A 60-year-old man with vomiting and diarrhea has serum creatinine of 2.4 mg/dL, BUN of 68 mg/dL, urine sodium of 12 mEq/L, and urine osmolality of 550 mOsm/kg. Which fractional excretion of sodium would be expected?
- A Less than 1 percent ✓
- B Between 1 and 2 percent
- C Greater than 2 percent
- D Exactly equal to the fractional excretion of urea
Correct answer: A. Less than 1 percent
Explanation
This is prerenal azotemia: volume depletion activates RAAS and sympathetic tone, so the kidney avidly reabsorbs sodium and water. The FENa is characteristically below 1 percent, urine sodium is low, urine osmolality is high, and the BUN to creatinine ratio exceeds 20. In acute tubular necrosis, tubular damage impairs sodium reabsorption and the FENa typically exceeds 2 percent, which distinguishes it from this presentation.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.