A 28-year-old woman presents to the emergency department with complaints of lower abdominal pain and vaginal spotting for the past three days. She describes the pain as sharp and intermittent, primarily on the right side, and denies any fever or chills. She reports a positive home pregnancy test taken two weeks ago but has not yet seen a healthcare provider. Her last menstrual period was six weeks ago. She denies significant medical history, prior surgeries, or sexually transmitted infections but mentions occasional irregular menstrual cycles. On physical examination, the patient appears mildly distressed due to pain. Her vital signs reveal a blood pressure of 110/70 mmHg, a pulse of 95 beats per minute, and no fever. Abdominal examination shows tenderness in the right lower quadrant without guarding or rebound tenderness. Pelvic examination reveals mild cervical motion tenderness and tenderness in the right adnexa, with no palpable masses. Speculum examination shows a small amount of dark blood in the vaginal vault. A urine pregnancy test performed in the emergency department is positive.You suspect ectopic pregnancy. Which of the following is the best initial diagnostic step for suspected ectopic pregnancy?
