**Core Concept**
Pure motor hemiparesis is a type of hemiparesis characterized by weakness of the face, arm, and leg, with relative preservation of sensation and other cranial nerves. This condition is typically caused by a lesion in the motor pathways of the brain, specifically affecting the corticospinal tract.

**Why the Correct Answer is Right**
The corticospinal tract is responsible for transmitting motor signals from the cerebral cortex to the spinal cord and ultimately to the muscles. In pure motor hemiparesis, the lesion typically occurs in the internal capsule, which is a crucial part of the motor pathway. The internal capsule is a structure that contains the corticospinal tract as it descends from the cerebral cortex to the spinal cord. A lesion in this area can disrupt the motor signals, resulting in weakness of the face, arm, and leg. Other areas such as the motor cortex, brainstem, or spinal cord can also be affected but internal capsule is the most common site of lesion in pure motor hemiparesis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Lesion in the sensory pathways would result in sensory deficits, not pure motor hemiparesis.
* **Option B:** Lesion in the brainstem would result in more widespread brainstem symptoms, including cranial nerve deficits and ataxia.
* **Option D:** Lesion in the spinal cord would result in more localized spinal cord symptoms, such as weakness or paralysis of the lower limbs.

**Clinical Pearl / High-Yield Fact**
When evaluating a patient with hemiparesis, it's essential to perform a thorough neurological examination to determine the distribution of weakness and any associated sensory or cranial nerve deficits. This can help localize the lesion to the motor pathways or other areas of the brain.

**Correct Answer: C. Internal capsule.**