A 28-year-old woman presents to the emergency department with severe lower abdominal pain and vaginal bleeding. She is 8 weeks pregnant by last menstrual period. She reports no fever or chills. On examination, her blood pressure is 100/60 mmHg, heart rate is 110 bpm, and she appears pale and diaphoretic. Abdominal examination reveals tenderness in the lower quadrants without rebound tenderness or guarding. A pelvic examination shows a closed cervical os with moderate bleeding. A bedside transvaginal ultrasound reveals an empty uterus. Which of the following is the most appropriate next step in management?