A 39-year-old woman with a history of long-standing ulcerative colitis (UC) undergoes an elective total proctocolectomy and ileoanal pouch anal anastomosis (IPAA). Two months postoperatively, she reports a persistent low-grade fever, dyspareunia, vaginal air, and mucopurulent rectal drainage. A gastrografin pouch study demonstrates a pouch vaginal fistula. What is the most likely etiology?