Regarding the structural and biochemical features of amyloid fibrils, all of the following are true EXCEPT:

Question

Accepted Answer

D. Amyloid fibrils are composed of randomly coiled polypeptide chains arranged in a parallel fashion. ## 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.

Answer

A. Amyloid deposits are resistant to proteolytic degradation due to their compact, cross-linked structure

Answer

B. The β-pleated sheet conformation is the hallmark structural feature of amyloid deposits

Answer

C. Amyloid fibrils exhibit characteristic apple-green birefringence under polarized light microscopy after Congo red staining

Regarding the structural and biochemical features of amyloid fibrils, all of the following are true EXCEPT:

Explanation

Structural Organization of Amyloid Fibrils

Correct Features of Amyloid

Why the Incorrect Option is Wrong

Clinical Pearl

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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