Renal Pathophysiology
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USMLE Step 1 › Renal Pathophysiology
A 70-year-old man with long-standing diabetes and hypertension presents with fatigue and ankle swelling. Meds: amlodipine, insulin, atorvastatin. Exam: BP 168/92 mmHg, periorbital edema, 2+ pitting edema. Labs: BUN 46 mg/dL, creatinine 2.6 mg/dL, K 5.2 mmol/L, HCO3− 19 mmol/L; eGFR 26 mL/min/1.73 m². Urine albumin-to-creatinine ratio: 980 mg/g. Renal ultrasound: bilaterally small echogenic kidneys. What is the underlying pathophysiological process causing these symptoms?
Diffuse nodular glomerulosclerosis from nonenzymatic glycosylation of basement membranes
Obstruction at the bladder outlet causing bilateral hydronephrosis
Anti–PLA2R antibody–mediated subepithelial immune complex deposition
Acute inhibition of prostaglandins causing afferent arteriolar constriction
Interstitial eosinophilic inflammation due to recent beta-lactam exposure
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on diabetic nephropathy. Understanding renal pathophysiology involves recognizing how chronic hyperglycemia causes glomerular changes and interpreting albuminuria. In this vignette, the patient's long-standing diabetes and high UACR suggest diabetic nephropathy. The correct answer is supported by nodular glomerulosclerosis from basement membrane glycosylation. A common distractor might incorrectly suggest membranous nephropathy based on proteinuria, failing to account for diabetes history. Teaching strategies include reinforcing the progression of diabetic kidney disease. Additionally, practice interpreting eGFR and UACR in chronic settings.
A 52-year-old man presents with 24 hours of oliguria after starting gentamicin for sepsis. History: type 2 diabetes, HTN; meds: lisinopril, metformin. Exam: euvolemic, mild flank tenderness. Urinalysis: muddy brown granular casts, no RBC casts; FeNa 3.2%. Labs: BUN 48 mg/dL, creatinine 4.1 mg/dL (baseline 1.0), K 5.8 mmol/L, HCO3− 18 mmol/L, phosphate 5.6 mg/dL. Renal ultrasound: normal-sized kidneys, no hydronephrosis. Which of the following mechanisms best explains this patient's renal findings?
Immune complex deposition with subepithelial humps and complement consumption
Anti–GBM antibody–mediated crescentic glomerulonephritis with linear IgG staining
Proximal tubular epithelial cell injury causing impaired reabsorption and cast formation
Obstruction of urinary flow at the ureteropelvic junction causing back pressure
Afferent arteriolar vasoconstriction causing decreased glomerular filtration fraction
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on mechanisms of acute kidney injury. Understanding renal pathophysiology involves recognizing how nephrotoxic drugs affect kidney function and interpreting lab results like FeNa and urine sediment. In this vignette, the patient's history of gentamicin use and acute oliguria with muddy brown casts suggest acute tubular necrosis. The correct answer is supported by the elevated FeNa >2% and granular casts, indicative of tubular epithelial injury. A common distractor might incorrectly suggest glomerulonephritis based on oliguria, failing to account for the absence of RBC casts and drug history. Teaching strategies include reinforcing the importance of correlating medication history with urine findings. Additionally, practice differentiating between prerenal, intrinsic, and postrenal AKI using FeNa and ultrasound results.
A 58-year-old man presents with oliguria after 5 days of high-dose ibuprofen for back pain. History: cirrhosis with ascites; meds: furosemide, spironolactone. Exam: hypotension, dry mucous membranes, cool extremities. Urinalysis: bland sediment, no casts; urine sodium 8 mmol/L; FeNa 0.4%. Labs: BUN 72 mg/dL, creatinine 2.8 mg/dL (baseline 1.0), K 5.1 mmol/L, HCO3− 20 mmol/L. Renal ultrasound: no obstruction. Which of the following mechanisms best explains this patient's renal findings?
Efferent arteriolar dilation causing decreased intraglomerular pressure and GFR
Afferent arteriolar constriction from prostaglandin inhibition decreasing renal perfusion
Tubular obstruction by hemoglobin casts after intravascular hemolysis
Immune-mediated glomerular basement membrane rupture with crescent formation
Bilateral ureteral obstruction causing hydronephrosis and elevated postvoid residual
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on prerenal acute kidney injury. Understanding renal pathophysiology involves recognizing how NSAIDs affect renal perfusion in volume-depleted states and interpreting low FeNa. In this vignette, the patient's cirrhosis, NSAID use, and hypotension with bland urine suggest prerenal azotemia. The correct answer is supported by afferent constriction from prostaglandin inhibition and low FeNa <1%. A common distractor might incorrectly suggest tubular obstruction based on oliguria, failing to account for the bland sediment. Teaching strategies include reinforcing the role of prostaglandins in renal autoregulation. Additionally, practice using FeNa to differentiate prerenal from intrinsic AKI.
