This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Gynaecology & Obstetrics
[{'content': 'Earliest fetal anomaly to be detected by USG : \nSelect and explain. \nA. Hydrocephalous, B. Anencephaly, C. Achondroplasia, D. Spina bifida', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Anencephaly', 'role': 'assistant'}]