This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Amino Acids Basics, Subject name: Biochemistry
Which of the following is not an essential amino add? 
Select and explain. 
A. Methionine, B. Tryptophan, C. Leucine, D. Alanine
D: Alanine
Answer: D: Alanine 
Explanation: ANSWER: (D) AlanineREF: Harper 27th ed chapter 3Repeat Biochemistry December 2010