**Question:** Protein involved in Alzheimer's disease

**Core Concept:** Alzheimer's disease (AD) is a progressive neurodegenerative disorder characterized by memory loss, cognitive decline, and behavioral changes. The primary pathological hallmarks of AD are the accumulation of amyloid-beta (Aβ) plaques and neurofibrillary tangles composed of hyperphosphorylated tau protein.

**Why the Correct Answer is Right:** Alzheimer's disease is primarily caused by the accumulation of amyloid-beta (Aβ) plaques in the brain, which results in neuronal dysfunction and death. Aβ is a peptide derived from the amyloid precursor protein (APP) cleavage by the sequential action of β-secretase (BACE1) and γ-secretase enzymes. The correct answer, **Aβ**, represents the amyloid-beta protein, which plays a central role in the pathogenesis of Alzheimer's disease.

**Why Each Wrong Option is Incorrect:**

A. **Tau (Protein Tau):** Tau protein is a microtubule-associated protein that helps stabilize microtubules in neurons. In Alzheimer's disease, hyperphosphorylation of tau protein leads to the formation of neurofibrillary tangles, contributing to neuronal dysfunction and death but is not the primary cause of AD.

B. **Amyloid precursor protein (APP):** APP is the precursor protein for Aβ peptides, which are cleaved by β-secretase (BACE1) and γ-secretase enzymes. While APP is involved in AD, the correct answer, Aβ, represents the amyloid-beta peptide derived from APP cleavage.

C. **Tau protein**: As explained earlier, the hyperphosphorylated tau protein forms neurofibrillary tangles in Alzheimer's disease. However, tau protein is not primarily responsible for Alzheimer's disease but contributes to its pathogenesis.

D. **Gamma-secretase enzyme**: Gamma-secretase is one of the enzymes involved in APP cleavage, leading to Aβ peptide formation. While gamma-secretase plays a role in Alzheimer's disease, the correct answer, Aβ, represents the amyloid-beta peptide derived from APP cleavage by gamma-secretase.

**Clinical Pearls:**

The accumulation of Aβ peptides, specifically Aβ42, is a defining pathological feature of Alzheimer's disease. While other proteins and factors contribute to AD pathogenesis, the primary role belongs to Aβ peptides, particularly Aβ42, which forms the amyloid plaques in the brain. Alzheimer's disease is characterized by the accumulation of extracellular Aβ plaques and intracellular neurofibrillary tangles composed of hyperphosphorylated tau protein.

**Correct Answer Explanation: Aβ** (Amyloid-beta) **protein is the primary culprit in Alzheimer's disease, as it forms the amyloid plaques in the brain. The other proteins and factors contribute to the disease, but Aβ peptides, particularly Aβ42, are essential in understanding the pathogenesis of Alzheimer's disease, which is why it is the correct answer.