A 32-year-old woman with a history of pelvic inflammatory disease presents with 8 weeks of amenorrhea and right-sided abdominal pain. Transvaginal ultrasound reveals a 2.8 cm gestational sac with a yolk sac in the right fallopian tube. β-hCG is 3,200 mIU/mL. She is hemodynamically stable. Which drug is the first-line pharmacologic treatment for this tubal ectopic pregnancy?