A 31-year-old woman comes to the emergency depament because of abdominal pain and vaginal spotting. She states that the pain began 2 days ago and has been worsening since. The spotting occurred this morning. Her last menstrual period was 6 weeks ago. She has no medical problems. She takes no medications and has no known drug allergies. Her temperature is 37 C (98.6 F), blood pressure is 90/50 mm Hg, pulse is 110/minute, and respirations are 14/minute. Abdominal examination reveals significant lower abdominal tenderness. Speculum examination shows scant blood in the vagina with a closed cervical os. Bimanual examination demonstrates significant left adnexal tenderness. Urine hCG is positive. Serum hCG is 5,000 mIU/mL. Pelvic ultrasound shows a normal uterus with a left adnexal mass surrounded by free fluid. In MOST cases, the disease process of this patient affects which of the following anatomic structures?