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
Prenatal diagnosis at 16 weeks of pregnancy can be performed using which of the following: 
Select and explain. 
A. Amniotic fluid, B. Maternal blood AFP, C. Chorionic villi, D. Fetal blood
A: Amniotic fluid
Answer: A: Amniotic fluid 
Explanation: Amniocentesis is done during 16th week of pregnancy. Fetal blood sampling is done after 17-18 weeks, chorionic villi during 11-12 weeks of pregnancy, maternal blood AFP during 15-20 weeks. Ref: Textbook of Gynaecology By Dutta, 6th Edition, Page 108; In-Vitro Feilization By Kay Elder, Brian Dale, Yves Menezo, Joyce Harper, John Huntriss, 2010, Page 243; Donald School Textbook of Ultrasound in Obstetrics and Gynecology By Asim Kurjak, 2008, Page 755.