A 32-year-old woman with a history of pelvic inflammatory disease presents with 8 weeks of amenorrhea, mild lower abdominal pain, and light vaginal spotting. Serum β-hCG is 3,200 mIU/mL. Transvaginal ultrasound shows no intrauterine gestational sac, a 2.5 cm mass in the left fallopian tube, and no free fluid. Vital signs are stable (BP 118/76, HR 82/min). Hemoglobin is 11.8 g/dL. She is counseled about all management options. What is the most appropriate next step?