**Core Concept**
Alzheimer's disease is the most common cause of dementia in adults, accounting for 60-80% of cases. It is a progressive neurodegenerative disorder characterized by the accumulation of amyloid-beta plaques and tau protein tangles in the brain, leading to neuronal loss and cognitive decline.

**Why the Correct Answer is Right**
The pathogenesis of Alzheimer's disease involves the abnormal metabolism of amyloid precursor protein (APP), resulting in the production of amyloid-beta peptides. These peptides aggregate to form insoluble fibrils that accumulate in the brain, leading to neuronal damage and death. The underlying mechanisms also involve the activation of various inflammatory pathways and the disruption of neurotransmitter systems, including the cholinergic system.

**Why Each Wrong Option is Incorrect**
**Option A:** Vascular dementia is the second most common cause of dementia, accounting for 10-20% of cases. However, it is caused by cerebrovascular disease, such as stroke or small vessel disease, rather than amyloid-beta accumulation.

**Option B:** Lewy body dementia is a neurodegenerative disorder characterized by the presence of Lewy bodies in the brain. While it can cause cognitive decline and dementia, it is much less common than Alzheimer's disease.

**Option C:** Frontotemporal dementia is a group of disorders that affect the frontotemporal regions of the brain. These disorders can cause significant cognitive decline and behavioral changes, but they are less common than Alzheimer's disease.

**Clinical Pearl / High-Yield Fact**
Alzheimer's disease is often associated with the development of amyloid-beta plaques and tau protein tangles in the brain. It is essential to recognize that Alzheimer's disease is a heterogeneous disorder, and its pathogenesis involves multiple genetic and environmental factors.

**Correct Answer:** A. Alzheimer's disease.