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
One of the following is not an amino acid: 
Select and explain. 
A. Glycine, B. Hydroxy proline, C. Glutamic acid, D. Choline
D: Choline
Answer: D: Choline 
Explanation: None