given.
• C. This substantially overestimates GFR and would suggest a calculation error in flow rate conversion.
• D. This value does not match the calculation using the given urine flow rate and concentrations.

NSAIDs inhibit prostaglandin synthesis, and prostaglandins normally vasodilate the afferent arteriole to maintain renal blood flow, especially under conditions of reduced effective circulating volume. NSAID-induced afferent arteriolar constriction reduces both renal plasma flow and GFR proportionally, so filtration fraction (GFR/RPF) remains unchanged. This contrasts with ACE inhibitors, which dilate the efferent arteriole, lowering GFR more than RPF and decreasing filtration fraction.

• A. Both parameters fall with afferent constriction rather than rising.
• B. Afferent constriction lowers renal plasma flow, not raises it.
• C. Afferent constriction lowers GFR along with renal plasma flow; it does not raise GFR.
• E. GFR falls with afferent constriction, it does not stay the same.

This patient has Sjögren syndrome causing distal (type 1) RTA through autoimmune lymphocytic infiltration that damages alpha intercalated cells in the collecting duct, impairing H+-ATPase-mediated hydrogen secretion. The inability to acidify urine despite systemic acidosis (urine pH persistently above 5.5) and the associated hypokalemia (due to increased distal Na+ delivery enhancing K+ secretion) are classic features. Amphotericin B causes the same defect by increasing distal tubular membrane permeability to H+, allowing back-leak of secreted protons.

• A. This describes vitamin D metabolism defects, unrelated to the acid-base disturbance described.
• B. This describes type 2 (proximal) RTA, which produces a self-limited acidosis with urine pH that can become appropriately acidic once filtered bicarbonate load falls below the reduced reabsorptive threshold.
• C. This describes type 4 RTA, typically associated with hyperkalemia rather than the hypokalemia seen here.
• D. Ammonia production defects occur in type 4 RTA, associated with hyperkalemia, not the picture described.

This child has Fanconi syndrome causing type 2 (proximal) RTA, a generalized defect of proximal tubule reabsorption affecting bicarbonate, phosphate, glucose, and amino acids. Normally about 85% of filtered bicarbonate is reabsorbed proximally via carbonic anhydrase-dependent mechanisms; when this fails, bicarbonate is wasted in the urine until plasma levels fall low enough that the reduced reabsorptive capacity can keep pace, at which point a new steady state with acidosis is reached. The concurrent phosphaturia, glucosuria (with normal serum glucose), and aminoaciduria reflect the global proximal tubular dysfunction, explaining the rickets from phosphate wasting.

• A. This describes type 4 RTA with hyperkalemia, not the multi-solute wasting pattern described.
• C. This would cause nephrogenic diabetes insipidus, not the pattern of bicarbonate, phosphate, glucose, and amino acid wasting seen here.
• D. This describes distal RTA, which would not explain the concurrent glucosuria, phosphaturia, and aminoaciduria.
• E. This describes a mechanism of distal RTA from Sjögren syndrome, not a generalized proximal tubulopathy.

Diabetic nephropathy damages the juxtaglomerular apparatus, reducing renin secretion (hyporeninemic hypoaldosteronism), which lowers aldosterone and impairs distal nephron sodium reabsorption and potassium/hydrogen secretion. The resulting hyperkalemia directly suppresses ammoniagenesis in the proximal tubule, reducing the kidney's capacity to buffer and excrete hydrogen ions as NH4+, producing a mild hyperkalemic nonanion gap metabolic acidosis (type 4 RTA). This is the most common RTA encountered clinically, typically seen in older diabetics with mild chronic kidney disease.

• B. This mechanism contributes to type 1 RTA pathophysiology (e.g., amphotericin B), not the hyporeninemic state described.
• C. This mechanism causes type 2 RTA, which does not typically present with hyperkalemia or low renin/aldosterone.
• D. This mechanism causes hypokalemic type 1 RTA, not the hyperkalemic picture seen here.
• E. Primary adrenal insufficiency would show elevated, not low, renin due to loss of negative feedback from cortisol and aldosterone deficiency.

Loop diuretics inhibit the Na+/K+/2Cl- cotransporter in the thick ascending limb, which is the engine driving countercurrent multiplication by pumping sodium and chloride into the medullary interstitium without water. Blocking this transporter abolishes the single effect responsible for generating the hypertonic medullary interstitium, collapsing the corticomedullary osmotic gradient from its normal range of up to 1200 mOsm/kg down toward isotonicity. Because the medullary gradient that antidiuretic hormone-mediated water reabsorption depends on is lost, the kidney's maximal urine concentrating ability falls accordingly, which is why loop diuretics are more potent than thiazides and impair the ability to produce maximally concentrated urine.

• B. The gradient is disrupted, not preserved, because the transporter generating it is blocked.
• C. Urine osmolality cannot rise while the underlying gradient that concentrates urine is destroyed.
• D. Both parameters fall rather than rise because loop diuretics abolish the single effect of the countercurrent multiplier.
• E. Inhibiting the thick ascending limb cannot increase urine osmolality since it eliminates the mechanism generating the medullary gradient.

Vasopressin (ADH) binds the V2 receptor, a Gs-coupled receptor, on the basolateral membrane of collecting duct principal cells. Gs activation stimulates adenylate cyclase, raising intracellular cAMP and activating protein kinase A. PKA phosphorylates aquaporin-2 channels stored in subapical vesicles, causing their translocation and insertion into the apical membrane, which increases water permeability and reabsorption. This pathway is distinct from the V1 receptor (Gq, vascular smooth muscle vasoconstriction).

