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
Which of the following is the investigation of choice in a pregnant lady at 18 weeks of pregnancy with past history 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: Answer is B (Amniocentesis): The ideal investigation in a mother with a pregnancy at risk (previous child with Down's) at 18 weeks gestation is amniocentesis.