This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Embryology and Histology, Subject name: Anatomy
Teiary stem villi consists of:- 
Select and explain. 
A. Central core of cytotrophoblast covered with synctiotrophoblast, B. Central core of extra-embryonic mesoderm covered by central cyto- and peripheral synctio- trophoblast, C. Central maternal blood vessels surrounded by mesoderm and trophoblast, D. Central fetal blood vessels surrounded by mesoderm and trophoblast
D: Central fetal blood vessels surrounded by mesoderm and trophoblast
Answer: D: Central fetal blood vessels surrounded by mesoderm and trophoblast 
Explanation: Secondary villi:- contains a central core of primary (extra embryonic) mesoderm and is covered by cyto- and syncytio- trophoblast cells. Teiary villicontains fetal blood vessels invading the extra embryonic mesoderm and is covered by trophoblastic cells.