A 60 year old man complains of difficulty arising from a chair and initiating new movements. On exam, you notice a resting hand tremor and cogwheel rigidity. Which of the following amino acids is the precursor for the neurotransmitter that is deficient in the brain of this patient?

This question requires three steps of logic. First, figure out the diagnosis (a classic case of Parkinson's disease), then remember which neurotransmitter is involved in the disease (dopamine), and finally, recall which amino acid serves as the precursor for that neurotransmitter (tyrosine). The hydroxylation of tyrosine by tyrosine hydroxylase results in DOPA, which is then decarboxylated to dopamine. Note that norepinephrine and epinephrine are also tyrosine derivatives, as are the melanins and the thyroid hormones, thyroxine and triiodothyronine. The other answer choices are also precursors of specialized products: Glutamate can be conveed to the inhibitory neurotransmitter GABA by the action of glutamate decarboxylase. Glycine is involved in the synthesis of both creatine (along with arginine and S-adenosylmethionine) and heme (along with succinyl-CoA). Histidine can be decarboxylated to histamine, an impoant inflammatory mediator. Ref: Beck-Yoo K. (2012). Chapter 19. The Nervous System. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.