Which ultrasound finding is the key diagnostic sign for posterior urethral valve obstruction?

• A. Keyhole sign
• B. Double bubble sign
• C. Sunray sign
• D. Apical sign

Answer: (A)

Posterior urethral valve obstruction causes bladder outlet obstruction in male fetuses and neonates. The main ultrasound clue is the keyhole sign, where a markedly distended bladder is seen with dilation of the proximal urethra, creating a cross-sectional silhouette that resembles a keyhole. This appearance directly results from the obstruction and is a classic, highly suggestive indicator of PUVO on fetal imaging, often accompanied by hydronephrosis or hydroureter. Other signs listed point to different conditions: the double bubble sign is classic for duodenal atresia, while the remaining signs are not characteristic features of PUVO. Therefore, the keyhole sign is the best diagnostic clue for posterior urethral valve obstruction.