This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Diagnosis of Pregnancy, Subject name: Gynaecology & Obstetrics
Investigation of choice in a pregnant lady at 16 weeks of pregnancy with past H/0 delivering a baby with Downs syndrome: 
Select and explain. 
A. Triple screen test, B. Amniocentesis, C. Chorionic villous sampling, D. USG
B: Amniocentesis
Answer: B: Amniocentesis 
Explanation: Dual marker and Triple marker tests are screening tests and since the patient here already has a past history of Downs syndrome fetus, she is at an increased risk of having an aneuploid fetus again, so it is advisable to undergo a diagnostic procedure rather than a screening test. Fetal chromosomes can be assessed only on amniocentesis and chorionic villous sampling But CVS is best done between 10 to 13 weeks of gestation and amniocentesis is best done between 16 to 18 weeks of pregnancy