A 58-year-old male bricklayer is admitted to the hospital with severe pain that radiates from his lower back to the pubic region. Ultrasound examination reveals that a kidney stone is partially obstructing his right ureter; the examination also indicates the presence of a second ureter on the right side. Which of the following is the most likely cause of this latter finding?

The ureteric bud is responsible for the development of the ureter, and thus an early splitting of the ureteric bud would result in formation of a second ureter on the ipsilateral side. Failure of the ureteric bud to form would cause a complete absence of the ureter, whereas failure of the ureteric bud to branch occurs normally during embryonic development and results in one ureter joined to each kidney. The urorectal septum is a section of tissue of mesenchymal origin that develops between the allantois and hind- gut. Failure of this structure to develop would not result in an additional ureter. Finally, a persistent urachus acts as an abnormal fistula that runs from the bladder to the umbilicus, resulting in urine leaking from the external abnormal wall.