A 29-year-old woman has 6 weeks of amenorrhoea and two days of increasing right iliac fossa pain. Transvaginal ultrasonography shows a 30-mm right tubal gestation without cardiac activity; serum human chorionic gonadotrophin is 2400 IU/L and has risen by 35% in 48 hours. She is stable, has normal liver function, but her estimated glomerular filtration rate is 28 mL/min/1.73 m² (greater than 60). The opposite fallopian tube appears normal. Which one of the following is the most appropriate treatment?