A 28-year-old woman presents with 7 weeks of amenorrhea, vaginal bleeding, and right-sided abdominal pain. Transvaginal ultrasound shows an empty uterine cavity with a thick decidual reaction, and the structure marked **D** is clearly visualized in the right adnexa. Her serum β-hCG is 3200 mIU/mL, and she is hemodynamically stable with no evidence of free fluid. Which of the following is the most appropriate next step in management?