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 nonessential amino acid: 
Select and explain. 
A. Tyrosine, B. Valine, C. Methionine, D. Tryptophan
A: Tyrosine
Answer: A: Tyrosine 
Explanation: None