The strategy for therapy for dopamine deficiency in the substantia nigra of individuals with Parkinson's disease is indicated by which of the following?

Dopamine is produced from L-dopa, which in turn is made from tyrosine. Therapy with the L-dopa precursor increases dopamine concentrations and improves the rigidity and immobility that occur in Parkinsons disease. Dopamine is degraded in the synaptic cleft by monoamine oxidases A and B (MAO-A and MAO-B), producing 3,4- dihydroxyphenylacetaldehyde (DOPAC). DOPAC is in turn broken down to homovanilhic acid, which can be measured in spinal fluid to assess dopamine metabolism. Inhibitors of MAO-A and MAO-B have some use in treating Parkinson's disease. The metabolism of histidine or alanine is not related to that of dopamine. but phenylalanine is a precursor of tyrosine and L-dopa.