A 26-year-old woman presents to the emergency department with acute lower abdominal pain and vaginal spotting. Her last menstrual period was 7 weeks ago. Her serum β-hCG is 2 500 IU/L, and transvaginal ultrasound does not show an intrauterine pregnancy but reveals a 2.5 cm adnexal mass with peripheral vascularity. She is haemodynamically stable. What is the most appropriate management?