This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of lipid, Subject name: Biochemistry
Hydroxyproline and hydroxylysine occur principally in 
Select and explain. 
A. Collagen, B. Elastin, C. Synovium, D. Hyaline
A: Collagen
Answer: A: Collagen 
Explanation: Hydroxyproline and hydroxylysine occur principally in collagen. Since there is no tRNA for either hydroxylated amino acid, neither dietary hydroxyproline nor dietary hydroxylysine is incorporated during protein synthesis. Ref: Harper&;s biochemistry; 30th edition; Chapter 27 Biosynthesis of the Nutritionally Nonessential Amino Acids