A 52-year-old male presents with progressive difficulty walking and muscle stiffness. He was previously diagnosed with a neurodegenerative disorder characterized by protein misfolding and aggregation in the brain. A brain biopsy reveals abnormal β-sheet-rich protein deposits, leading the physician to suspect amyloid plaques as a key pathological feature. Which of the following secondary protein structures is primarily involved in protein misfolding diseases such as Alzheimer’s and prion diseases?
