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
When are the axons of the coicospinal tracts fully myelinated? 
Select and explain. 
A. In the late embryonic period, B. In the mid-fetal period, C. At bih, D. By the end of the second postnatal year
D: By the end of the second postnatal year
Answer: D: By the end of the second postnatal year 
Explanation: 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.