A male neonate presents with poor urinary stream and bilateral palpable kidneys. Ultrasound shows bilateral hydronephrosis with a dilated posterior urethra and a thick-walled bladder. Which finding on micturating cystourethrography confirms the diagnosis?

• A Ectopic ureter inserting below the bladder neck
• B Ureterocele filling defect at the bladder base
• C Keyhole sign with dilated posterior urethra ✓
• D Bladder diverticulum at the trigone

Explanation

Posterior urethral valves are the commonest cause of lower urinary tract obstruction in male infants. The classical MCU appearance is the keyhole sign, produced by dilation of the prostatic urethra proximal to the valve with a distended bladder. Treatment begins with catheter drainage followed by endoscopic valve ablation. An ectopic ureter in a boy inserts above the sphincter, and a ureterocele produces a round filling defect rather than a dilated posterior urethra, excluding these distractors.

Reference: Campbell-Walsh Urology, 12th ed.

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Written and medically reviewed by the StethoPrep medical team.