**Question:** Stain used to diagnose amyloidosis: September 2008

**Core Concept:** Amyloidosis is a disorder characterized by the extracellular deposition of amyloid fibrils, which are misfolded proteins that disrupt normal tissue function. Diagnosis of amyloidosis involves histopathological examination of tissue samples using appropriate stains.

**Why the Correct Answer is Right:** Congo Red stain is a vital tool used in histopathology to diagnose amyloidosis. It is a dyestuff that exhibits characteristic birefringence when viewed under polarized light, due to its interaction with the beta-sheet structure of amyloid fibrils.

**Why Each Wrong Option is Incorrect:**

A. **Option A:** H&E (Hematoxylin and Eosin) stain is a common histopathological stain used to visualize cellular structures and nuclei. However, it is not specific for amyloidosis diagnosis.

B. **Option B:** Gomori's methenamine silver stain is primarily used to visualize elastic fibers and reticular fibers, which are not relevant to the diagnosis of amyloidosis.

C. **Option C:** Periodic Acid Schiff (PAS) stain is commonly used for histopathological examination of tissues to visualize glycoproteins and glycogen. It is not specific for amyloidosis diagnosis.

D. **Option D:** Alcian blue stain is used to visualize mucopolysaccharides, particularly sialic acid and sulfated glycosaminoglycans. It is not specific for amyloidosis diagnosis.

**Clinical Pearl / High-Yield Fact:** Congo Red stain is essential for diagnosing amyloidosis, as it provides the unique birefringence property under polarized light, which helps in distinguishing amyloid from other tissue deposits. The red birefringent appearance under polarized light is known as the "apple-green birefringence" and is a characteristic feature of amyloid deposits, which helps in diagnosing amyloidosis.

**Correct Answer:** A. Congo Red stain + potassium iodide counterstain

Explanation: Congo Red stain, when combined with potassium iodide counterstain, exhibits a characteristic apple-green birefringence under polarized light, which is specific to amyloid deposits and helps in diagnosing amyloidosis. This combination of stains distinguishes amyloid from other tissue deposits and is essential in the diagnosis of amyloidosis.