<prompt> Which of the following enzymes is believed to be deficient in Parkinsonism?DOPA DecarboxylaseTyrosine HydroxylasePhenylalanine HydroxylaseDopamine oxidase  <response>: Answer is B Tyrosine Hydroxylase: Tyrosine Hydroxylase is the most impoant critical enzyme, deficiency which leads to Parkinsons disease The main biochemical charachteristic of Parkinsons Disease is the reduction of neurotransmitter and the dopamine synthesizing enzyme system including Tyrosine Hydroxylase 7H and Tetrahydrobiopterin BH4 Cofactor in nigrostriatal neurons. The loss of Tyrosine Hydroxylase represents the critical failure in Parkinsons Disease because this enzyme is the rate limiting enzyme dopamine synthesis. With a deficiency of tyrosine hydroxylase the ever-shrinking pool of nigrostriatal tract neurons cannot sustain the essential synthetic pathway. Once approximately 80 of these neurons degenerate, the nigrostriatal tract cannot synthesize adequate dopamine and Parkinsons disease symptoms appear - Clinical Neurology for Psychiatrits. 