A 28-year-old G1P0 woman at 22 weeks gestation is referred for abnormal fetal ultrasound. The amniotic fluid index is low. Ultrasound demonstrates bilateral hydronephrosis and a markedly distended bladder with a thickened wall and a characteristic "keyhole" appearance of the bladder/urethra. The fetus is male. Which of the following embryologic abnormalities most likely underlies this condition?