## Core Concept
Alzheimer's disease is a progressive neurodegenerative disorder characterized by the accumulation of amyloid-beta plaques and neurofibrillary tangles in the brain. The amyloid plaques are extracellular deposits primarily composed of amyloid-beta peptides. This core concept revolves around the pathological features of Alzheimer's disease.

## Why the Correct Answer is Right
The correct answer, , indicates that the plaque in Alzheimer's disease is made up of amyloid beta. Amyloid beta peptides are fragments of the amyloid precursor protein (APP). When APP is processed by beta-secretase and gamma-secretase enzymes, it releases amyloid beta peptides, which then aggregate to form insoluble fibrils. These fibrils accumulate extracellularly to form senile plaques, a hallmark of Alzheimer's disease pathology. The accumulation of amyloid beta is thought to play a critical role in the pathogenesis of Alzheimer's disease, contributing to neuronal damage and death.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify amyloid beta, which is the primary component of plaques in Alzheimer's disease.
- **Option B:** This option is incorrect for similar reasons; it does not accurately represent the composition of Alzheimer's plaques.
- **Option D:** This option is incorrect as it does not correctly identify amyloid beta as the main constituent of Alzheimer's plaques.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the amyloid cascade hypothesis is a leading theory in the pathogenesis of Alzheimer's disease, suggesting that the accumulation of amyloid beta peptides is a critical early step. This hypothesis has led to the development of several therapeutic strategies aimed at reducing amyloid beta levels in the brain.

## Correct Answer: . Amyloid beta.