An infant is noted to have a left flank mass shortly after birth and an ultrasound examination demonstrates left hydronephrosis. The most common cause of this finding is which of the following?

b. Congenital ureteropelvic junction obstruction

The most common cause of neonatal hydronephrosis is congenital ureteropelvic junction obstruction. It is important to rule out bilateral ureteropelvic junction obstruction when this is found as this can present in 20% to 30% of neonatal cases. The management requires establishment of dependent drainage of the renal pelvis, and this is usually accomplished by a dismembered pyeloplasty with resection of the obstructing segment. A successful outcome is expected in over 90% of patients.

Neonatal Wilm’s tumor is exceedingly rare and does not ordinarily present with obstruction of a functioning kidney, but rather as a solid mass. Multicystic dysplastic kidney occurs in about 1 in 4000 births and is usually unilateral. It is the most common form of renal cystic disease in infants, but these cysts do not communicate with a functional renal pelvis and there is little or no functional renal cortex. Vesicoureteral reflux, while common, does not present in the newborn period with hydronephrosis and a palpable kidney.