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
Myelin sheath is absent in:- 
Select and explain. 
A. Pre-axon, B. Axons of CNS, C. Optic nerve., D. Motor roots.
A: Pre-axon
Answer: A: Pre-axon 
Explanation: 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.