What enzyme deficiency is most likely responsible for the symptoms of a 6-year-old boy with leg stiffness and dystonia, shown by decreased homovanillic acid in cerebrospinal fluid?

• A. CoQ reductase
• B. Cytochrome c reductase
• C. Tyrosine hydroxylase
• D. Tryptophan hydroxylase

Answer: (C)

The symptoms described in the 6-year-old boy, including leg stiffness and dystonia, combined with the finding of decreased homovanillic acid (HVA) in cerebrospinal fluid, suggest a specific metabolic condition related to neurotransmitter synthesis.

Homovanillic acid is a metabolite of dopamine, which is primarily produced through the action of the enzyme tyrosine hydroxylase. This enzyme converts the amino acid tyrosine into L-DOPA, which is then converted into dopamine. A deficiency of tyrosine hydroxylase would lead to reduced production of dopamine, resulting in lower levels of its metabolite, homovanillic acid. Consequently, the symptoms of dystonia and rigidity may arise from insufficient dopaminergic signaling, particularly affecting motor control.

Given this context, the correct answer points to a deficiency in tyrosine hydroxylase as the underlying problem for the child's presentation and the biochemical findings.