**Core Concept**
Decreased inhibitory input to the internal segment of the globus pallidus disrupts the normal regulatory mechanisms that modulate the activity of the motor area of the cerebral cortex. The globus pallidus is a key component of the basal ganglia, a group of structures involved in the control of voluntary motor movements.

**Why the Correct Answer is Right**
The internal segment of the globus pallidus receives inhibitory input from the striatum, which is a major output structure of the basal ganglia. When this inhibitory input is decreased, the activity of the internal segment increases, leading to an excessive release of glutamate onto the motor area of the cerebral cortex. This results in an abnormal increase in the activity of the motor cortex, leading to excessive movement or hyperkinesia. The affected motor area of the cerebral cortex is typically the primary motor cortex (M1) and the supplementary motor area (SMA).

**Why Each Wrong Option is Incorrect**
* **Option A:** Increased inhibitory input to the internal segment of the globus pallidus would actually decrease the activity of the motor area of the cerebral cortex, leading to hypokinesia or decreased movement.
* **Option B:** Increased activity of the subthalamic nucleus (STN) would lead to increased excitatory input to the internal segment of the globus pallidus, resulting in increased motor activity, but this is not the direct effect of decreased inhibitory input to the globus pallidus.
* **Option D:** Increased activity of the substantia nigra pars compacta (SNc) would lead to increased dopaminergic input to the striatum, which would actually decrease the activity of the globus pallidus and the motor area of the cerebral cortex.

**Clinical Pearl / High-Yield Fact**
The basal ganglia are a group of structures that play a critical role in the regulation of voluntary motor movements. Disruptions to the normal functioning of the basal ganglia can lead to a range of movement disorders, including Parkinson's disease, Huntington's disease, and dystonia.

**Correct Answer: C.**