**Core Concept**
Hypothyroidism is a reversible cause of cognitive impairment and dementia, particularly when treated early. It affects brain function through reduced metabolic activity and impaired neurotransmitter synthesis, especially in the hippocampus and cortex.

**Why the Correct Answer is Right**
Hypothyroidism causes cognitive decline due to decreased cerebral blood flow, reduced neuronal metabolism, and impaired synaptic function. Symptoms include memory loss, confusion, and slowed thinking—classic features of reversible dementia. With proper thyroid hormone replacement (levothyroxine), cognitive function can improve significantly within weeks to months. This makes it a key example of a treatable, reversible cause of dementia.

**Why Each Wrong Option is Incorrect**
Option A: Alzheimer's disease is a progressive, irreversible neurodegenerative disorder characterized by amyloid plaques and neurofibrillary tangles. It does not respond to treatment.
Option B: Parkinsonism involves motor symptoms like tremors and rigidity, with cognitive decline as a late feature, but it is not a reversible cause of dementia.
Option D: Vascular dementia results from cerebrovascular disease and is typically irreversible, with poor response to therapy.

**Clinical Pearl / High-Yield Fact**
Hypothyroidism should be suspected in any patient with dementia, especially if they have fatigue, constipation, or cold intolerance. A simple TSH test can confirm and treat it—making early diagnosis and treatment critical.

✓ Correct Answer: C. Hypothyroidism