A 65-year-old woman presents with a 4-month history of progressively worsening fecal incontinence. The patient initially noted the intermittent uncontrolled passage of gas and liquid stools. However, she now reports daily uncontrolled passage of solid stools. She notes that this has been debilitating, preventing her from interacting with friends and family during her usual activities. The patient reports three vaginal births, with a grade 3 perineal tear occurring during her last delivery requiring repair. The best modality to evaluate her fecal incontinence is