A 5-year-old girl is brought to the clinic for recurrent febrile urinary tract infections. Her mother reports intermittent daytime urinary dribbling and that the child “always seems to have accidents” despite being toilet trained. Physical exam is unremarkable. Urinalysis shows positive leukocyte esterase and nitrites. Renal ultrasound suggests duplication of the collecting system on the right, and a voiding cystourethrogram demonstrates vesicoureteral reflux into one right-sided ureter. Which of the following embryologic events most likely produced this patient’s anatomic abnormality?