• B. Natriuretic peptides, not ADH, act through guanylate cyclase and cGMP.
• C. Gi coupling would decrease cAMP, opposite to the stimulatory effect of ADH at V2 receptors.
• D. This is the V1 receptor pathway on vascular smooth muscle, mediating vasoconstriction, not the V2 pathway in the collecting duct.
• E. V2 receptors are G protein-coupled receptors, not receptor tyrosine kinases.

This patient has SIADH due to ectopic ADH secretion from small cell lung carcinoma, causing euvolemic hyponatremia with inappropriately concentrated urine. Tolvaptan is a selective V2 receptor antagonist that blocks ADH's action on the collecting duct, preventing aquaporin-2 insertion and promoting free water excretion (aquaresis) without significant natriuresis. This class of drugs, known as vaptans, is used for symptomatic or severe hyponatremia due to SIADH when fluid restriction alone is inadequate. Tolvaptan should be initiated in a hospital setting with close monitoring of serum sodium to avoid overly rapid correction and osmotic demyelination syndrome.

• A. Carbonic anhydrase inhibitors like acetazolamide are used for conditions such as metabolic alkalosis or altitude sickness, not SIADH.
• B. ENaC blockers like amiloride are potassium-sparing diuretics, not used for SIADH-related hyponatremia.
• C. Mineralocorticoid receptor antagonists like spironolactone treat hyperaldosteronism, not ADH excess.
• D. Loop diuretics inhibit this transporter but are not the primary mechanism of SIADH-specific pharmacotherapy; they are sometimes used adjunctively but do not directly antagonize ADH action.

This patient has severe hyperkalemia with ECG evidence of cardiac membrane instability (widened QRS, loss of P waves), indicating imminent risk of ventricular arrhythmia or asystole. Intravenous calcium gluconate acts within minutes by raising the threshold potential at the cardiac myocyte membrane, stabilizing the membrane and antagonizing the arrhythmogenic effects of hyperkalemia, though it does not lower serum potassium. This buys time for subsequent therapies, such as insulin with dextrose, which shift potassium intracellularly via the Na-K-ATPase pump, and definitive treatment with hemodialysis, which actually removes potassium from the body.

• A. Dialysis is the definitive treatment to remove potassium but takes time to arrange; membrane stabilization with calcium must occur first given the ECG changes.
• B. Insulin with dextrose shifts potassium intracellularly and lowers serum levels, but it does not immediately stabilize the cardiac membrane, which is the priority given the dangerous ECG changes.
• C. Bicarbonate has a modest and delayed effect on potassium shift and is not first-line for immediate membrane stabilization.
• D. Sodium polystyrene sulfonate acts slowly over hours via the gastrointestinal tract and is not useful in an acute cardiac emergency.

Recurrent vomiting causes loss of gastric hydrochloric acid, producing hypochloremic metabolic alkalosis, as reflected by this patient's elevated bicarbonate and low chloride. Volume depletion from vomiting activates the renin-angiotensin-aldosterone system, and the resulting hyperaldosteronism increases distal tubular sodium reabsorption in exchange for potassium and hydrogen secretion, causing significant renal potassium wasting; this, not direct gastric potassium loss, is the main driver of hypokalemia. The alkalosis also promotes intracellular potassium shift. Hypokalemia impairs myocardial repolarization, producing flattened T waves and the characteristic U wave seen on ECG.

• B. Gastric fluid contains relatively little potassium; the hypokalemia is driven predominantly by secondary hyperaldosteronism causing renal potassium loss, not direct vomiting losses.
• C. Volume depletion from vomiting increases, rather than decreases, aldosterone secretion via activation of the renin-angiotensin system.
• D. Metabolic alkalosis promotes, rather than impairs, renal potassium excretion by increasing distal tubular flow and aldosterone-mediated secretion.
• E. This patient has metabolic alkalosis from vomiting, not primary respiratory alkalosis, and the main mechanism is renal loss rather than intracellular redistribution alone.

Parathyroid hormone-related protein (PTHrP), commonly secreted by squamous cell carcinomas (particularly lung), shares N-terminal sequence homology with PTH and binds the same PTH/PTHrP receptor (PTH1R) on osteoblasts and renal tubular cells. This mimics PTH's actions: increasing osteoblast-mediated RANKL expression (which stimulates osteoclastic bone resorption), enhancing distal tubular calcium reabsorption, and decreasing proximal phosphate reabsorption, producing hypercalcemia with hypophosphatemia. Because PTHrP suppresses endogenous PTH through negative feedback on the parathyroid glands, intact PTH levels are low in this paraneoplastic syndrome, distinguishing it from primary hyperparathyroidism.

• A. Calcitonin suppression is not the mechanism of PTHrP-mediated hypercalcemia of malignancy.
• C. PTHrP suppresses, rather than activates, endogenous PTH secretion through negative feedback from the resulting hypercalcemia, which is why this patient's intact PTH is low.
• D. PTHrP does not significantly increase 1-alpha-hydroxylase activity or calcitriol levels, unlike PTH; this is a key distinguishing feature between PTHrP-mediated and PTH-mediated hypercalcemia.
• E. PTHrP works through the PTH receptor pathway, which does act via RANKL-mediated osteoclast stimulation rather than a RANKL-independent mechanism.

In chronic starvation, the body shifts to fat metabolism and intracellular phosphate stores become depleted, though serum levels may remain normal. With reintroduction of carbohydrates during refeeding, a surge in insulin secretion drives cellular uptake of glucose along with phosphate, magnesium, and potassium, as these are required for glycolysis and synthesis of ATP and 2,3-diphosphoglycerate. This rapid intracellular shift causes a sharp fall in serum phosphate, producing the hallmark hypophosphatemia of refeeding syndrome, which can lead to cardiac and respiratory muscle dysfunction, confusion, and rhabdomyolysis.

