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
[{'content': 'Teiary stem villi consists of:- \nSelect and explain. \nA. 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', 'role': 'user'}, {'content': 'Answer: D \nExplanation: 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.', 'role': 'assistant'}]