A 32-year-old primigravida delivers vaginally after a prolonged second stage of labor. Intrapartum, she sustains a 3rd-degree perineal tear with direct trauma to the structure marked **B** in the diagram. Six weeks postpartum, she reports involuntary leakage of stool with straining and inability to defer defecation. Which of the following best describes the mechanism of her fecal incontinence and the nerve injured?

Explanation

## Why "Loss of voluntary continence due to injury to the inferior rectal branch of the pudendal nerve (S2-S4)" is right The structure marked **B** is the external anal sphincter, a skeletal muscle innervated by the inferior rectal branch of the pudendal nerve (S2-S4). This muscle is responsible for VOLUNTARY continence — the ability to consciously defer defecation. During obstetric trauma (3rd- or 4th-degree perineal tears), direct injury to the external sphincter and/or its nerve supply results in loss of voluntary control and fecal incontinence with straining. This is the classic mechanism of post-partum fecal incontinence and is repaired surgically by overlapping anal sphincteroplasty (Gray's Anatomy 42e Ch 65; Bailey & Love 28e). ## Why each distractor is wrong - **Loss of resting anal tone due to injury to the autonomic innervation of the internal anal sphincter**: The internal anal sphincter (smooth muscle, autonomic) maintains 80% of resting anal tone, not the external sphincter. Isolated internal sphincter injury causes seepage at rest, not loss of voluntary control with straining. The clinical presentation here is loss of voluntary continence, not resting incontinence. - **Loss of anorectal angle due to injury to the puborectalis sling**: The puborectalis (part of levator ani) maintains the anorectal angle, which is important for continence mechanics. However, the question explicitly states trauma to structure **B** (external sphincter), not the puborectalis (structure **C**). Puborectalis injury causes different biomechanical dysfunction, not the nerve-mediated loss of voluntary control described. - **Loss of pelvic floor support due to injury to the levator ani complex**: While pelvic floor trauma can occur in severe perineal tears, the direct injury to structure **B** (external sphincter) and its pudendal nerve supply is the primary mechanism of post-partum fecal incontinence in this case. Levator ani injury (structure **D**) contributes to pelvic organ prolapse, not the acute loss of voluntary continence. **High-Yield:** External anal sphincter = skeletal muscle + pudendal nerve (S2-S4) = voluntary continence; obstetric injury → fecal incontinence → overlapping sphincteroplasty. [cite: Gray's Anatomy 42e Ch 65; Bailey & Love 28e]