What causes Parkinson's disease?

• A. Degeneration of acetylcholine neurons.
• B. Loss of dopamine neurons.
• C. Increased GABA activity in the cortex.
• D. Autoimmune destruction of motor neurons.

Answer: (B)

Parkinson's disease is caused by degeneration of dopaminergic neurons in the substantia nigra pars compacta, leading to reduced dopamine in the striatum. Dopamine normally promotes movement by activating the direct pathway and inhibiting the indirect pathway within the basal ganglia. When dopamine is depleted, the indirect pathway becomes more active, increasing inhibitory output to the thalamus and decreasing thalamocortical drive to motor areas, which produces the hallmark motor symptoms like bradykinesia, rigidity, and tremor. Pathology often shows Lewy bodies with alpha-synuclein. Other options don’t capture the primary driver: degeneration of acetylcholine neurons isn’t the cause, changes in GABA activity are downstream effects, and autoimmune destruction of motor neurons is characteristic of ALS, not Parkinson’s.