A 19-year-old woman comes to the physician because of a 5-day history of increasingly severe right lower abdominal pain and bloody vaginal discharge. Her last menstrual period was 8 weeks ago. Abdominal examination shows exquisite tenderness of the right lower quadrant. A serum pregnancy test result is positive. Ultrasonography shows no gestational sac in the uterus. Which of the following is the most likely location of this patient’s fertilized egg?