A 32-year-old woman with a history of pelvic inflammatory disease presents with 7 weeks of amenorrhea, mild lower abdominal pain, and light vaginal spotting. Serum β-hCG is 3,200 mIU/mL. Transvaginal ultrasound reveals an empty uterus, no free fluid, and no adnexal mass visualized. She is hemodynamically stable with a normal abdominal examination. What is the most appropriate next step in management?