This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General obstetrics, Subject name: Gynaecology & Obstetrics
[{'content': 'Most impoant investigation for ectopic pregnancy: \nSelect and explain. \nA. Serial f3-hCG levels, B. Transvaginal USG, C. Progesterone measurement, D. Culdocentesis', 'role': 'user'}, {'content': 'Answer: B \nExplanation: If TVS reveals an intrauterine gestational sac,the diagnosis of ectopic can usually be ruled out .sometimes a psuedosac may be seen in an ectopic due to bleeding produced by separation of a decidual cast.(refer pgno:159 sheila 2 nd edition)', 'role': 'assistant'}]