A 26-year-old woman presents to the Emergency Department with a 12-hour history of worsening right iliac fossa pain and mild vaginal bleeding. Her last menstrual period was 7 weeks ago. On examination, she is tender in the RIF with guarding. Her blood pressure is 105/70 mmHg and heart rate is 95 bpm. A urine pregnancy test is positive. What is the single most important immediate investigation?