This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney, Subject name: Medicine
Fractional excretion of sodium < 1 is seen in - 
Select and explain. 
A. Pre renal azotemia, B. Acute tubular necrosis, C. Acute ureteral obstruction, D. Interstitial nephritis
A: Pre renal azotemia
Answer: A: Pre renal azotemia 
Explanation: The fractional excretion of filtered sodium (FENa) has been demonstrated to be a reliably discriminating test between prerenal azotemia and acute tubular necrosis. One paicular urinary index cannot be expected to reliably discriminate between prerenal azotemia and acute renal failure in all cases Prerenal azotemia Oliguria (urine volume < 500 mL/day) or anuria (< 100 mL/day), high urine specific gravity (>1.015), normal urinary sediment, and low urinary sodium (< 20 mEq/L; fractional excretion of sodium < 1%) are seen Ref Harrison20th edition pg 267