In an elderly patient with postrenal azotemia, what is the most likely cause?

In an elderly patient with postrenal azotemia, what is the most likely cause?

• A. Renal artery stenosis
• B. NSAID use
• C. Dehydration
• D. Bladder outlet obstruction

Answer: (D)

Postrenal azotemia occurs when urine flow is blocked downstream of the kidneys, causing backpressure that reduces glomerular filtration and leads to rising nitrogenous wastes. In an elderly man, bladder outlet obstruction from an enlarged prostate (benign prostatic hyperplasia) is the most common cause, because it directly blocks the outflow of urine, leading to urinary retention, distended bladder, and eventually impaired kidney function if the obstruction persists. This scenario fits well with the elderly patient who develops postrenal azotemia, since an enlarged prostate commonly narrows the urethral channel and creates an obstructive uropathy. The other possibilities don’t fit as well because they cause problems with kidney function without an obstruction behind the kidneys. Renal artery stenosis reduces renal perfusion and tends toward prerenal or renovascular patterns. NSAID use can impair kidney function through afferent arteriolar constriction or nephrotoxic effects, not a downstream blockage. Dehydration lowers effective circulating volume and causes prerenal azotemia rather than an obstruction of urine outflow.

Postrenal azotemia occurs when urine flow is blocked downstream of the kidneys, causing backpressure that reduces glomerular filtration and leads to rising nitrogenous wastes. In an elderly man, bladder outlet obstruction from an enlarged prostate (benign prostatic hyperplasia) is the most common cause, because it directly blocks the outflow of urine, leading to urinary retention, distended bladder, and eventually impaired kidney function if the obstruction persists. This scenario fits well with the elderly patient who develops postrenal azotemia, since an enlarged prostate commonly narrows the urethral channel and creates an obstructive uropathy.

The other possibilities don’t fit as well because they cause problems with kidney function without an obstruction behind the kidneys. Renal artery stenosis reduces renal perfusion and tends toward prerenal or renovascular patterns. NSAID use can impair kidney function through afferent arteriolar constriction or nephrotoxic effects, not a downstream blockage. Dehydration lowers effective circulating volume and causes prerenal azotemia rather than an obstruction of urine outflow.