A 42-year-old man presents with progressive involuntary movements, cognitive decline, and psychiatric symptoms. His father died at age 55 with similar manifestations. Examination reveals widespread chorea, motor impersistence, abnormal saccades, and gait instability. MRI shows bilateral caudate atrophy with widening of the frontal horns. Genetic testing reveals a heterozygous CAG repeat expansion (47 repeats) in exon 1 of the HTT gene on the structure marked **A** in the diagram. Which of the following best explains the pathophysiology of the movement disorder in this patient?
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