A 29-year-old woman presents with severe lower abdominal pain and vaginal bleeding at 7 weeks gestation. Her last menstrual period was 7 weeks ago. She has left iliac fossa tenderness and cervical motion tenderness. Her serum β-hCG is 2,500 IU/L and a transvaginal ultrasound shows no intrauterine pregnancy. What is the most likely diagnosis?