<prompt> Amino acid phenylalanine is the precursor of all the following, except:TyrosineEpinephrineThyroxineMelatonin  <response>: 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: Harpers Illustrated Biochemistry, 26th Edition, Pages 239, 255, 266-267; Textbook of Biochemistry By D M Vasudevan, 3rd Edition, Pages 185, 187 