Physical examination of a baby boy shortly after birth reveals a large bladder and palpable kidneys. The nurses note that he produces a weak urinary stream. A voiding cystourethrogram is shown below. He appears to be otherwise normal. Which of the following is the most likely diagnosis?

(b) Source: (Hay et al, pp 678-679. Kliegman, pp 1845-1846. McMillan, pp 1824-1826. Rudolph, p 1743.) The findings described point to posterior urethral valves; the radiograph shows an area of obstruction and proximal dilatation of the bladder. The clinical picture of posterior valves ranges from that described in the question to severe renal obstruction with renal failure and pulmonary hypoplasia. UTIs are common complications in older children, and sepsis occasionally can be the presenting sign in afflicted newborns. Despite early recognition and correction of the obstruction, the prognosis in these children for normal renal function is guarded. Prenatal diagnosis is often accomplished because of widespread use of ultrasound. Ureteropelvic junction obstruction, a common urinary obstruction in children that results in isolated hydronephrosis, can be found either on a prenatal ultrasound or after birth when the child is noted to have a mass, pain, or infection. Serial ultrasounds of the kidney assist in following the associated hydronephrosis. Prune belly syndrome occurs almost exclusively in males; it is diagnosed in a child with absent abdominal muscular tone, undescended testes, and urinary tract anomalies including hydronephrosis and dilated ureters and bladder. Duplication of the collecting system is often asymptomatic, but can present with symptoms of obstruction and reflux. Horseshoe kidney is often an asymptomatic condition, and results when both kidneys are joined at the lower pole by a fibrous band of tissue or by renal parenchyma; symptoms may arise resulting from obstruction and/or reflux.