A 29-year-old woman presents to the emergency department with lower abdominal pain and vaginal spotting. Her last menstrual period was 7 weeks ago, and she has a positive pregnancy test. Transvaginal ultrasound shows no intrauterine gestational sac and a small adnexal mass on the right. β-hCG is 2,500 IU/L. She is haemodynamically stable. What is the most likely diagnosis?