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?
Obstruction at the bladder outlet causing bilateral hydronephrosis
Acute inhibition of prostaglandins causing afferent arteriolar constriction
Diffuse nodular glomerulosclerosis from nonenzymatic glycosylation of basement membranes
Anti–PLA2R antibody–mediated subepithelial immune complex deposition
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?
Obstruction of urinary flow at the ureteropelvic junction causing back pressure
Immune complex deposition with subepithelial humps and complement consumption
Afferent arteriolar vasoconstriction causing decreased glomerular filtration fraction
Proximal tubular epithelial cell injury causing impaired reabsorption and cast formation
Anti–GBM antibody–mediated crescentic glomerulonephritis with linear IgG staining
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 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?
Acute urate crystal deposition in tubules causing obstructive nephropathy
Autoantibodies against GBM causing rapidly progressive crescentic nephritis
Loss of collecting duct responsiveness to ADH causing nephrogenic diabetes insipidus
Decreased 1α-hydroxylation of vitamin D causing hypocalcemia and secondary hyperparathyroidism
Increased renal prostaglandin production causing afferent arteriolar dilation
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 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?
Proximal tubular necrosis causing impaired sodium reabsorption and FeNa >2%
Bilateral urinary tract obstruction causing hydronephrosis and postrenal azotemia
Decreased renal perfusion with intact tubules increasing sodium and water reabsorption
Immune-mediated glomerular injury causing RBC casts and low complement levels
Drug-induced interstitial nephritis causing eosinophiluria and WBC casts
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
Acute interstitial nephritis with eosinophiluria after antibiotic exposure
Acute tubular necrosis due to nephrotoxic injury with FeNa >2%
Prerenal azotemia due to decreased prostaglandin-mediated afferent arteriolar dilation
Postrenal acute kidney injury due to bilateral ureteral obstruction and hydronephrosis
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?
Acute tubular necrosis from direct proximal tubular toxicity
Obstructive uropathy due to bilateral ureteral calculi
Poststreptococcal glomerulonephritis due to immune complex deposition
Acute interstitial nephritis due to drug hypersensitivity reaction
Prerenal azotemia due to decreased effective arterial blood volume
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?
Poststreptococcal glomerulonephritis due to immune complexes and low complement
Acute interstitial nephritis due to drug hypersensitivity with eosinophiluria
Membranous nephropathy causing nephrotic syndrome due to subepithelial deposits
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 26-year-old man presents with hematuria and mild edema 10 days after a skin infection. No medications. Exam: BP 146/90 mmHg. Labs: BUN 30 mg/dL, creatinine 1.7 mg/dL; C3 low. Urinalysis: RBC casts, protein 2+. Biopsy: hypercellular glomeruli; EM shows subepithelial humps. Which of the following mechanisms best explains this patient's renal findings?
Circulating immune complexes activate complement leading to inflammatory glomerular injury
Afferent arteriolar vasoconstriction reduces GFR with FeNa <1% and bland urine
Eosinophil-rich interstitial infiltrates cause tubular dysfunction after antibiotics
T-cell cytokines cause podocyte effacement with selective albuminuria
Antibodies against podocyte phospholipase A2 receptor cause nephrotic syndrome
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on postinfectious GN. Understanding renal pathophysiology involves recognizing immune complexes and interpreting biopsy humps. In this vignette, the patient's post-skin infection hematuria and low C3 suggest PSGN. The correct answer is supported by complement activation causing injury. A common distractor might incorrectly suggest MCD based on edema, failing to account for RBC casts. Teaching strategies include reinforcing immune-mediated GN. Additionally, practice mechanisms in nephritic syndrome.
A 22-year-old man presents with cola-colored urine and periorbital edema 2 weeks after impetigo. No medications. Exam: BP 148/92 mmHg, mild edema. Labs: BUN 34 mg/dL, creatinine 1.9 mg/dL; C3 low, anti-DNase B elevated. Urinalysis: RBC casts, protein 2+. Kidney biopsy: enlarged hypercellular glomeruli with neutrophils; EM shows subepithelial humps. What is the most likely diagnosis based on the lab results?
IgA nephropathy due to mesangial IgA deposition after mucosal infection
Acute interstitial nephritis due to drug-induced hypersensitivity reaction
Membranous nephropathy due to subepithelial anti-PLA2R immune deposits
Minimal change disease due to podocyte foot process effacement
Poststreptococcal glomerulonephritis due to immune complex deposition
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on postinfectious glomerulonephritis. Understanding renal pathophysiology involves recognizing immune complex deposition and interpreting low complement. In this vignette, the patient's post-impetigo hematuria and RBC casts suggest PSGN. The correct answer is supported by subepithelial humps and low C3. A common distractor might incorrectly suggest IgA nephropathy based on hematuria, failing to account for timing and low complement. Teaching strategies include reinforcing infection-related GN. Additionally, practice biopsy interpretation in nephritic syndrome.
A 33-year-old man develops oliguria after receiving amphotericin B for fungal pneumonia. History: kidney transplant recipient; meds: tacrolimus, amphotericin B. Exam: euvolemic, no edema. Labs: BUN 44 mg/dL, creatinine 3.7 mg/dL (baseline 1.3), K 4.9 mmol/L, Mg 1.2 mg/dL. Urinalysis: granular casts; FeNa 2.4%. Renal ultrasound: no hydronephrosis. Which of the following mechanisms best explains this patient's renal findings?
Direct tubular epithelial injury increasing membrane permeability and causing ATN
Obstruction of urine flow causing hydronephrosis and postrenal azotemia
Immune complex deposition in mesangium causing recurrent hematuria after URI
Reduced renal perfusion with intact tubular function causing FeNa <1%
Allergic interstitial nephritis with eosinophiluria and WBC casts
Explanation
This question tests USMLE Step 1 renal pathophysiology, focusing on drug-induced ATN. Understanding renal pathophysiology involves recognizing tubular toxicity and interpreting FeNa. In this vignette, the patient's amphotericin B use and granular casts suggest ATN. The correct answer is supported by direct epithelial injury increasing permeability. A common distractor might incorrectly suggest prerenal based on transplant history, failing to account for high FeNa. Teaching strategies include reinforcing nephrotoxic drugs. Additionally, practice electrolyte monitoring in toxicity.