A 32-year-old woman with a history of 2 episodes of pelvic inflammatory disease (treated 18 months and 6 months ago) presents with 2 weeks of mild lower abdominal pain, irregular menses, and infertility for 8 months despite regular unprotected intercourse. On examination, she is afebrile, has mild adnexal tenderness, and no cervical discharge. Pelvic ultrasound shows normal ovaries and uterus with no free fluid. Cervical swabs for gonorrhea and chlamydia PCR are negative. What is the most likely cause of her infertility?