In male fetuses, a membrane in the posterior urethra causing urinary obstruction with a keyhole sign is characteristic of which condition?

• A. Posterior urethral valve obstruction
• B. Ureteropelvic junction obstruction
• C. Ureterovesical junction obstruction
• D. Gastroschisis

Answer: (A)

In this scenario, the main concept is fetal bladder outlet obstruction caused by posterior urethral valves. In male fetuses, a membrane in the posterior urethra can block urine outflow, leading to a distended bladder and dilation of the posterior urethra. On prenatal ultrasound, this creates the characteristic keyhole sign—a large, full bladder with an elongated posterior urethra forming a distinctive silhouette.

This sign is most specific for posterior urethral valve obstruction because it directly reflects the outflow blockage at the level of the posterior urethra. Other obstructions, like a UPJ obstruction or a UVJ obstruction, tend to cause hydronephrosis and hydroureter without the bladder outlet obstruction pattern that produces the keyhole appearance. Gastroschisis is an abdominal wall defect and does not relate to urinary tract obstruction or the keyhole sign.

So, the finding described points to posterior urethral valve obstruction.