A 40-year-old man presents with acute kidney injury: creatinine has risen from a baseline of 0.9 to 4.2 mg/dL over 3 days. He had a 7-day course of gentamicin for gram-negative bacteremia. Urinalysis shows muddy brown granular casts and renal tubular epithelial cells. Urine sodium is 52 mEq/L and fractional excretion of sodium (FENa) is 3.8%. What is the MOST likely diagnosis?

• A Pre-renal azotemia from sepsis-related volume depletion
• B Acute tubular necrosis (ATN) due to aminoglycoside nephrotoxicity ✓
• C Acute interstitial nephritis from gentamicin
• D Contrast-induced nephropathy

Explanation

Muddy brown granular casts and renal tubular epithelial (RTE) cells are the pathognomonic urinary findings of acute tubular necrosis. High urine sodium (> 40 mEq/L) and FENa > 2% indicate that the tubules have lost their ability to concentrate and reabsorb sodium, confirming intrinsic renal injury. Aminoglycosides accumulate in proximal tubular cells causing direct mitochondrial toxicity, and injury typically manifests on days 7–10 of therapy. Pre-renal azotemia shows FENa < 1% and urine sodium < 20 mEq/L because tubular function is preserved. Acute interstitial nephritis would show WBC casts and eosinophiluria on urinalysis. Contrast-induced nephropathy is excluded as no contrast was given.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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Written and medically reviewed by the StethoPrep medical team.