A 28-year-old woman presents to the emergency department with lower abdominal pain and light vaginal bleeding. Her last menstrual period was 7 weeks ago. Serum beta-hCG is 2000 IU/L. Transvaginal ultrasound does not show an intrauterine gestational sac but reveals a 1.5 cm adnexal mass. She is haemodynamically stable. According to Australian early pregnancy guidelines, what is the most appropriate management?