A 28-year-old woman presents with 6 weeks of amenorrhea, unilateral lower abdominal pain, and scanty dark vaginal bleeding. Transvaginal ultrasound shows an empty uterine cavity, and the structure marked **A** (hyperechoic adnexal tubal ring) is identified separate from the ipsilateral ovary. Serum β-hCG is 2800 mIU/mL. Which of the following is the MOST appropriate next step in management for this hemodynamically stable patient with no fetal cardiac activity and an ectopic mass measuring 2.8 cm?