Which finding most supports prerenal azotemia due to dehydration?

• A. Urine osmolality >500 mOsm/kg with urine sodium <20 mEq/L
• B. Urine osmolality <250 mOsm/kg with high urine sodium
• C. Urine sediment with red blood cells
• D. White blood cell casts

Answer: (A)

When dehydration causes prerenal azotemia, the kidneys respond by conserving volume. They reabsorb water and sodium aggressively to maintain intravascular volume, which makes the urine very concentrated and low in sodium. That results in urine osmolality well above 500 mOsm/kg and urine sodium under 20 mEq/L. This pattern — high concentrating ability with sodium retention — is a classic sign of prerenal azotemia due to reduced renal perfusion from dehydration, often accompanied by an elevated BUN-to-creatinine ratio as a supporting clue.

If the kidney were intrinsically damaged, it would struggle to concentrate urine, yielding low osmolality (<250 mOsm/kg) and higher urine sodium. Red blood cells in the urine sediment point toward glomerular disease or nephritic processes, while white blood cell casts suggest infection or tubulointerstitial nephritis. These scenarios do not fit dehydration-driven prerenal azotemia.