• B. Secondary hyperparathyroidism is not the acute mechanism; the hypophosphatemia of refeeding syndrome occurs within days due to intracellular shift, not increased renal losses.
• C. Refeeding syndrome does not involve bone remineralization as the primary driver of hypophosphatemia; the phosphate moves intracellularly rather than binding to calcium.
• D. Vitamin D deficiency may contribute to chronic malnutrition but is not the acute mechanism causing the rapid drop in phosphate seen with refeeding.
• E. Refeeding causes an anabolic shift with phosphate moving into cells, the opposite of catabolism-driven extracellular redistribution.

Poststreptococcal glomerulonephritis follows infection with nephritogenic strains of group A streptococcus, typically after a latent period of 1-3 weeks. Circulating immune complexes containing streptococcal antigens (such as nephritis-associated plasmin receptor) deposit in the subepithelial space, forming the characteristic electron-dense "humps" seen on electron microscopy. Complement activation by these deposits consumes C3, producing the low serum C3 level seen in this patient. The resulting inflammation damages the glomerular capillary wall, causing a nephritic picture with hematuria and red cell casts.

• A. ANCA-associated vasculitis produces pauci-immune GN with little to no immune deposits, unlike the immune complex deposits of PSGN.
• B. This describes Goodpasture syndrome, which shows linear IF staining, not the granular pattern of PSGN.
• D. IgA nephropathy presents with synpharyngitic hematuria (within 1-2 days of infection) and normal complement levels, unlike this patient's delayed onset and low C3.
• E. Streptococci do not directly invade the kidney; the glomerular injury is immune-mediated, occurring after the infection has resolved.

Goodpasture syndrome is caused by autoantibodies against the alpha-3 chain of type IV collagen, a structural component distributed evenly along the glomerular and alveolar basement membranes. Because this antigen is continuous and uniformly present, antibody binding produces a smooth, linear (ribbon-like) pattern on immunofluorescence, in contrast to the granular, "lumpy-bumpy" pattern seen with immune complex deposition. Antibody binding to both pulmonary and glomerular basement membranes explains the combined pulmonary hemorrhage and rapidly progressive glomerulonephritis. Plasmapheresis combined with immunosuppression is used to remove and halt production of the pathogenic antibody.

• B. Nonspecific complement deposition does not produce the organized linear staining seen with anti-GBM disease.
• C. Fibrin crescents can form in severe disease but do not produce the linear capillary wall staining described.
• D. Goodpasture antibodies target basement membrane collagen, not a podocyte cell-surface receptor.
• E. Random immune complex deposition produces a granular, not linear, IF pattern, as in lupus nephritis or PSGN.

This patient's triad of upper respiratory tract involvement (sinusitis, nasal septal perforation), lower respiratory tract involvement (cavitary nodules), and rapidly progressive glomerulonephritis is classic for granulomatosis with polyangiitis (GPA). GPA is strongly associated with c-ANCA, which targets proteinase-3 (PR3) and produces a cytoplasmic staining pattern on indirect immunofluorescence. The renal biopsy typically shows a pauci-immune, necrotizing crescentic glomerulonephritis with little to no immune complex or complement deposition. Treatment involves immunosuppression with cyclophosphamide or rituximab plus glucocorticoids.

• B. Cryoglobulinemia is associated with hepatitis C infection and presents with palpable purpura, not cavitary granulomatous lung disease.
• C. Anti-GBM disease causes a pulmonary-renal syndrome but not sinusitis with septal perforation or cavitary lung nodules.
• D. p-ANCA/MPO is associated with microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis, which typically lack prominent upper airway granulomatous destruction.
• E. ANA and anti-dsDNA are seen in lupus nephritis, not in the pauci-immune vasculitis described here.

This patient's history of asthma, marked peripheral eosinophilia, mononeuritis multiplex, and p-ANCA positivity is characteristic of eosinophilic granulomatosis with polyangiitis (EGPA, Churg-Strauss syndrome). Both EGPA and microscopic polyangiitis (MPA) are associated with p-ANCA directed against myeloperoxidase (MPO), distinguishing them from granulomatosis with polyangiitis, which is associated with c-ANCA/PR3. EGPA is distinguished from MPA clinically by the presence of asthma, allergic rhinitis, and striking eosinophilia, while MPA lacks these allergic features and granulomas. Both conditions produce a pauci-immune, necrotizing crescentic glomerulonephritis.

• A. Anti-GBM antibodies are not ANCA and are not associated with asthma or eosinophilia.
• B. PR3 is the target antigen in c-ANCA, associated with granulomatosis with polyangiitis, which does not typically feature prominent eosinophilia or asthma.
• C. Anti-dsDNA is specific for systemic lupus erythematosus, not ANCA-associated vasculitis.
• E. Anticentromere antibodies are associated with limited cutaneous systemic sclerosis (CREST syndrome), not vasculitis.

Diffuse proliferative (class IV) lupus nephritis is characterized by subendothelial immune complex deposition, located between the endothelial cells and the glomerular basement membrane. These bulky subendothelial deposits thicken the capillary wall and appear as "wire-loop" lesions on light microscopy and as granular deposits on immunofluorescence ("full house" pattern: IgG, IgA, IgM, C3, C1q). This location triggers an inflammatory response from the adjacent capillary lumen, producing an endocapillary proliferative pattern with hypocomplementemia from immune complex-mediated complement consumption. Class IV is the most severe and common form of lupus nephritis and carries the highest risk of progression to renal failure.

