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': 'Single best prognostic indicator of successful treatment with single-dose Methotrexate is \nSelect and explain. \nA. Gestational sac size, B. Serum b-hCG level, C. Absence of fetal cardiac activity, D. Pain abdomen', 'role': 'user'}, {'content': "Answer: B \nExplanation: The best candidate for medical therapy is the woman who is asymptomatic, motivated, and compliant. With medical therapy, some classic predictors of success include a low initial serum b-hCG level, small ectopic pregnancy size, and absent fetal cardiac activity. Of these, initial serum b-hCG level is the single best prognostic indicator of successful treatment with single-dose MTX. Specifically, repoed failure rates are 1.5 percent if the initial serum b-hCG concentration is <1000 mIU/mLReference: William's Obstetrics; 25th edition; Chapter 19; Ectopic pregnancy", 'role': 'assistant'}]