## Core Concept
Subcortical dementia primarily affects the subcortical regions of the brain, which include structures such as the basal ganglia, thalamus, and brainstem. This type of dementia is characterized by disruptions in the regulation of thought and movement. Subcortical dementias often present with prominent motor symptoms, such as tremors, rigidity, and bradykinesia.

## Why the Correct Answer is Right
Alzheimer's disease is primarily a cortical dementia. It predominantly affects the cortical regions of the brain, leading to symptoms such as memory loss, language difficulties, and problems with reasoning and judgment. The key features of Alzheimer's include amyloid plaques, neurofibrillary tangles, and neuronal loss in the cerebral cortex.

## Why Each Wrong Option is Incorrect
**Option A:** Huntington's disease is a classic example of a subcortical dementia. It is characterized by chorea (involuntary movements), psychiatric symptoms, and cognitive decline. The pathology involves atrophy of the caudate nucleus and putamen, which are subcortical structures.

**Option B:** Parkinson's disease dementia is considered a subcortical dementia. It presents with parkinsonian motor symptoms (tremor, rigidity, bradykinesia, postural instability) and cognitive decline. The dementia in Parkinson's disease is associated with Lewy bodies and involves subcortical structures.

**Option C:** Progressive supranuclear palsy (PSP) is another disorder that can be classified under subcortical dementia. It presents with parkinsonism, supranuclear vertical gaze palsy, and cognitive decline. PSP affects subcortical structures, including the brainstem and basal ganglia.

## Clinical Pearl / High-Yield Fact
A key clinical pearl is that subcortical dementias often present with more pronounced motor symptoms and less severe memory impairments compared to cortical dementias. Alzheimer's disease, on the other hand, is the most common cause of dementia and is characterized by prominent memory loss early in the disease course.

## Correct Answer: . Alzheimer's disease