**Question:** Special stain useful in the following condition

A. Gomori's methamine silver stain for demonstrating argyrophilic structures like neurofibrillary tangles in Alzheimer's disease
B. PAS (Picric acid-Schiff) stain for demonstrating carbohydrates in tissues
C. Giemsa stain for demonstrating nucleic acids in cells
D. Orcein stain for demonstrating elastin in tissues

**Core Concept:** Special stains are techniques used in histopathology to enhance the visibility of specific structures in tissue sections. They are utilized to gain insights into various pathological conditions and aid in diagnosis.

**Why the Correct Answer is Right:** Gomori's methamine silver stain is specifically designed to demonstrate argyrophilic structures like neurofibrillary tangles in Alzheimer's disease. In this condition, the accumulation of hyperphosphorylated tau protein forms neurofibrillary tangles, which are not visible with routine Haematoxylin and Eosin (H&E) staining. Gomori's stain uses silver nitrate to react with the argyrophilic structures, resulting in a distinct silver precipitate. This enhances the visibility of neurofibrillary tangles, allowing pathologists to diagnose Alzheimer's disease more accurately.

**Why Each Wrong Option is Incorrect:**

B. PAS (Picric acid-Schiff) stain is used to demonstrate carbohydrates (glycoproteins and glycolipids) in tissues, not argyrophilic structures like neurofibrillary tangles.

C. Giemsa stain is designed to demonstrate nucleic acids, particularly DNA and RNA, in cells, not argyrophilic structures like neurofibrillary tangles.

D. Orcein stain is utilized for demonstrating elastin in tissues, not argyrophilic structures like neurofibrillary tangles. Orcein stain reacts with elastin fibers, which are distinct from neurofibrillary tangles in Alzheimer's disease.

**Clinical Pearl:** Understanding the appropriate stain for specific structures in histopathology is crucial for accurate diagnosis of various diseases. Gomori's methamine silver stain, specifically designed for argyrophilic structures, is essential in diagnosing Alzheimer's disease, while other stains like PAS, Giemsa, and Orcein are not suitable for this purpose.