<prompt> Chorionic villous sampling is done in all exceptPhenylketonuriaDowns syndromeNeural tube defectThalassemiasickle cell anemia  <response>: Chorionic villi are extremely good source of fetal DNA and thus ideal for PCR and DNA analysis to diagnose single gene defects like hemoglobinopathies,cysticfibrosis,duchenne muscular dystrophy.Represents confined placental mosaicism.It can be used to diagnose storage diseases like Tay Sachs and Gaucher;s disease. TEXTBOOK OF OBSTETRICS,SHEILA BALAKRISHNAN,Pg no:503,2nd edition 