Question

Pre-renal azotemia is characterized by all of the following except:

• A. Fractional excretion of Na < 1%
• B. Urinary osmolality > 500 mosm/kg
• C. Urinary sodium concentration > 40 meq/l
• D. Reversible with replacement fluids

Hint:

A salt-starved pre-renal kidney hoards sodium, so a high urine sodium is the exception.

Answer 1

The Correct Option is C

Step 1: Understand pre-renal azotemia. It is a fall in renal perfusion (volume loss, hypotension, heart failure) with intact tubular function. The kidney responds by avidly conserving sodium and water to defend blood volume.

Step 2: Predict the urine indices. Because the tubules reabsorb sodium hard, the urine has very little sodium. Therefore fractional excretion of sodium falls below 1 percent (option A is true) and urinary sodium concentration is low, typically less than 20 meq/l.

Step 3: Predict urine concentration. With intact tubules and high ADH drive, the kidney concentrates urine strongly, so urinary osmolality rises above 500 mosm/kg (option B is true).

Step 4: Apply reversibility. Pre-renal azotemia, by definition, corrects once perfusion is restored with fluids, before tubular injury sets in (option D is true).

Step 5: Find the exception. A urinary sodium concentration above 40 meq/l indicates sodium wasting, which is a feature of intrinsic acute tubular necrosis, not pre-renal disease. So option C is the odd one out.

The answer is option C.