Which mechanism best describes prerenal AKI?

• A. Direct tubular epithelial injury
• B. Glomerular basement membrane thickening
• C. Decreased renal perfusion leading to reduced GFR without intrinsic injury
• D. Obstruction of urinary outflow

Answer: (C)

Prerenal AKI is driven by reduced blood flow to the kidneys rather than damage to the kidney tissue itself. When perfusion falls, the filtration pressure drops and the kidneys respond by conserving sodium and water and by constricting the efferent arteriole (via angiotensin II) to help maintain GFR. The result is a lower GFR due to less driving pressure, with tubule cells largely intact at this stage. If perfusion is restored promptly, kidney function can recover because there is no intrinsic tubular injury. However, prolonged underperfusion can cause ischemic injury to the tubules, leading to intrinsic AKI.

Direct tubular epithelial injury would reflect damage to the kidney tissue itself. Glomerular basement membrane thickening points to a glomerular disease affecting filtration at the level of the glomerulus rather than perfusion. Obstruction of urinary outflow causes postrenal AKI, where the problem is downstream drainage rather than blood flow to the kidney.