Mild hydronephrosis detected prenatally is most likely caused by which mechanism?

• A. Vesicoureteral reflux
• B. UPJ obstruction
• C. Delay in maturation of the ureters
• D. Wilms tumor

Answer: (C)

Mild hydronephrosis detected before birth is most often due to delayed maturation of the urinary tract, a transient physiologic immaturity that allows temporary dilation of the renal collecting system as the fetus develops. This maturation lag means urine can pool slightly in the pelvis and calyces without a fixed blockage or reflux, and many cases resolve after birth as the ureters and vesicoureteral junction finish maturing.

Vesicoureteral reflux can cause hydronephrosis but is more likely to present after birth with urinary tract infections and signs of reflux, not as a common isolated prenatal finding. Ureteropelvic junction obstruction is a real obstruction that tends to produce more persistent or more pronounced dilation, sometimes with postnatal symptoms. Wilms tumor is a pediatric renal tumor presenting as a mass later in childhood, not as a cause of prenatal hydronephrosis.