A 19-year-old student presents to the university health service with lower abdominal pain and a painful swollen right knee. She denies any trauma to the knee. Pelvic examination is exquisitely painful and reveals an ill-defined thickening in the right and left adnexae. A vaginal discharge is noted. The patient is febrile (38.7degC/103degF). Examination of her right knee reveals an enlarged, tender, and warm joint. The WBC count is 18,500/mL (normal =4,000 to 11,000/mL). If untreated, which of the following would be the most likely complication in this patient?