A 32-year-old woman with a history of pelvic inflammatory disease presents with 6 weeks amenorrhea and mild right-sided pelvic pain. Urine pregnancy test is positive. Transvaginal ultrasound shows an empty uterus and a 2.8 cm mass in the right adnexa without fetal heart activity. Serum β-hCG is 3,200 IU/L. Hemodynamic parameters are normal (BP 120/80 mmHg, HR 82/min). What is the most appropriate next step in management?