A 30-year-old woman presents with 6 weeks of amenorrhoea and lower abdominal pain. Serum β-hCG is 2 000 IU/L and transvaginal ultrasound shows no intrauterine gestational sac but a 2 cm adnexal mass with minimal free fluid. She is haemodynamically stable and has no contraindications to medical therapy. Which management is most appropriate?