This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Physiology
[{'content': 'Chorionic villous sampling is useful in all the following except \nSelect and explain. \nA. Tay-Sachs disease, B. Spina bifida, C. Thalassanemia, D. Down syndrome', 'role': 'user'}, {'content': 'Answer: B \nExplanation: Chorionic villous sampling (CVS) is generally performed at 10 to 13 weeks. 1. Second-trimester amniocentesis - for genetic diagnosis is usually performed between 15 and 20 weeks. 2. Early amniocentesis - this is performed between 11 and 14 weeks CVS for trophoblastic cells (derived embryologically from the same feilized eggs as the fetus is usually done during gestational weeks 11-13 The advantage of CVS is that the results are available early in pregnancy', 'role': 'assistant'}]