Postrenal AKI results from which mechanism?

• A. Obstruction of urinary outflow
• B. Decreased renal perfusion
• C. Direct renal injury
• D. Glomerular immune complex deposition

Answer: (A)

Postrenal AKI occurs when urine flow is blocked downstream from the kidneys. This obstruction raises pressure in the urinary tract, which backs up into the renal tubules and Bowman's space. The back-pressure lowers the net filtration pressure across the glomerulus, reducing the glomerular filtration rate and causing a rise in creatinine with decreased urine output. If the obstruction is relieved promptly, renal function can recover, which is a hallmark of the postrenal pattern.

This differs from prerenal AKI, where the problem is reduced renal perfusion rather than an obstruction; from intrinsic renal injury, where the damage is inside the kidney itself (such as tubular injury or interstitial nephritis); and from glomerular diseases like immune complex–mediated glomerulonephritis, where the issue is within the glomeruli, not due to obstructed urine flow.