A 65-year-old woman with CKD presents with bone pain and muscle cramps. History: diabetes, HTN; meds: insulin, nifedipine. Exam: mild edema. Labs: BUN 62 mg/dL, creatinine 3.6 mg/dL; eGFR 15 mL/min/1.73 m²; phosphate 6.1 mg/dL, calcium 8.0 mg/dL, PTH elevated; K 5.4 mmol/L, HCO3− 17 mmol/L. What is the underlying pathophysiological process causing these symptoms?
Autoantibodies against GBM causing rapidly progressive crescentic nephritis
Increased renal prostaglandin production causing afferent arteriolar dilation
Loss of collecting duct responsiveness to ADH causing nephrogenic diabetes insipidus
Acute urate crystal deposition in tubules causing obstructive nephropathy
Decreased 1α-hydroxylation of vitamin D causing hypocalcemia and secondary hyperparathyroidism
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on CKD-mineral bone disorder. Understanding renal pathophysiology involves recognizing impaired vitamin D activation and interpreting PTH and electrolytes. In this vignette, the patient's CKD with hypocalcemia and high PTH suggest secondary hyperparathyroidism. The correct answer is supported by decreased 1α-hydroxylation. A common distractor might incorrectly suggest anti-GBM disease based on CKD, failing to account for bone symptoms. Teaching strategies include reinforcing CKD complications. Additionally, practice managing metabolic imbalances in CKD.
A 46-year-old woman presents with fever and flank pain 10 days after starting methicillin for endocarditis. Meds: methicillin. Exam: diffuse maculopapular rash, mild costovertebral angle tenderness. Labs: BUN 38 mg/dL, creatinine 2.4 mg/dL (baseline 0.8), eosinophils 11%. Urinalysis: WBC casts, eosinophils, mild proteinuria. Renal ultrasound: kidneys normal size, no hydronephrosis. What is the underlying pathophysiological process causing these symptoms?
Obstruction at the ureters causing hydronephrosis and colicky pain
Immune complex deposition in glomeruli causing subepithelial humps and low C3
Afferent arteriolar constriction from prostaglandin inhibition causing prerenal azotemia
Type IV hypersensitivity causing interstitial inflammation and tubular dysfunction
Type I hypersensitivity with IgE cross-linking and mast cell degranulation
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on acute interstitial nephritis. Understanding renal pathophysiology involves recognizing hypersensitivity types and interpreting eosinophiluria. In this vignette, the patient's methicillin exposure, rash, and WBC casts suggest AIN. The correct answer is supported by type IV hypersensitivity causing inflammation. A common distractor might incorrectly suggest PSGN based on endocarditis, failing to account for eosinophils. Teaching strategies include reinforcing hypersensitivity classifications. Additionally, practice urinalysis in drug reactions.
A 44-year-old man presents with oliguria after 2 days of severe vomiting and diarrhea. Meds: none. Exam: tachycardia, orthostatic hypotension, dry mucous membranes. Urinalysis: bland sediment; urine sodium 6 mmol/L; FeNa 0.3%. Labs: BUN 64 mg/dL, creatinine 2.0 mg/dL, K 5.0 mmol/L, HCO3− 18 mmol/L. Renal ultrasound: no hydronephrosis. Which of the following mechanisms best explains this patient's renal findings?
Drug-induced interstitial nephritis causing eosinophiluria and WBC casts
Decreased renal perfusion with intact tubules increasing sodium and water reabsorption
Immune-mediated glomerular injury causing RBC casts and low complement levels
Proximal tubular necrosis causing impaired sodium reabsorption and FeNa >2%
Bilateral urinary tract obstruction causing hydronephrosis and postrenal azotemia
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on prerenal azotemia. Understanding renal pathophysiology involves recognizing volume depletion effects and interpreting low FeNa. In this vignette, the patient's vomiting, hypotension, and bland urine suggest prerenal AKI. The correct answer is supported by intact tubular reabsorption. A common distractor might incorrectly suggest ATN based on oliguria, failing to account for low FeNa. Teaching strategies include reinforcing dehydration signs. Additionally, practice FeNa calculations in AKI.
A 47-year-old woman presents with oliguria after taking large doses of naproxen for migraines. History: heart failure; meds: furosemide, naproxen. Exam: mild hypotension, cool extremities, no rash. Urinalysis: bland sediment; urine sodium 9 mmol/L; FeNa 0.6%. Labs: BUN 70 mg/dL, creatinine 2.9 mg/dL, K 5.3 mmol/L. Renal ultrasound: no hydronephrosis. What is the most likely diagnosis based on the lab results?
