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
[{'content': 'Amino acid phenylalanine is the precursor of all the following, except: \nSelect and explain. \nA. Tyrosine, B. Epinephrine, C. Thyroxine, D. Melatonin', 'role': 'user'}, {'content': "Answer: D \nExplanation: Phenylalanine hydroxylase conves phenylalanine to tyrosine. The amino acid tyrosine is the staing point in the synthesis of the catecholamines and of the thyroid hormones tetra iodothyronine (thyroxine; T4) and triiodothyronine (T3). Tyrosine is also conveed into DOPA, dopamine, norepinephrine and finally epinephrine. Ref: Harper's Illustrated Biochemistry, 26th Edition, Pages 239, 255, 266-267; Textbook of Biochemistry By D M Vasudevan, 3rd Edition, Pages 185, 187", 'role': 'assistant'}]