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': 'Radiation exposure during which period of Pregnancy is highly lethal \nSelect and explain. \nA. During fetal cardiac development, B. Before implantation, C. During closure of proximal neural tube, D. None of the above', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Lethality risk is highest during the pre-implantation period - up to 10 days post conception. This is likely due to blastomere destruction caused by chromosomal damage.', 'role': 'assistant'}]