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?