A 52-year-old woman presents with a 3-year history of chronic constipation, straining at stool, and sensation of incomplete evacuation despite normal colonoscopy. Defecography shows failure of straightening of the anorectal angle during straining. The structure marked **B** in the diagram fails to relax appropriately during defecation. Which of the following best describes the pathophysiology of her obstructed defecation?

Explanation

## Why "Paradoxical contraction of puborectalis (anismus) preventing straightening of the anorectal angle during defecation" is right The structure marked **B** is the puborectalis, which forms a U-shaped muscular sling around the anorectal junction. At rest, it maintains an anorectal angle of approximately 90°, which is essential for fecal continence. During normal defecation, the puborectalis must relax, allowing the anorectal angle to straighten (become more obtuse) to permit passage of stool. When the puborectalis fails to relax and paradoxically contracts during straining—a condition called anismus—the anorectal angle remains acute, mechanically obstructing stool passage. This is the pathophysiology of obstructed defecation in this patient, as evidenced by defecography showing failure of angle straightening. (Gray's Anatomy 42e Ch 77; Williams Gynecology 4e) ## Why each distractor is wrong - **Loss of elasticity of the external anal sphincter with impaired accommodation of rectal contents**: The external anal sphincter is a striated muscle under voluntary control; loss of elasticity is not the primary mechanism of anismus. Anismus is a functional disorder of puborectalis relaxation, not sphincter elasticity. - **Decreased contractility of the rectosigmoid smooth muscle reducing propulsive peristalsis**: Rectosigmoid contractility is not the primary issue in anismus. The defecography finding of failure of anorectal angle straightening specifically implicates the puborectalis, not the smooth muscle of the rectum itself. - **Excessive relaxation of the internal anal sphincter causing fecal urgency and incomplete evacuation**: Excessive internal sphincter relaxation would cause incontinence and urgency, not obstructed defecation with straining. The clinical presentation and defecography findings point to failure of relaxation, not excessive relaxation. **High-Yield:** Anismus = paradoxical puborectalis contraction during defecation → acute anorectal angle → obstructed defecation; defecography is diagnostic; biofeedback and pelvic floor PT are first-line treatment. [cite:Gray's Anatomy 42e Ch 77; Williams Gynecology 4e]