This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
Karyotyping of a fetus is done routinely by all the following except 
Select and explain. 
A. Chorionic villus sampling, B. Cordocentesis, C. Amniocentesis, D. Fetal skin biopsy
D: Fetal skin biopsy
Answer: D: Fetal skin biopsy 
Explanation: Karyotyping: the Visual study of chromosomes after metaphase arrest by the use of ColchicineFetal karyotyping :1)Chorionic villus sampling: Biopsy taken transcervically at 8 to 12 weeks of gestation2)Amniocentesis: 20 ml of amniotic fluid is aspirated at 14 to 20 weeks of gestation3)Cordocentesis (not preferable): Fetal blood aspirated from the umbilical cord after 18weeks of gestation(Ref: Kawthalkar's essentials of hematology 2/e p162)