**Core Concept**
Subcortical dementia refers to a clinical syndrome characterized by cognitive impairments, particularly in executive function, memory, and mood regulation, with relative preservation of cortical functions such as language and praxis. Subcortical dementia is often associated with lesions or diseases affecting the basal ganglia, thalamus, and brainstem.

**Why the Correct Answer is Right**
The correct answer is associated with Alzheimer's disease, which primarily causes cortical dementia. Cortical dementia is characterized by the degeneration of cortical neurons, leading to impairments in language, praxis, and other higher-order cognitive functions. Alzheimer's disease typically presents with early symptoms of memory loss, language difficulties, and visuospatial impairments, which are indicative of cortical involvement.

**Why Each Wrong Option is Incorrect**
**Option A:** Huntington's disease is a neurodegenerative disorder that primarily affects the basal ganglia, leading to subcortical dementia. It is characterized by choreiform movements, cognitive decline, and psychiatric symptoms.
**Option B:** Vascular dementia is a condition caused by reduced blood flow to the brain, often resulting from strokes or small vessel disease. It can lead to subcortical dementia, particularly if the lesions affect the subcortical structures such as the basal ganglia or thalamus.
**Option C:** Normal pressure hydrocephalus (NPH) is a condition characterized by the accumulation of cerebrospinal fluid in the ventricles of the brain, leading to subcortical dementia. Symptoms of NPH include gait disturbance, dementia, and urinary incontinence.

**Clinical Pearl / High-Yield Fact**
When differentiating between cortical and subcortical dementia, remember that cortical dementia tends to present with language and praxis impairments, whereas subcortical dementia is characterized by executive function, memory, and mood regulation impairments.

**Correct Answer: D. Alzheimer's disease. Alzheimer's disease is a primary cause of cortical dementia, not subcortical dementia.**