**Core Concept**
Subcortical dementia refers to a type of cognitive decline that primarily affects subcortical structures, including the basal ganglia, thalamus, and brainstem. This form of dementia is characterized by slowed cognitive processing, difficulty with executive function, and prominent motor symptoms.

**Why the Correct Answer is Right**
Huntington's chorea is a genetic disorder that causes progressive degeneration of the basal ganglia, leading to subcortical dementia. The disease is characterized by the presence of chorea, which is a type of involuntary movement disorder, as well as cognitive decline, particularly in executive function. The subcortical involvement in Huntington's chorea results in a distinct clinical profile, which includes difficulty with planning, organization, and decision-making, in addition to motor symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** Alzheimer's disease is a classical example of cortical dementia, characterized by degeneration of the cerebral cortex, particularly in the temporal and parietal lobes. While Alzheimer's disease can also involve subcortical structures, it is primarily a cortical disorder.
**Option C:** Normal pressure hydrocephalus (NPH) is a condition characterized by the accumulation of cerebrospinal fluid in the brain, leading to ventricular enlargement. While NPH can cause cognitive decline, it is not typically classified as a subcortical dementia.
**Option D:** Vitamin B12 deficiency can cause subacute combined degeneration, a condition that affects the spinal cord and peripheral nerves, but it is not typically classified as a subcortical dementia.

**Clinical Pearl / High-Yield Fact**
It's essential to distinguish between cortical and subcortical dementias, as they have different underlying pathologies and clinical presentations. Cortical dementias, such as Alzheimer's disease, tend to present with early memory loss, whereas subcortical dementias, such as Huntington's chorea, present with slowed cognitive processing and prominent motor symptoms.

✓ Correct Answer: B. Huntington's chorea