Acute Kidney Injury Quiz

See Post-Renal Failure. Option C is correct: in post-renal AKI, an obstruction to urine outflow causes back-pressure that damages the kidneys and stops waste being excreted. The obstruction lies after the kidney, in the ureters, bladder or urethra. It might be a stone, a tumour, an enlarged prostate or a blocked catheter. Urine that cannot drain backs up, so pressure rises in the urinary tract, the collecting system dilates (hydronephrosis) and the pressure is transmitted to the filtering units. This opposes filtration, so the glomerular filtration rate falls and creatinine and urea rise. The mechanism also involves local ischaemia (reduced blood supply) in the kidney. To cause AKI, the obstruction usually has to be bilateral, or affect a single functioning kidney, because one healthy kidney can compensate. The practical point is that ultrasound looks for hydronephrosis, and early relief of the blockage by catheter, stent or nephrostomy (a tube placed through the skin into the kidney to drain it) can recover function. Memory aid: pre-renal means not enough blood in, renal means damage inside, post-renal means urine cannot get out.

• Cause: an obstruction to urine outflow after the kidney, in the ureters, bladder or urethra.
• Mechanism: urine backs up and the pressure is transmitted to the filtering units, so filtration falls and creatinine and urea rise.
• Requirement: the obstruction is usually bilateral or affects a single functioning kidney, because one healthy kidney can compensate.