This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Intra Uterine Growth Restriction, Intrapaum and Antepaum Fetal Surviellance, Subject name: Gynaecology & Obstetrics
Which of the following is the investigation of choice in a pregnant lady at 17 weeks of pregnancy, with past history of delivering a baby with Down syndrome? 
Select and explain. 
A. Triple screen test, B. Amniocentesis, C. Chorionic villus biopsy, D. Ultrasonography
B: Amniocentesis
Answer: B: Amniocentesis 
Explanation: Because there is a past history of Down syndrome, a confirmatory test should be done. Amniocentesis and karyotyping is the best choice here. It is generally done around 16-18 weeks and gives confirmatory results. The triple marker is only a screening test and not a confirmatory test. USG can pick up soft tissue markers of Down syndrome, but it is not confirmatory and USG can be normal in a fetus with Down syndrome. If the same patient presents at 11-12 weeks, then the answer is CVS and karyotyping.