• B. Tubulointerstitial lupus nephritis can occur but does not explain the glomerular wire-loop finding described.
• C. Subepithelial deposits (humps) are characteristic of poststreptococcal GN and membranous nephropathy, not the wire-loop lesions of class IV lupus nephritis.
• D. Mesangial-only deposits characterize class II (mesangial proliferative) lupus nephritis, a milder form without wire-loop lesions.
• E. Crescents can occur in severe lupus nephritis but form in Bowman's space secondary to capillary rupture, not as the primary explanation for wire-loop capillary thickening.

Alport syndrome is most commonly caused by X-linked mutations in COL4A5, which encodes the alpha-5 chain of type IV collagen, a major structural component of the glomerular basement membrane, cochlea, and lens. Defective type IV collagen leads to an abnormally thin, thickened, and split basement membrane with the characteristic "basket-weave" appearance on electron microscopy. Because type IV collagen is also present in the cochlea and eye, patients develop sensorineural hearing loss and ocular abnormalities such as anterior lenticonus, in addition to progressive glomerulonephritis. The X-linked inheritance pattern explains why the maternal uncle, but not necessarily the mother, is affected.

• B. Type I collagen mutations cause osteogenesis imperfecta, not Alport syndrome.
• C. Nephrin mutations cause congenital nephrotic syndrome of the Finnish type, presenting with massive proteinuria in infancy, not hematuria with hearing loss.
• D. Fibrillin-1 mutations cause Marfan syndrome, which presents with lens dislocation and aortic root dilation, not hematuria with basement membrane splitting.
• E. PKD1 mutations cause autosomal dominant polycystic kidney disease, which presents with renal cysts, not glomerular basement membrane splitting.

This patient's presentation of heavy proteinuria (greater than 3.5 g/24 hr), hypoalbuminemia, and edema, together with fatty casts and oval fat bodies showing a Maltese cross pattern under polarized light, is characteristic of nephrotic syndrome. Fatty casts form when lipoprotein from the heavy proteinuria is reabsorbed and sloughed by tubular epithelial cells. In contrast, nephritic syndrome presents with hematuria, red cell casts, and variable proteinuria due to glomerular capillary wall inflammation and disruption, which this patient's sediment lacks. The absence of hematuria or red cell casts argues against a nephritic process despite the significant proteinuria.

• A. Acute interstitial nephritis typically presents with white cell casts, often eosinophiluria, and a history of drug exposure, not fatty casts and massive proteinuria.
• B. Nephritic syndrome is characterized by hematuria and red cell casts, which are absent in this patient; fatty casts point to nephrotic-range proteinuria instead.
• D. Acute tubular necrosis presents with granular, muddy brown casts and signs of acute kidney injury, not fatty casts with heavy proteinuria.
• E. Postrenal obstruction presents with signs of urinary retention or hydronephrosis, not fatty casts and nephrotic-range proteinuria.

This patient's prior episode of hemorrhagic shock caused renal ischemia, leading to ischemic acute tubular necrosis (ATN). Ischemic injury predominantly damages the proximal tubule and thick ascending limb, which are most metabolically active and susceptible to hypoperfusion; necrotic tubular epithelial cells slough into the tubular lumen and form the characteristic granular, muddy brown casts. The elevated FENa (greater than 2%) and urine sodium reflect impaired tubular sodium reabsorption due to tubular cell damage, distinguishing ATN from prerenal azotemia, in which tubules remain intact and avidly reabsorb sodium. Urine osmolality near that of plasma reflects loss of the kidney's concentrating ability due to tubular dysfunction.

• A. Bladder outlet obstruction would cause a postrenal pattern with hydronephrosis, not granular casts following a shock episode.
• B. Immune complex deposition would produce a nephritic sediment with red cell casts, not the granular casts and ischemic clinical setting described here.
• D. Acute interstitial nephritis is drug-induced and typically shows white cell casts with eosinophiluria, not granular casts following a hypotensive episode.
• E. Uric acid nephropathy occurs with tumor lysis syndrome and produces urate crystals, not granular muddy brown casts, and there is no history of malignancy or chemotherapy.

In chronic kidney disease, peritubular fibroblast-like cells of the kidney lose the ability to produce erythropoietin, leading to normocytic normochromic anemia. The failing kidney also cannot convert 25-hydroxyvitamin D to its active form, 1,25-dihydroxyvitamin D (calcitriol), via 1-alpha-hydroxylase, causing decreased intestinal calcium absorption, hypocalcemia, and secondary hyperparathyroidism, which produces renal osteodystrophy. The elevated phosphorus results from decreased renal excretion and further suppresses calcium and stimulates PTH. This combination of decreased erythropoietin and decreased calcitriol explains both the anemia and the bone disease in this patient.

• B. Pernicious anemia causes macrocytic anemia, not the normocytic anemia seen here, and does not explain the bone findings.
• C. Urea is not directly toxic to marrow stem cells or osteoclasts in a manner that explains this specific anemia-bone disease pairing.
• D. Uremic platelet dysfunction causes bleeding tendency, not the chronic normocytic anemia and bone disease described.
• E. Thrombopoietin deficiency would cause thrombocytopenia, not anemia, and is not the mechanism of renal osteodystrophy.

Autosomal dominant polycystic kidney disease is most commonly caused by mutations in PKD1 (chromosome 16, encoding polycystin-1) or less commonly PKD2 (chromosome 4, encoding polycystin-2), both of which localize to the primary cilium of renal tubular epithelial cells and regulate cell proliferation and tubular morphology. Loss of normal polycystin function leads to progressive bilateral cyst formation, massive kidney enlargement, hypertension, and extrarenal manifestations including intracranial (berry) aneurysms, hepatic cysts, and mitral valve prolapse. The family history of a fatal aneurysm in this patient's father is a classic clue pointing toward ADPKD. Screening with MR or CT angiography is considered in patients with a family history of intracranial aneurysm or subarachnoid hemorrhage.

