**Core Concept**
The basal ganglia are a complex group of subcortical structures involved in motor control and cognition, and they receive afferent inputs from various sources. The primary afferent pathway to the basal ganglia is the striatum, which receives inputs from the cerebral cortex via the thalamus.

**Why the Correct Answer is Right**
The striatum is the main input structure of the basal ganglia, receiving glutamatergic inputs from the cortex via the thalamus. This input is primarily mediated by the cortico-striatal pathway, which plays a crucial role in motor control and cognition. The striatum is divided into two main parts: the caudate nucleus and the putamen. The afferent inputs to the striatum are essential for the normal functioning of the basal ganglia circuitry.

**Why Each Wrong Option is Incorrect**
**Option B:** The globus pallidus is an output structure of the basal ganglia, not a primary afferent input. It receives inputs from the striatum and the subthalamic nucleus but does not receive direct afferent inputs from the cortex.
**Option C:** This is a duplicate of Option A and is therefore incorrect for the same reasons.
**Option D:** The subthalamic nucleus is an intrinsic part of the basal ganglia circuitry and receives inputs from the cortex via the thalamus, but it is not the primary afferent input to the basal ganglia.

**Clinical Pearl / High-Yield Fact**
The basal ganglia circuitry is highly complex and involves multiple feedback loops. Understanding the primary afferent pathway to the basal ganglia is essential for grasping the pathophysiology of movement disorders such as Parkinson's disease.

**✓ Correct Answer: A. Striatum**