Which peptide fragment is derived from the amyloid precursor protein and forms extracellular deposits in Alzheimer's disease?

• A. Tau protein
• B. Prion protein
• C. β-Amyloid (Aβ) peptide
• D. Alpha-synuclein

Answer: (C)

Alzheimer disease features extracellular deposits formed from a peptide produced when amyloid precursor protein (APP) is cut by secretases in the amyloidogenic pathway. Beta-secretase cleaves APP to generate a fragment that, with gamma-secretase, releases β-amyloid peptides—especially Aβ42, which is prone to misfolding and aggregation outside neurons to form plaques. This extracellular accumulation of β-amyloid is a hallmark of the disease. By contrast, tau is an intracellular protein that forms neurofibrillary tangles, prion protein misfolding underlies prion diseases, and alpha-synuclein aggregates into Lewy bodies in synucleinopathies.