A 2-week-old male infant born at term is found to have bilateral renal hydronephrosis on postnatal ultrasound performed at day 7 of life. The structure marked **B** in the diagram shows Society for Fetal Urology (SFU) grade 3 hydronephrosis with dilation of the renal pelvis and all calyces without parenchymal thinning. Voiding cystourethrogram (VCUG) reveals bilateral vesicoureteral reflux and a dilated posterior urethra with a characteristic "keyhole" appearance. Which of the following is the most likely underlying diagnosis?
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