## Structural Organization of Amyloid Fibrils **Key Point:** Amyloid fibrils are NOT composed of randomly coiled polypeptide chains. They have a highly organized, ordered structure with **β-pleated sheet conformation** as the defining feature. ### Correct Features of Amyloid | Feature | Details | |---------|----------| | **Conformation** | β-pleated sheet (antiparallel or parallel arrangement of β-strands) | | **Staining** | Congo red positive with apple-green birefringence under polarized light | | **Electron microscopy** | Non-branching, rigid fibrils of 7–10 nm diameter | | **Protease resistance** | Highly resistant to degradation; cross-linked structure | | **Organization** | Highly ordered, crystalline arrangement — NOT random | ### Why the Incorrect Option is Wrong **High-Yield:** The statement "randomly coiled polypeptide chains" contradicts the fundamental definition of amyloid. Amyloid structure is characterized by **strict organization and order**, not randomness. The β-pleated sheet geometry requires precise alignment of polypeptide chains in a regular, repeating pattern. ### Clinical Pearl The ordered structure of amyloid explains both its diagnostic hallmark (Congo red birefringence) and its pathological consequence: resistance to normal protein turnover and proteolytic clearance, leading to progressive tissue infiltration and organ dysfunction. **Mnemonic: BRACE** — **B**-pleated sheet, **R**esistant to proteases, **A**pple-green birefringence, **C**ongo red positive, **E**lectron-dense fibrils.
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