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
Following is a non-essential amino acid: 
Select and explain. 
A. Phenylalanine, B. Proline, C. Tryptophan, D. Threonine
B: Proline
Answer: B: Proline 
Explanation: None