A 60-year-old woman develops AKI after contrast exposure for CT angiography (creatinine rises from 1.0 to 1.9 mg/dL at 48 hours). Urinalysis shows muddy brown granular casts. Fractional excretion of sodium (FENa) is 3.2%. What type of AKI is this?

• A Pre-renal AKI
• B Post-renal AKI
• C Interstitial nephritis
• D Intrinsic renal AKI — contrast-induced acute tubular necrosis (CI-AKN) ✓

Explanation

Muddy brown granular casts are hallmarks of acute tubular necrosis (ATN), and FENa >2% indicates tubular dysfunction (inability to reabsorb sodium) consistent with intrinsic renal injury. Contrast-induced nephropathy (now termed contrast-induced AKN) peaks at 48-72 hours post-exposure. Pre-renal AKI has FENa <1%, concentrated urine, and hyaline casts. Post-renal AKI has features of obstruction. Interstitial nephritis shows WBC casts and eosinophiluria.

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

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