Which of the following best describes ATN urine findings?

• A. Low urine sodium and high osmolality
• B. Urine Na >40 mEq/L and osmolality <350 mOsm/kg
• C. BUN:Cr ratio >20:1
• D. Hyaline casts only

Answer: (B)

In acute tubular necrosis, tubular injury disrupts the kidney’s ability to reabsorb sodium and to concentrate urine. That means more sodium stays in the filtrate and is excreted, and water reabsorption is impaired, so the urine becomes more dilute. The result is urine sodium that is high (typically greater than 40 mEq/L) and urine osmolality that is relatively low (usually below 350 mOsm/kg). This combination specifically reflects tubular damage with loss of concentrating ability.

This pattern helps distinguish ATN from prerenal causes. In prerenal states, the kidney tries to conserve sodium and water, so urine sodium is low (often <20–40 mEq/L), urine osmolality is high (often >500 mOsm/kg), and the BUN:Cr ratio tends to be elevated. Hyaline casts are more typical of prerenal azotemia, whereas ATN more commonly shows granular (muddy brown) casts.