A 14-month-old girl presents with a febrile urinary tract infection (E. coli, >100,000 CFU/mL). Renal-bladder ultrasound reveals a left duplex collecting system with hydronephrosis of the upper pole moiety and a thin-walled cystic intravesical mass at the left ureteral orifice that varies in size with bladder filling. VCUG shows ipsilateral lower pole vesicoureteric reflux (Grade III) and a filling defect at the ureterocele. The structure marked **A** in the diagram is the cystic mass at the bladder trigone. Which of the following best describes the embryological defect responsible for this lesion?
A. Failure of the ureteric bud to separate from the mesonephric duct
B. Incomplete dissolution of the Chwalla membrane during development of the distal ureter
C. Abnormal migration of the mesonephric duct during weeks 5–6 of gestation
D. Persistence of the common mesonephric-ureteric duct beyond week 12
Explanation
Why incomplete dissolution of the Chwalla membrane is right
Ureteroceles are cystic dilations of the submucosal intravesical distal ureter that result from incomplete dissolution of the Chwalla membrane during embryogenesis. The Chwalla membrane is the tissue that normally separates the ureteric orifice from the bladder mucosa during development. When this membrane fails to dissolve completely, the distal ureter becomes trapped in a cystic, thin-walled configuration at the bladder trigone—exactly as seen in this patient's imaging. This is the pathognomonic embryological defect underlying ureterocele formation and is the foundation of the diagnosis (Campbell-Walsh-Wein Urology 12e; AUA Pediatric Urology Guidelines).
Why each distractor is wrong
Failure of the ureteric bud to separate from the mesonephric duct: This would result in a single system with ectopic ureter, not a duplex system with ureterocele. This patient has a duplex collecting system, which requires two separate ureteric buds.
Abnormal migration of the mesonephric duct during weeks 5–6: This describes the embryological basis of lateral ectopia and renal dysplasia, not the specific defect in ureterocele formation. The Weigert-Meyer rule (medial and inferior insertion of the upper pole ureter) is a consequence of duplex anatomy, not the cause of the ureterocele itself.
Persistence of the common mesonephric-ureteric duct beyond week 12: This would result in a single system with a common channel, not the cystic intravesical mass seen here. Ureteroceles occur in both single and duplex systems, but the cystic appearance is always due to Chwalla membrane pathology.
High-YieldNEET PG
Ureterocele = incomplete Chwalla membrane dissolution → thin-walled cystic intravesical distal ureter; ectopic ureteroceles are almost always associated with the upper pole moiety of a duplex kidney in pediatric females.
