Ectopic Pregnancy (Tubal) with Pseudosac

A 28-year-old G2P1 woman presents with right lower quadrant pain and light vaginal spotting at 7 weeks amenorrhea. She has a history of chlamydial pelvic inflammatory disease. Serum β-hCG is 4,200 mIU/mL. Transvaginal ultrasound shows an empty uterus with a thin pseudosac, a 2.5 cm right adnexal heterogeneous mass, and free fluid in the cul-de-sac. The structure marked **B** in the diagram—the right adnexal tubal ring—is identified as the site of pregnancy implantation. Which of the following is the most appropriate IMMEDIATE management for this patient?