• A. Triplet repeat expansion on chromosome 19 is the mechanism of myotonic dystrophy, unrelated to polycystic kidney disease.
• B. Chromosome 3p deletion is associated with von Hippel-Lindau disease and clear cell renal carcinoma, not ADPKD.
• D. Fibrillin-1 mutations cause Marfan syndrome, which does not present with polycystic kidneys.
• E. Type IV collagen alpha-5 mutations cause Alport syndrome, which presents with hematuria, sensorineural hearing loss, and ocular defects rather than massive bilateral cysts.

This infant has autosomal recessive polycystic kidney disease (ARPKD), caused by mutations in PKHD1 encoding fibrocystin, leading to bilaterally enlarged kidneys with innumerable small cysts arising from the collecting ducts. Severely reduced fetal urine output causes oligohydramnios, which in turn causes Potter sequence: fetal compression leads to the characteristic facial features (flattened nose, low-set ears, micrognathia), limb deformities (clubfoot), and critically, pulmonary hypoplasia from restricted chest wall and lung development in utero. Pulmonary hypoplasia, not a primary lung or surfactant disorder, is the cause of respiratory failure in these infants and is often the immediate cause of neonatal death. This mechanical/physiologic pathway links the renal cystic disease to oligohydramnios to pulmonary hypoplasia.

• A. Congenital diaphragmatic hernia is a distinct structural anomaly not caused by or associated with ARPKD in this manner.
• B. Surfactant protein mutations cause a primary surfactant deficiency unrelated to the renal cystic disease and oligohydramnios described here.
• D. Transient tachypnea is a benign, self-limited condition unrelated to the severe respiratory distress from pulmonary hypoplasia described.
• E. Meconium aspiration is unrelated to the oligohydramnios-driven pulmonary hypoplasia mechanism in Potter sequence.

Medullary sponge kidney, seen in the man, is a generally benign, usually sporadic congenital condition characterized by cystic dilation of the collecting ducts in the medulla, producing a striated or brush-like appearance on imaging; it predisposes to nephrocalcinosis and calcium stone formation but typically does not impair renal function. Medullary cystic kidney disease (nephronophthisis complex), seen in the woman, is an inherited disorder, most commonly autosomal recessive (nephronophthisis) or autosomal dominant, characterized by small kidneys with medullary cysts and tubulointerstitial fibrosis that progresses to end-stage renal failure. The key distinguishing feature tested is that medullary sponge kidney is a benign, usually non-progressive condition, while medullary cystic kidney disease progresses to chronic kidney failure. Distinguishing these two entities by name and prognosis is a classic USMLE Step 1 point.

• A. Berry aneurysms and hepatic cysts are associated with ADPKD, not with medullary sponge kidney or medullary cystic kidney disease.
• B. The progression pattern described is reversed; medullary cystic disease, not medullary sponge kidney, progresses to renal failure.
• D. Medullary sponge kidney is usually sporadic, not autosomal dominant, and medullary cystic disease has recognized inherited patterns, not always sporadic.
• E. Both conditions primarily involve the renal medulla/papillae, not the cortex or pelvis exclusively.

Bilateral renal agenesis results in failure of fetal urine production, which is the major contributor to amniotic fluid volume in the second half of pregnancy, producing severe oligohydramnios. The resulting mechanical compression of the fetal thorax restricts normal chest wall and lung expansion during critical periods of lung development, causing pulmonary hypoplasia, which together with the facial and limb deformities constitutes Potter sequence. Pulmonary hypoplasia is typically the cause of death in infants with bilateral renal agenesis, as the lungs cannot sustain postnatal respiration. This same oligohydramnios-driven mechanism applies to any condition causing severely reduced fetal urine output, including bilateral renal agenesis and ARPKD.

• B. There is no mention of diaphragmatic agenesis; the pulmonary hypoplasia here is due to extrinsic compression from oligohydramnios, not diaphragmatic defect.
• C. Pulmonary vein atresia is not a feature of Potter sequence and does not explain hypoplastic lungs in this context.
• D. A primary surfactant gene defect is a distinct cause of respiratory distress unrelated to the renal agenesis and oligohydramnios described.
• E. Tracheoesophageal fistula is an unrelated congenital anomaly not caused by or linked to renal agenesis in this vignette.

In patients with Crohn disease or ileal resection, malabsorption of bile acids and fat leads to excess free fatty acids in the intestinal lumen. These fatty acids preferentially bind luminal calcium, which normally would bind dietary oxalate and limit its absorption. With calcium unavailable, free oxalate remains soluble and is absorbed in excess by the colon, leading to hyperoxaluria and an increased risk of calcium oxalate stone formation, a process known as enteric hyperoxaluria. This is distinct from primary hyperoxaluria (an inherited enzyme defect) and from the oxalate toxicity seen in ethylene glycol poisoning, but all three converge on increased urinary oxalate as the stone-forming mechanism. Calcium oxalate stones are radiopaque and the most common type of kidney stone overall.

• B. Metabolic acidosis increases urinary calcium excretion by mobilizing bone calcium and reducing tubular calcium reabsorption; it does not primarily act on oxalate reabsorption.
• C. Oxalobacter formigenes depletion can contribute to hyperoxaluria in some settings but is not the principal mechanism in enteric hyperoxaluria from fat malabsorption.
• D. Vitamin B6 deficiency can increase oxalate production via impaired glyoxylate metabolism, but this is not the primary mechanism in a patient with fat malabsorption from ileal disease.
• E. Ileal resection reduces, rather than increases, the ileum's surface area and function, so it does not directly increase oxalate absorption; the mechanism is colonic absorption due to fat malabsorption.

