Ovarian torsion

A 32-year-old woman comes to the emergency room with nausea, vomiting, and severe abdominal pain. The pain began 6 hours ago and has become more severe over the last 2 hours. Past medical history includes seasonal depression and placement of a progestin-releasing IUD a month ago. Her last menstrual period was two weeks ago. Temperature is 37.0°C (98.6°F), pulse is 110/min, respirations are 20/min, and blood pressure is 90/75 mmHg. Abdominal examination reveals tenderness in the left lower quadrant. Bimanual examination reveals a tender, palpable left adnexal mass and visible IUD strings at the vaginal orifice. The urine pregnancy test is negative. The total leukocyte count is 8000/mm3. A transabdominal ultrasound reveals a hyperechoic adnexal mass with the decreased arterial flow. An explorative laparotomy is performed. The infundibulopelvic ligament is ligated during the procedure, and the pathological specimen is removed.  Which of the following is the most likely diagnosis?