Posterior urethral valves (PUV) in a male neonate produce bladder outlet obstruction. Which antenatal ultrasound finding most specifically suggests this diagnosis?
- A Bilateral hydronephrosis with oligohydramnios
- B Keyhole sign — dilated posterior urethra with distended bladder ✓
- C Unilateral renal agenesis
- D Echogenic kidneys with cysts
Correct answer: B. Keyhole sign — dilated posterior urethra with distended bladder
Explanation
The keyhole sign on fetal ultrasound — a dilated posterior urethra continuous with a distended bladder — is the pathognomonic antenatal finding of posterior urethral valves. It represents incomplete drainage of urine through the obstructed proximal urethra. Bilateral hydronephrosis and oligohydramnios are secondary consequences (from reduced fetal urine production/drainage) that can occur in other causes of bilateral obstruction. Echogenic kidneys with cysts suggest renal dysplasia, often a consequence of longstanding obstructive uropathy.
Reference: Bailey & Love's Short Practice of Surgery, 27th ed.
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