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?

Explanation

## Why option 1 is right The membranous urethra (structure **C**) is the shortest (~1.5 cm) and narrowest segment of the male urethra, surrounded by the external urethral sphincter (skeletal muscle responsible for voluntary continence). Critically, it passes through the urogenital diaphragm with minimal mobility and is therefore fixed in position. During pelvic fracture, the bladder is displaced upward by hematoma, and the immobile membranous urethra is sheared at the level of the urogenital diaphragm. This results in extravasation of urine above the urogenital diaphragm (Type I–V Goldman injury), producing the characteristic "drooping lily" sign on VCUG — a superiorly displaced, teardrop-shaped bladder outline. This anatomical vulnerability is the reason membranous urethral strictures are a common sequela of pelvic fracture. ## Why each distractor is wrong - **Option 2**: The prostatic urethra (structure **B**), not the membranous urethra, is the widest segment (~3 cm). The membranous urethra is the narrowest, and it is well-supported by the external sphincter and urogenital diaphragm — its vulnerability lies in its fixed position and narrow caliber, not lack of support. - **Option 3**: This describes the bulbar urethra (structure **D**), which is the common site of straddle injury (fall onto bicycle crossbar), not pelvic fracture. Bulbar injury results in extravasation *below* the urogenital diaphragm into the superficial perineal pouch, producing a "butterfly hematoma," not the drooping lily sign. - **Option 4**: The navicular fossa is the terminal dilated segment within the glans, not the membranous urethra. It is not typically injured in pelvic fracture and does not produce the drooping lily sign. **High-Yield:** Membranous urethra = narrowest, most fixed segment → vulnerable in pelvic fracture (drooping lily); bulbar urethra = vulnerable in straddle injury (butterfly hematoma). [cite: Sutton Radiology 7e Ch 32; Bailey & Love 28e Ch 76]