Proteus mirabilis is a motile, gram-negative rod that produces urease, an enzyme that hydrolyzes urinary urea into ammonia and carbon dioxide. The ammonia alkalinizes the urine, raising the pH above 7.2, which is confirmed in this patient's urine pH of 8.2, and promotes precipitation of magnesium ammonium phosphate (struvite) crystals. Over time, struvite stones can enlarge to fill the entire renal pelvis and calyceal system, forming a classic staghorn calculus visible on imaging as a branching radiopaque mass. Indwelling catheters are a major risk factor for urease-producing organism infection and subsequent struvite stone formation, and these stones often require surgical removal because they harbor persistent infection.

• A. Escherichia coli is the most common cause of urinary tract infection overall but is not a strong urease producer and is not typically associated with struvite staghorn calculi.
• C. Staphylococcus saprophyticus causes uncomplicated UTIs in young sexually active women and is not associated with struvite stone formation.
• D. Enterococcus faecalis can cause catheter-associated UTIs but is not a major urease producer and is not classically linked to staghorn calculus formation.
• E. Klebsiella species can produce some urease activity but are a far less common and less classic cause of staghorn calculi compared to Proteus mirabilis.

Uric acid stones form in the setting of persistently acidic urine (pH < 5.5) and high purine turnover, as seen in gout, and are classically radiolucent on plain X-ray because they lack calcium. Noncontrast helical CT is the imaging modality of choice for suspected nephrolithiasis because it detects uric acid stones (and virtually all other stone types) with high sensitivity and specificity, unlike KUB which misses radiolucent stones. Ultrasound can show hydronephrosis but is less sensitive for small ureteral stones and cannot reliably characterize stone composition. Definitive stone-type confirmation ultimately requires stone analysis after passage or removal, but CT is the best initial confirmatory test for the acute diagnosis.

• B. 24-hour urine calcium assesses calcium stone risk, not the uric acid stone suspected here.
• C. Retrograde pyelography is invasive and reserved for cases where CT is contraindicated or ureteral anatomy must be defined before intervention.
• D. Ultrasound is useful for hydronephrosis screening but is less sensitive than CT for detecting ureteral calculi.
• E. Serum uric acid supports a metabolic predisposition but does not confirm the presence or location of a stone.

Cystinuria is an autosomal recessive disorder caused by mutations in SLC3A1 or SLC7A9, which encode subunits of the amino acid transporter responsible for reabsorbing cystine and dibasic amino acids (ornithine, arginine, lysine) in the proximal tubule and small intestine. Loss of this transporter leads to excess cystine excretion, which precipitates in acidic urine to form hexagonal, pathognomonic crystals and recurrent stones. The cyanide-nitroprusside test is a screening tool that detects urinary cystine. Treatment includes hydration, urinary alkalinization, and chelating agents if needed.

• B. Defective phosphate reabsorption describes disorders like X-linked hypophosphatemic rickets, unrelated to cystine crystals.
• C. Urate transporter defects cause hyperuricosuria and uric acid stones, not the hexagonal crystals described here.
• D. This describes Fanconi syndrome or glucose-galactose malabsorption, not cystinuria.
• E. This describes the defect in Bartter syndrome, which causes a salt-wasting picture, not stone formation from amino acid excess.

Thiazide diuretics such as hydrochlorothiazide reduce urinary calcium excretion by inhibiting the Na+-Cl- cotransporter in the distal convoluted tubule, which lowers intracellular sodium and promotes increased basolateral Na+/Ca2+ exchange, enhancing calcium reabsorption and thus reducing calciuria. This makes thiazides first-line prophylaxis for recurrent calcium-containing stones in patients with idiopathic hypercalciuria. In contrast, potassium citrate prevents both calcium stones (by binding urinary calcium and raising urine pH to inhibit crystallization) and uric acid stones (by alkalinizing urine to increase uric acid solubility).

• A. This describes the mechanism of potassium citrate, not thiazides.
• B. Proximal tubule calcium reabsorption is passive and linked to sodium/water reabsorption, not the site of thiazide action.
• D. Loop diuretics inhibit this transporter and actually increase urinary calcium excretion, worsening calcium stone risk.
• E. Calcium channel blockade in the collecting duct is not a recognized stone-prevention mechanism.

Posterior urethral valves are the most common cause of bladder outlet obstruction in male infants, resulting from abnormal folds of tissue in the posterior urethra that obstruct urine outflow. This produces the classic voiding cystourethrogram finding of a dilated posterior urethra with distal narrowing ("keyhole sign" correlate), bilateral hydronephrosis, oligohydramnios from reduced fetal urine output, and a palpably distended bladder. Severe cases can cause pulmonary hypoplasia due to oligohydramnios (Potter sequence) and progressive renal dysplasia. Early diagnosis and valve ablation are important to preserve renal function.

• A. Neurogenic bladder is usually associated with spinal cord anomalies like myelomeningocele, not isolated urethral obstruction findings on VCUG.
• B. Ureteropelvic junction obstruction is typically unilateral and does not cause the urethral findings or weak stream described.
• C. Hypospadias involves abnormal urethral meatus position on the penis and does not cause bladder outlet obstruction or hydronephrosis.
• E. Vesicoureteral reflux causes hydronephrosis from retrograde urine flow but does not produce a dilated posterior urethra with distal narrowing or bladder outlet obstruction signs.

