This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Introduction to Central Nervous System, Subject name: Physiology
[{'content': 'Myelin sheath is absent in:- \nSelect and explain. \nA. Pre-axon, B. Axons of CNS, C. Optic nerve., D. Motor roots.', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Myelin sheath is absent at following sites:- 1) Nodes of Ranvier 2) Proximal pa of Axon (Pre axon) 3) Near the termination of Axon. 4) At the clefts of Schmidt- Lantermann.', 'role': 'assistant'}]