A 29-year-old woman at 7 weeks gestation presents with vaginal spotting and mild lower abdominal pain. Serum beta-hCG is 2500 IU per litre. Transvaginal ultrasound shows no intrauterine gestational sac and a 2 cm adnexal mass on the right with minimal free fluid. She is haemodynamically stable. What is the most appropriate management?