Which structural feature of amyloid fibrils is responsible for their characteristic appearance on Congo red staining under polarized light microscopy?

Explanation

## Amyloid Fibril Structure and Congo Red Staining **Key Point:** Amyloid fibrils are composed of **β-pleated sheets** arranged in a parallel, cross-β configuration. This rigid, crystalline structure is the hallmark of amyloid and is responsible for the pathognomonic Congo red birefringence. ### Structural Characteristics 1. **β-pleated sheet arrangement** - Protein molecules align in parallel orientation - Cross-β structure: β-strands perpendicular to fibril axis - Creates a rigid, insoluble polymer 2. **Congo Red Binding** - Congo red dye binds to the regular β-sheet structure - Creates a linear dichroism effect - Under **polarized light microscopy**: exhibits **apple-green birefringence** (pathognomonic) 3. **Why β-sheets are unique to amyloid** - Most proteins fold into α-helices or random coils (soluble) - Amyloid's β-sheet configuration is **insoluble and resistant to proteolysis** - This explains amyloid's accumulation and toxicity **High-Yield:** The **apple-green birefringence** on Congo red staining under polarized light is the **gold standard diagnostic feature** of amyloidosis. **Mnemonic:** **ABBA** — **A**myloid = **B**eta-**B**irefringence + **A**pple-green **Clinical Pearl:** Congo red staining followed by polarized light microscopy is the diagnostic gold standard; electron microscopy shows characteristic 7–10 nm unbranched fibrils.