A 40-year-old woman, gravida 2, para 2 (both vaginal deliveries), initially presents with a transsphincteric anal fistula. She undergoes a ligation of intersphincteric fistula tract (LIFT) procedure. The patient now reports persistent seropurulent drainage from the surgical incision approximately 4 weeks postoperatively. Magnetic resonance imaging of the pelvis reveals an intersphincteric fistula but no associated abscess. Of the following options, which is the best next step in management of this patient’s fistula?