This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Highest concentration of hydroxy proline is seen in- 
Select and explain. 
A. Elastin, B. Fibrous tissue, C. Gelatin, D. Collagen
D: Collagen
Answer: D: Collagen 
Explanation: None