Prerenal AKI is best defined as which of the following?

• A. AKI due to systemic infection with glomerular injury.
• B. AKI due to intrinsic renal parenchymal damage.
• C. AKI due to decreased renal perfusion without intrinsic kidney damage.
• D. AKI caused by obstruction of urinary outflow.

Answer: (C)

Prerenal AKI is caused by decreased blood flow to the kidneys without the kidney tissue itself being damaged. When perfusion drops, the glomerular filtration rate falls even though the renal parenchyma remains largely intact. If perfusion is restored promptly, kidney function can recover because there’s no intrinsic injury to the tubules or other structures.

This definition fits best because the problem lies in reduced renal perfusion, not in intrinsic kidney damage (like tubular injury or glomerular disease) or in urinary obstruction. Clinically, prerenal AKI often shows signs of volume depletion or low effective circulating volume, and lab patterns reflect reduced flow: a relatively high BUN to creatinine ratio and a low fractional excretion of sodium (<1%), since the kidneys conserve sodium and water to preserve volume. Imaging typically shows no intrinsic obstruction or structural damage to the kidneys.

In contrast, intrinsic renal AKI involves damage to the kidney tissue itself (e.g., tubular injury, glomerulonephritis), and obstruction causes postrenal AKI, both of which are not described by decreased perfusion alone.