Prerenal azotemia is caused by

• A. Acute renal failure
• B. Chronic renal failure
• C. Congestive heart failure
• D. Urinary tract obstruction

Answer: (C)

Prerenal azotemia arises when kidney perfusion falls, so the glomerular filtration rate drops even though the kidney tissue itself is not damaged. In congestive heart failure, the heart can’t pump effectively, which lowers the effective arterial blood volume reaching the kidneys. That reduced flow triggers the kidney’s preservation responses (like activating the renin-angiotensin system and antidiuretic hormone) to maintain perfusion, leading to azotemia with preserved tubular function. This distinguishes it from intrinsic kidney disease or postrenal obstruction, where the problem lies inside the kidney tissue or with urinary flow, respectively. Acute or chronic renal failure and urinary tract obstruction describe problems within the kidney or beyond it, not the decreased perfusion that defines prerenal azotemia.