A 32-year-old man presents with a history of pelvic fracture sustained in a motor vehicle accident 6 months ago. He now complains of urinary retention and a history of gross hematuria immediately after the injury. A voiding cystourethrogram (VCUG) is performed. The structure marked **C** in the diagram is identified as the site of injury with extravasation confined above the urogenital diaphragm, producing a characteristic "drooping lily" appearance of the bladder. Which of the following best explains why the structure marked **C** is the most vulnerable to injury in pelvic trauma?