A 24-year-old woman reports feculent vaginal discharge 8 months after spontaneous vaginal delivery complicated by a 4th-degree perineal laceration, which was previously repaired. Physical examination reveals a 3-mm rectovaginal fistula 2 cm proximal to the anal verge with significant thinning of the perineal body and a palpable sphincter defect anteriorly. Endoanal ultrasound confirms a 90° defect of the external sphincter at the anterior midline. The best next step in management is