This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Chemistry of Lipids, Subject name: Biochemistry
All of the following are Isoprene derivates (Polyprenoids), except ? 
Select and explain. 
A. Ubiquitin, B. Dolichol, C. b-carotene, D. a-tocopherol
A: Ubiquitin
Answer: A: Ubiquitin 
Explanation: Polyprenoids are a class of compounds synthesized from 5 carbon isoprene units. They include: Ubiquinone - electron carrier in respiratory chain Dolichol - long-chain alcohol involved in N-glycosylation Plant-derived Polyprenoids - rubber, camphor, the fat-soluble vitamins A, D, E, and K, and b-carotene (provitamin A). Ubiquitin is not a pa of polyprenoids. It is a protein (polypeptide).