This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Biochemistry
With help of the drug shown below, serotonin is synthesized from which of the following precursor amino acid? 
Select and explain. 
A. Tryptophan, B. Glycine, C. Threonine, D. Methionine
A: Tryptophan
Answer: A: Tryptophan 
Explanation: Tryptophan. Serotonin is synthesized from the essential amino acid tryptophan. The rate-limiting step is the conversion of the amino acid to 5-hydroxytrptophan by tryptophan hydroxylase. This is then conveed to serotonin by the aromatic L-amino acid decarboxylase which needs Pyridoxal Phosphate as a cofactor Ref : vasudevan 7th ed