A 32-year-old woman from rural India presents with vaginal bleeding and a uterine size larger than expected for gestational age at 12 weeks. Ultrasound shows a heterogeneous mass with multiple small cystic spaces ('bunch of grapes' appearance) and no fetal parts. β-hCG is 250,000 mIU/mL. She has no history of prior molar pregnancy or chemotherapy. What is the most appropriate immediate next step?