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
[{'content': "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. \nSelect and explain. \nA. Triple screen test, B. Amniocentesis, C. Chorionic villous biopsy, D. Ultrasonography", 'role': 'user'}, {'content': "Answer: B \nExplanation: 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.", 'role': 'assistant'}]