A 42-year-old woman from Mumbai presents with involuntary choreiform movements of her limbs, emotional lability, and cognitive decline over the past 18 months. Her father died at age 55 with similar symptoms. Genetic testing confirms a CAG trinucleotide repeat expansion in the huntingtin gene. Which basal ganglia circuit dysfunction best explains her hyperkinetic movement disorder, and what is the underlying neurochemical imbalance?