A 24-year-old woman comes to the office for evaluation of abno | Obstetrics & Gynecology

A 24-year-old woman comes to the office for evaluation of abnormal vaginal bleeding. The patient has a history of heavy, regular menstrual periods for which a progestin-releasing intrauterine device (IUD) was placed 2 years ago. After IUD insertion, the patient became amenorrheic. However, for the last month, she has had light vaginal spotting with wiping. The patient also recently developed intermittent pelvic pain that has now become constant and unrelieved with ibuprofen. She has no chronic medical conditions and does not use tobacco, alcohol, or illicit drugs. Family history is unremarkable. On bimanual pelvic examination, there is uterine and bilateral adnexal tenderness. The IUD strings are palpable at the cervix. Urine pregnancy test is negative. Which of the following is the most likely cause of this patient's symptoms?