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
Thyroid hormone formation involves many steps. Which of the following amino acid is the precursor for thyroxine? 
Select and explain. 
A. Glycine, B. Tyrosine, C. Phenylalanine, D. Iodine
B: Tyrosine
Answer: B: Tyrosine 
Explanation: Tyrosine: Tyrosine is the precursor of triiodothyronine and thyroxine. Neural cells conve tyrosine to epinephrine and norepinephrine. While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes. Dopa decarboxylase, a pyridoxal phosphate-dependent enzyme, forms dopamine. Subsequent hydroxylation by dopamine -oxidase then forms norepinephrine. In the adrenal medulla, phenylethanolamine-N-methyltransferase utilizes S-adenosylmethionine to methylate the primary amine of norepinephrine, forming epinephrine. Ref: Rodwell V.W. (2011). Chapter 30. Conversion of Amino Acids to Specialized Products. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper's Illustrated Biochemistry, 29e.