A 42-year-old woman from Bangalore presents with involuntary, jerky, purposeless movements of her limbs and trunk that worsen with stress and improve during sleep. She has a family history of similar illness in her mother and maternal uncle. Cognitive decline and emotional lability are noted. Genetic testing confirms a CAG trinucleotide repeat expansion. On functional neuroimaging, there is selective loss of medium spiny neurons in the striatum, particularly affecting neurons that express D2 dopamine receptors. Which basal ganglia circuit dysfunction best explains her hyperkinetic movement disorder?