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
Chorionic villous sampling is useful in all the following except 
Select and explain. 
A. Tay-Sachs disease, B. Spina bifida, C. Thalassanemia, D. Down syndrome
B: Spina bifida
Answer: B: Spina bifida 
Explanation: 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