This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Fertilization and Development of Embryo, Subject name: Anatomy
Primitive streak is initiated and maintained by 
Select and explain. 
A. Nodal gene, B. BMP4, C. FGF, D. Brachyury gene
A: Nodal gene
Answer: A: Nodal gene 
Explanation: Ans. is 'a' i.e. Nodal gene "The primitive streak in itself is initiated and maintained by expression of Nodal, a member of the transforming growth factor b (TGFb) family." - Longman's Medical embryology 10/e, p 57