Which imaging finding most strongly supports post-renal obstruction due to bladder outlet obstruction?

• A. Ureteral stone with no dilation
• B. Hydronephrosis
• C. Normal ultrasound
• D. Decreased renal size without dilation

Answer: (B)

Post-renal obstruction causes urine to back up into the kidneys, so the most telling imaging sign is hydronephrosis—dilation of the renal pelvis and calyces. This reflects urine accumulating upstream of the obstruction, which fits bladder outlet obstruction where drainage is impaired at the level of the bladder or urethra.

A ureteral stone with no dilation is inconsistent with obstruction, since obstruction usually produces proximal dilation. A normal ultrasound argues against obstruction entirely, and decreased renal size without dilation suggests chronic kidney disease rather than an active obstructive process. Hydronephrosis, in contrast, directly signals back pressure from impaired outflow.