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?