**Question:** Which of the following is a special stain used to diagnose fungal hyphae in tissues?

A. Periodic Acid-Schiff (PAS)
B. Gomori-Grocott Methenamine Silver (GMS) stain
C. Picro-Sirius Red
D. Van Geison stain

**Correct Answer:** B. Gomori-Grocott Methenamine Silver (GMS) stain

**Core Concept:**
Diagnosing fungal infections is crucial in clinical medicine, particularly in immunocompromised patients and in regions with high fungal endemicity. Special stains are used to enhance the visualization of fungal elements in tissue sections, allowing pathologists to make a definitive diagnosis.

**Why the Correct Answer is Right:**
The Gomori-Grocott Methenamine Silver (GMS) stain is a powerful tool for detecting fungal hyphae in tissue sections due to its ability to selectively stain the chitinous wall of fungi, particularly those of the dermatophytes, yeasts, and some moulds. This stain is based on the silver nitrate technique and uses methenamine as the chromogen. The GMS stain provides a dark blue to black precipitate in the fungal hyphae, distinguishing them from host tissue elements.

**Why Each Wrong Option is Incorrect:**

A. Periodic Acid-Schiff (PAS) stain: PAS stain is commonly used to demonstrate carbohydrates (e.g., glycogen, glycoproteins, and polysaccharides) in tissues. It does not specifically stain fungal hyphae.

C. Picro-Sirius Red (PSR) stain: PSR stain is primarily used for staining collagen in tissue sections. It does not stain fungal hyphae.

D. Van Gieson stain: Van Gieson stain is a general staining technique used for demonstrating elastic and collagen fibers in tissues. It does not specifically stain fungal hyphae.

**Clinical Pearl:**

The Gomori-Grocott Methenamine Silver (GMS) stain is essential for diagnosing dermatophyte infections, chromoblastomycosis, and certain mycoses like histoplasmosis, coccidioidomycosis, and blastomycosis. In contrast, other stains, such as PAS, Picro-Sirius Red, and Van Gieson, are not specific for fungal stains and are not routinely used for this purpose.