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
Which of the following is not true about tryptophan? 
Select and explain. 
A. Involved in niacin synthesis, B. Involved in serotonin synthesis, C. Involved in melatonin synthesis, D. It is a non essential amino acid.
D: It is a non essential amino acid.
Answer: D: It is a non essential amino acid. 
Explanation: Tryptophan is an essential aminoacid. It is used in the synthesis of Melatonin, serotonin and niacin.