Acute kidney injury is classified into prerenal, intrinsic (renal), and postrenal categories. Which of the following is an example of postrenal AKI?

• A. Dehydration
• B. Acute tubular necrosis
• C. Urinary tract obstruction
• D. Glomerulonephritis

Answer: (C)

Postrenal AKI occurs when urine flow is blocked downstream of the kidneys. That blockage increases pressure in the urinary tract, reduces the filtration gradient in the glomerulus, and lowers the glomerular filtration rate. Obstruction can be due to stones, clots, tumors, or strictures anywhere along the ureters, bladder outlet, or urethra. An example of this scenario is urinary tract obstruction, which directly fits the definition of postrenal causes. By contrast, dehydration lowers blood volume and renal perfusion (prerenal), acute tubular necrosis is injury to the renal tubules (intrinsic), and glomerulonephritis affects the glomeruli (intrinsic).