A 70-year-old man undergoes elective CABG. On post-operative day 2, urine output drops to 0.3 mL/kg/hr for 8 hours and creatinine rises from a baseline of 1.0 to 1.8 mg/dL. Urine sodium is 12 mEq/L, fractional excretion of sodium (FeNa) is 0.4%, urine osmolality 480 mOsm/kg. Which KDIGO AKI stage is this, and what is the most likely aetiology?
- A AKI Stage 1; prerenal azotemia (FeNa <1% confirms reduced renal perfusion) ✓
- B AKI Stage 1; intrinsic renal (acute tubular necrosis)
- C AKI Stage 2; obstructive uropathy
- D AKI Stage 3; contrast nephropathy
Correct answer: A. AKI Stage 1; prerenal azotemia (FeNa <1% confirms reduced renal perfusion)
Explanation
KDIGO AKI Stage 1 is defined as creatinine ≥1.5× baseline (1.8/1.0 = 1.8×) or increase ≥0.3 mg/dL within 48 h, or urine output <0.5 mL/kg/hr for 6–12 h. This patient qualifies on both criteria. FeNa <1% with concentrated urine (osmolality >400), low urine sodium, and the post-operative setting (reduced cardiac output, hypovolemia) are hallmarks of prerenal azotemia with intact tubular function. ATN (intrinsic AKI) typically shows FeNa >2%, urine sodium >40, and isosthenuric urine with granular casts.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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