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': 'Modern diagnostic aid to diagnose ectopic pregnancy: \nSelect and explain. \nA. hCG, B. Transvaginal USG, C. AFP, D. Gravindex', 'role': 'user'}, {'content': 'Answer: A \nExplanation: When diagnosis is in doubt a serum beta hcg is done .The advent of radioimmunoassay has allowed for very accurate quantification and is able to detect 1 IU /L .(refer pgno:159 sheila 2 nd edition)', 'role': 'assistant'}]