A 28-year-old woman presents to the Emergency Department with sudden onset of severe, sharp, left-sided lower abdominal pain that started 6 hours ago. Her last menstrual period (LMP) was 7 weeks ago. She has had minimal vaginal spotting. Examination reveals localised tenderness in the left iliac fossa, cervical motion tenderness, and left adnexal tenderness. A urine pregnancy test is positive. What is the most critical diagnosis to exclude urgently?