## **Core Concept**
The question tests knowledge of microbiological staining techniques, specifically for identifying metachromatic granules. Metachromatic granules, also known as Babes-Ernst bodies, are characteristic of certain bacteria, notably *Corynebacterium diphtheriae*. The staining technique used for such structures involves dyes that change color when bound to specific cellular components.

## **Why the Correct Answer is Right**
The correct answer, **Albert stain**, is a specific type of stain used for demonstrating metachromatic granules in bacteria like *Corynebacterium diphtheriae*. This stain works by using a combination of dyes that results in the metachromatic granules appearing red or purple against a contrasting background. The process typically involves applying a series of dyes and decolorizers to differentiate the granules.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While methylene blue is a stain used in microbiology, it is not specifically known for staining metachromatic granules.
- **Option B:** Similarly, this option does not accurately represent a stain specifically used for metachromatic granules.
- **Option D:** This option is also incorrect as it does not correspond to a commonly recognized stain for metachromatic granules.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that metachromatic granules are a diagnostic feature of *Corynebacterium diphtheriae*, the causative organism of diphtheria. The ability to identify these granules using the **Albert stain** is crucial for the laboratory diagnosis of diphtheria.

## **Correct Answer:** B. Albert stain