Clear cell renal cell carcinoma, associated with biallelic inactivation of the VHL tumor suppressor gene, leads to constitutive activation of hypoxia-inducible factor (HIF) even in normoxic conditions. This drives ectopic secretion of erythropoietin, causing secondary polycythemia, and in some cases PTH-related peptide, causing hypercalcemia with appropriately suppressed native PTH. Other paraneoplastic products include renin (causing hypertension) and ACTH. These ectopic hormone effects are distinct from the direct tumor burden or metastatic effects on surrounding organs.

• A. There are no autoantibodies targeting parathyroid tissue in this paraneoplastic syndrome; PTHrP acts directly on PTH receptors.
• B. Venous congestion from renal vein invasion does not explain erythropoietin-driven erythrocytosis or hypercalcemia; the mechanism is hormonal, not mechanical.
• C. Marrow replacement by metastases would cause anemia from myelophthisis, not polycythemia.
• E. Renal artery stenosis causing secondary hyperreninemic hypertension is a distinct paraneoplastic mechanism in RCC but does not explain the polycythemia or hypercalcemia seen here.

Wilms tumor (nephroblastoma) arises from mutations in the WT1 tumor suppressor gene on chromosome 11p13, which normally directs normal genitourinary development. It typically presents in children under age 5 as an asymptomatic abdominal mass and shows the classic triphasic histology of blastemal, epithelial, and stromal elements, representing favorable histology when anaplasia is absent. WT1 mutations underlie associated syndromes: WAGR syndrome (Wilms tumor, Aniridia, Genitourinary anomalies, Range of developmental delay) and Beckwith-Wiedemann syndrome (macroglossia, hemihypertrophy, organomegaly), both seen with overgrowth features like this child's hemihypertrophy and macroglossia.

• B. RET mutations are associated with multiple endocrine neoplasia type 2 and medullary thyroid carcinoma, unrelated to Wilms tumor.
• C. RB1 mutations cause retinoblastoma and osteosarcoma, not Wilms tumor.
• D. VHL mutations cause clear cell renal cell carcinoma and von Hippel-Lindau syndrome, which presents in adults, not this pediatric overgrowth syndrome tumor.
• E. MEN1 mutations cause multiple endocrine neoplasia type 1, involving pituitary, parathyroid, and pancreatic tumors, not Wilms tumor.

Cigarette smoke contains aromatic amines (such as 2-naphthylamine) that are absorbed systemically, metabolized by the liver, and excreted in the urine, where they come into direct contact with and damage the urothelium lining the renal pelvis, ureters, and bladder. This chronic carcinogen exposure is the leading risk factor for urothelial carcinoma in industrialized countries. Other causes with distinct mechanisms include cyclophosphamide, whose metabolite acrolein causes hemorrhagic cystitis and mutagenesis, and Schistosoma haematobium infection, which induces chronic inflammation and squamous metaplasia leading to squamous cell carcinoma rather than urothelial carcinoma.

• A. Catheter-related chronic irritation is linked to squamous cell carcinoma risk, not the mechanism of smoking-related urothelial carcinoma.
• B. This describes the mechanism of cyclophosphamide-induced bladder cancer via its toxic metabolite acrolein, not smoking.
• D. Nephrolithiasis-related trauma is not an established major mechanism for urothelial carcinoma development.
• E. This describes the mechanism of Schistosoma haematobium-associated squamous cell carcinoma, not smoking-related urothelial carcinoma.

Renal oncocytoma is a benign tumor composed of oncocytes, large cells packed with mitochondria that produce abundant eosinophilic granular cytoplasm, arranged in nests or tubules within a characteristic hypocellular, hyalinized or edematous stroma. Both oncocytoma and chromophobe RCC share eosinophilic cytoplasm and CD117 positivity, but oncocytoma is classically CK7-negative while chromophobe RCC is CK7-positive, and oncocytoma lacks the perinuclear halos and prominent cell membranes ('plant cell' appearance) of chromophobe RCC. The uniform, bland round nuclei and negative CK7 here favor benign oncocytoma over the malignant chromophobe variant.

• A. Chromophobe RCC would typically show CK7 positivity and perinuclear cytoplasmic halos, not the CK7-negative pattern described.
• B. Papillary RCC shows papillary architecture with fibrovascular cores, not nested eosinophilic cells in hyalinized stroma.
• C. Clear cell RCC shows cells with clear, lipid-laden cytoplasm due to glycogen and lipid content, not eosinophilic granular cytoplasm.
• D. Collecting duct carcinoma is a rare, highly aggressive tumor arising from medullary collecting ducts with infiltrative, desmoplastic growth, not this bland nested pattern.

Furosemide is a loop diuretic that inhibits the Na-K-2Cl cotransporter in the thick ascending limb of the loop of Henle. It can disrupt the ionic gradient in the endolymph of the inner ear, which contains a similar cotransporter, causing reversible or occasionally permanent ototoxicity. Risk is dose- and rate-dependent and is markedly increased with high-dose intravenous infusion, renal failure, and concurrent use of other ototoxic drugs such as aminoglycosides. Thiazides, potassium-sparing diuretics, and carbonic anhydrase inhibitors do not act on this cotransporter and are not associated with ototoxicity.

• A. Amiloride blocks epithelial sodium channels in the collecting duct and is not ototoxic.
• B. Thiazides inhibit the NCC in the distal convoluted tubule and are not ototoxic.
• D. Acetazolamide inhibits carbonic anhydrase and causes metabolic acidosis, not ototoxicity.
• E. Spironolactone blocks aldosterone receptors and is not associated with hearing loss.

