A 68-year-old man with CKD stage 3b (eGFR 32 mL/min) is on lisinopril 10 mg for diabetic nephropathy. He is admitted with a 3-day history of vomiting and diarrhea. Serum creatinine has risen from 1.9 to 3.6 mg/dL. Urine sodium is 48 mEq/L, FeNa is 2.8%, and urine osmolality is 290 mOsm/kg. What type of AKI is present?
- A Intrinsic renal AKI (acute tubular necrosis) ✓
- B Pre-renal AKI (volume-responsive)
- C Post-renal AKI (obstructive uropathy)
- D Contrast-induced nephropathy
Explanation
In AKI, a FeNa >2% with urine sodium >40 mEq/L and urine osmolality approaching plasma (isosthenuria, ~300 mOsm/kg) indicates tubular injury with impaired concentrating and sodium-conserving ability, consistent with intrinsic AKI (acute tubular necrosis). Pre-renal AKI shows FeNa <1%, urine Na <20, urine osmolality >500 mOsm/kg. The history of volume depletion (vomiting/diarrhea) in a CKD patient on an ACE inhibitor (which further reduces GFR by blocking efferent arteriolar tone) created conditions for ischemic ATN. ACEI/ARBs should be held during volume depletion in CKD patients.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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