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: Medicine
A 35 year old female with a previous history of bih of a child with Down's syndrome, presents with 18 weeks Amenorrhoea. The Investigation of choice rule out Down's syndrome in the present pregnancy is: 
Select and explain. 
A. Triple test, B. Amniocentesis, C. Cordocentesis, D. Chorionic villous biopsy
B: Amniocentesis
Answer: B: Amniocentesis 
Explanation: The Answer is B (Amniocentesis) : At 18 weeks of pregnancy the best confirmatory test available is amniocentesis 'Triple test only gives a risk ratio. For confirmation of diagnosis, amniocentesis has to be done' - Dutta