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
Which of the following is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with pasthistory of delivering a baby with Down's syndrome: 
Select and explain. 
A. Triple screen test, B. Amniocentesis, C. Chorionic villous biopsy, D. Ultrasonography
B: Amniocentesis
Answer: B: Amniocentesis 
Explanation: None