Postoperative oliguria with an increased BUN:creatinine ratio and a fractional excretion of sodium less than 1% is most consistent with which diagnosis?

• A. Prerenal azotemia
• B. Acute tubular necrosis
• C. Acute glomerulonephritis
• D. Obstructive uropathy

Answer: (A)

When the kidney is underperfused but the tubules are still intact, it conserves water and sodium to preserve circulating volume. This produces an elevated BUN relative to creatinine because urea is reabsorbed more avidly in the proximal tubule during low flow, while creatinine is not. The kidneys also actively reabsorb sodium, so the fractional excretion of sodium stays very low, typically less than 1%. In the postoperative setting, this pattern points to reduced renal perfusion or volume depletion causing prerenal azotemia.

This differs from intrinsic kidney injury, where the tubules are damaged and cannot reclaim sodium effectively, leading to a higher fractional excretion of sodium (often >2%). Glomerular disease would present with different urine findings such as proteinuria or RBC casts, and obstructive (postrenal) causes usually involve signs of obstruction or impaired urine flow with variable sodium handling as the condition evolves.