This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Anatomy
[{'content': 'Primitive streak is initiated and maintained by? \nSelect and explain. \nA. Nodal gene, B. BMP4, C. FGF, D. Brachyury gene', 'role': 'user'}, {'content': "Answer: A \nExplanation: The primitive streak itself is initiated and maintained by expression of Nodal, a member of the transforming growth factor-b (TGF-b) family. Once the streak is formed, Nodal upregulates a number of genes responsible for formation of dorsal and ventral mesoderm and head and tail structures. Another member of the TGF-b family, bone morphogenetic protein 4 (BMP4), is secreted throughout the embryonic disc. Ref: Langman's Medical embryology 10/e, p 52-53.", 'role': 'assistant'}]