This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Neuroanatomy 3, Subject name: Anatomy
[{'content': 'When are the axons of the coicospinal tracts fully myelinated? \nSelect and explain. \nA. In the late embryonic period, B. In the mid-fetal period, C. At bih, D. By the end of the second postnatal year', 'role': 'user'}, {'content': "Answer: D \nExplanation: Axons of the coicospinal tracts are fully myelinated by the end of the second postnatal year *Babinski's sign (extensor plantar reflex) is usually elicitable before myelination of the coicospinal tracts -reflects poor coical control of motor function by the immature brain.", 'role': 'assistant'}]