A 25-year-old man was diagnosed with perianal Crohn’s disease 5 years ago at which time he had developed a complex high transsphincteric fistula in ano. He was initiated on biologic therapy with infliximab and entered endoscopic remission. He has been relatively asymptomatic except for bothersome, persistent, nonpurulent fluid drainage from the fistula site. He has concerns about operative complications such as incontinence. Which of the following treatments has the lowest risk of complications and has proven to be effective in clinical trials for the treatment of nonhealing perianal fistulas from Crohn’s disease?