Nephritic syndrome due to IgA deposition with normal complement levels
Prerenal azotemia due to decreased prostaglandin-mediated afferent arteriolar dilation
Acute interstitial nephritis with eosinophiluria after antibiotic exposure
Postrenal acute kidney injury due to bilateral ureteral obstruction and hydronephrosis
Acute tubular necrosis due to nephrotoxic injury with FeNa >2%
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on NSAID-induced AKI. Understanding renal pathophysiology involves recognizing prostaglandin roles and interpreting low FeNa. In this vignette, the patient's heart failure, naproxen use, and bland urine suggest prerenal azotemia. The correct answer is supported by decreased afferent dilation. A common distractor might incorrectly suggest ATN based on oliguria, failing to account for low FeNa. Teaching strategies include reinforcing risks in low EAV states. Additionally, practice differentiating AKI types by history.
A 54-year-old man develops oliguria and rising creatinine after starting trimethoprim-sulfamethoxazole for cellulitis. History: HIV well controlled; meds: tenofovir, TMP-SMX. Exam: afebrile, mild maculopapular rash. Urinalysis: WBCs, WBC casts, eosinophils; mild proteinuria. Labs: BUN 42 mg/dL, creatinine 3.0 mg/dL (baseline 1.1), K 5.0 mmol/L, HCO3− 21 mmol/L; eosinophils 9%. Renal ultrasound: normal kidneys, no hydronephrosis. What is the most likely diagnosis based on the lab results?
Poststreptococcal glomerulonephritis due to immune complex deposition
Acute interstitial nephritis due to drug hypersensitivity reaction
Prerenal azotemia due to decreased effective arterial blood volume
Acute tubular necrosis from direct proximal tubular toxicity
Obstructive uropathy due to bilateral ureteral calculi
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on drug-induced acute interstitial nephritis. Understanding renal pathophysiology involves recognizing hypersensitivity reactions and interpreting urine eosinophils and WBC casts. In this vignette, the patient's TMP-SMX exposure, rash, and eosinophiluria suggest acute interstitial nephritis. The correct answer is supported by the eosinophil-rich infiltrates and WBC casts. A common distractor might incorrectly suggest ATN based on rising creatinine, failing to account for the rash and eosinophils. Teaching strategies include reinforcing common drugs causing AIN. Additionally, practice correlating clinical history with urinalysis findings.
A 30-year-old man presents with frothy urine and leg swelling. History: chronic hepatitis B; meds: entecavir. Exam: BP 138/86 mmHg, 3+ pitting edema. Labs: BUN 28 mg/dL, creatinine 1.4 mg/dL; albumin 2.2 g/dL. Urine protein-to-creatinine ratio: 7.0 g/g. Kidney biopsy: thickened capillary walls; immunofluorescence shows granular IgG and C3 along GBM; EM shows subepithelial deposits with spike formation. What is the most likely diagnosis based on the lab results?
Membranous nephropathy causing nephrotic syndrome due to subepithelial deposits
Poststreptococcal glomerulonephritis due to immune complexes and low complement
Acute interstitial nephritis due to drug hypersensitivity with eosinophiluria
Minimal change disease due to T-cell cytokine–mediated podocyte injury
Acute tubular necrosis due to ischemia with muddy brown granular casts
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on membranous nephropathy. Understanding renal pathophysiology involves recognizing subepithelial deposits and interpreting biopsy findings. In this vignette, the patient's hepatitis B, nephrotic syndrome, and spikes on EM suggest membranous nephropathy. The correct answer is supported by granular IgG and subepithelial deposits. A common distractor might incorrectly suggest MCD based on proteinuria, failing to account for biopsy. Teaching strategies include reinforcing secondary causes like HBV. Additionally, practice EM features in glomerular diseases.
A 19-year-old woman has recurrent episodes of gross hematuria that occur within 1 day of upper respiratory infections. No medications. Exam: BP 132/84 mmHg, no edema. Labs: BUN 22 mg/dL, creatinine 1.2 mg/dL; C3 normal. Urinalysis: RBC casts, protein 1+. Kidney biopsy: mesangial proliferation with granular IgA deposition on immunofluorescence. Which of the following mechanisms best explains this patient's renal findings?
Aberrant IgA immune complex deposition in the mesangium activating complement
Anti–GBM antibodies binding type IV collagen with linear IgG staining
Subepithelial immune complex deposition with low C3 and humps on EM
Eosinophil-rich interstitial inflammation due to recent antibiotic exposure
Podocyte injury causing selective albuminuria responsive to corticosteroids
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on IgA nephropathy. Understanding renal pathophysiology involves recognizing mesangial deposition and interpreting normal complement. In this vignette, the patient's synpharyngitic hematuria and IgA on biopsy suggest IgA nephropathy. The correct answer is supported by aberrant IgA complexes activating complement. A common distractor might incorrectly suggest PSGN based on hematuria, failing to account for normal C3. Teaching strategies include reinforcing triggers like URI. Additionally, practice differentiating GN types by timing and labs.