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: Biochemistry
The amino acid, which is used in the estimation of collagen is: 
Select and explain. 
A. Hydroxyproline, B. Proline, C. Lysine, D. Glycine
A: Hydroxyproline
Answer: A: Hydroxyproline 
Explanation: None