Thiazide diuretics inhibit the Na-Cl cotransporter (NCC) in the distal convoluted tubule, which lowers intracellular sodium and favors increased basolateral Na-Ca exchange, ultimately increasing calcium reabsorption and causing mild hypercalcemia. This is the opposite of the effect seen with loop diuretics, which increase calcium excretion. Thiazides also commonly cause hyperglycemia (via decreased insulin secretion and increased insulin resistance), hyperlipidemia, and hyperuricemia (via competition with urate for proximal tubule organic anion transporters). These metabolic effects are important when choosing antihypertensive therapy in patients with diabetes, gout, or dyslipidemia.

• A. This describes the effect of loop diuretics, which increase calcium excretion, not thiazides.
• B. Thiazides do not alter vitamin D hydroxylation; this is not the mechanism of thiazide-induced hypercalcemia.
• C. Thiazides act on the distal convoluted tubule, not the proximal tubule, and do not primarily work through calcium-binding protein synthesis.
• D. Thiazides do not directly suppress parathyroid hormone release; hypercalcemia is due to a renal tubular effect.

Spironolactone is a competitive aldosterone receptor antagonist used for ascites and hypertension, but it also has antiandrogenic activity by blocking androgen receptors and weak progestogenic activity, which together cause gynecomastia and can cause erectile dysfunction and menstrual irregularities. This antiandrogenic effect is distinct from its diuretic mechanism (blocking mineralocorticoid receptors in the collecting duct, which also explains the hyperkalemia seen here). Eplerenone, a more selective aldosterone antagonist, causes less gynecomastia because it has minimal affinity for androgen and progesterone receptors. Recognizing this adverse effect is important when choosing a potassium-sparing diuretic in male patients.

• B. Spironolactone does not act by increasing SHBG synthesis.
• C. Spironolactone does not suppress GnRH secretion as its mechanism of gynecomastia.
• D. Spironolactone does not directly stimulate estrogen receptors.
• E. Spironolactone does not inhibit aromatase; this is not its mechanism of gynecomastia.

Acetazolamide inhibits carbonic anhydrase in the proximal tubule, preventing the conversion of carbonic acid to CO2 and water, which reduces bicarbonate reabsorption and causes a mild hyperchloremic metabolic acidosis with an alkaline urine. This same mechanism reduces aqueous humor formation (used in glaucoma) and reduces cerebrospinal fluid production (used in high-altitude illness and idiopathic intracranial hypertension). The resulting bicarbonaturia explains the elevated urine pH seen in this patient. Chronic use requires monitoring of serum bicarbonate and potassium because of ongoing acid-base and electrolyte effects.

• B. Acetazolamide's acidosis is due to bicarbonate loss, not accumulation of organic acids.
• C. Acetazolamide does not increase collecting duct hydrogen secretion; it impairs proximal bicarbonate reabsorption.
• D. Acetazolamide does not act on the thick ascending limb or chloride reabsorption there.
• E. Acetazolamide does not primarily affect distal ammonium excretion.

ACE inhibitors block the enzyme that converts angiotensin I to angiotensin II, but this same enzyme (kininase II) normally degrades bradykinin. Reduced bradykinin breakdown leads to its accumulation in the lungs, which stimulates sensory nerve fibers and causes a dry cough in up to 10-20% of patients, and can rarely cause angioedema of the lips, tongue, or larynx. This adverse effect is a class effect of ACE inhibitors and typically resolves within days of discontinuation; switching to an angiotensin receptor blocker, which does not affect bradykinin metabolism, usually avoids recurrence. Angioedema due to bradykinin is a medical emergency requiring airway monitoring and drug discontinuation.

• B. ACE inhibitors cause vasodilation through decreased angiotensin II and increased bradykinin, not a reflex bronchoconstriction mechanism.
• C. This cough is not an IgE-mediated allergic reaction.
• D. ACE inhibitors do not block beta-2 receptors; this describes a beta-blocker effect.
• E. ACE inhibitors decrease, not increase, angiotensin II levels.

In bilateral renal artery stenosis, glomerular filtration distal to the stenosis depends on angiotensin II-mediated constriction of the efferent arteriole to maintain filtration pressure. ACE inhibitors and ARBs block this compensatory mechanism, causing a sharp drop in GFR and acute kidney injury, reflected by the rising creatinine in this patient. The appropriate next step is to stop the offending drug, which typically allows renal function to recover. Continuing or increasing the dose would worsen renal ischemia, and dialysis or diuretics are not indicated for this reversible, drug-induced hemodynamic effect.

• A. Increasing the dose would worsen efferent arteriolar dilation and further reduce GFR.
• B. A potassium-sparing diuretic would worsen hyperkalemia risk and does not address the underlying problem.
• D. Emergency dialysis is not indicated for this reversible, drug-induced reduction in GFR before conservative measures are tried.
• E. A loop diuretic does not address the underlying hemodynamic cause and could worsen renal perfusion if volume depletion occurs.

Mannitol is an osmotic diuretic that is filtered at the glomerulus but poorly reabsorbed, increasing tubular fluid osmolarity and promoting water excretion, used to acutely lower intracranial pressure and intraocular pressure. Because mannitol is osmotically active, it initially draws water from the intracellular and interstitial space into the vasculature, transiently expanding intravascular volume before diuresis occurs; this can precipitate acute pulmonary edema in patients with heart failure who cannot tolerate the increased preload. Mannitol is also contraindicated in anuria, since without adequate renal excretion the drug will remain in the vasculature and worsen volume overload rather than produce a diuresis. Careful volume status assessment is required before administration in patients with cardiac or renal compromise.

• A. Mannitol does not cause direct vasodilation or hypotension as its primary concerning effect in heart failure.
• B. Mannitol does not block calcium channels or directly impair contractility.
• D. Mannitol is not associated with severe hyperkalemia as its mechanism of harm in heart failure.
• E. Mannitol does not stimulate aldosterone release; its effect is independent of the renin-angiotensin-aldosterone system.