**Question:** Congo-red with amyloid produces -

**Core Concept:** Congo-red is a dye that binds to amyloid fibrils, a type of protein aggregate found in various diseases like Alzheimer's and systemic amyloidosis. Amyloidosis is a group of disorders characterized by the extracellular deposition of amyloid proteins, leading to organ dysfunction.

**Why the Correct Answer is Right:** When Congo-red is applied to amyloid deposits, it forms a characteristic apple-green birefringence under polarized light due to its interaction with the protein structure. This is known as Congo-red birefringence.

**Why Each Wrong Option is Incorrect:**

A. **False positive**: This option is incorrect because Congo-red does not bind to all protein aggregates. It specifically interacts with amyloid fibrils, not other protein aggregates like alpha-synuclein in Parkinson's disease.

B. **False negative**: This option is incorrect because Congo-red does bind to amyloid fibrils, as explained above. It is not a non-binding test for amyloidosis.

C. **No birefringence**: This option is incorrect because Congo-red does produce a characteristic birefringence under polarized light when applied to amyloid deposits.

D. **No staining**: This option is incorrect because Congo-red does stain amyloid fibrils, making it useful in diagnosing amyloidosis and differentiating it from other protein aggregates.

**Clinical Pearl:** Congo-red staining with polarized light is a valuable tool in diagnosing and differentiating between various protein aggregates like amyloid, alpha-synuclein (Parkinson's disease), and tau (Alzheimer's disease). It helps in narrowing down the differential diagnosis and guiding further investigations and treatment options.

**Correct Answer:** C. Congo-red produces birefringence under polarized light when applied to amyloid fibrils. This characteristic staining pattern helps in diagnosing amyloidosis and distinguishing it from other protein aggregates.