**Question:** Capsule of Bacillus Anthracis is stained by -

A. Gram stain
B. Ziehl-Neelsen stain
C. Warthin-Starry stain
D. Silver stain

**Core Concept:** Bacillus anthracis is a Gram-positive, spore-forming bacterium that causes anthrax, a potentially fatal disease in humans and animals. The capsule of Bacillus anthracis is an essential virulence factor that protects the bacteria from phagocytosis and enhances their survival.

**Why the Correct Answer is Right:** The correct answer, D. Silver stain, is chosen because of its ability to stain the polysaccharide capsule of Bacillus anthracis. Silver stains, particularly the Warthin-Starry stain, are effective in staining capsules due to the presence of silver nanoparticles that bind to the polysaccharide capsules. This helps in visualizing the capsule and differentiating Bacillus anthracis from other closely related bacteria.

**Why Each Wrong Option is Incorrect:**

A. Gram stain: While the Gram stain classifies bacteria based on their cell wall structure, it does not specifically stain the capsule of Bacillus anthracis. Gram staining helps differentiate between Gram-positive and Gram-negative bacteria, but it does not highlight the capsule.

B. Ziehl-Neelsen stain: This stain is primarily used for identifying mycobacteria, especially Mycobacterium tuberculosis, by staining acid-fast bacteria. It does not highlight the capsule of Bacillus anthracis.

C. Warthin-Starry stain: Although this stain is effective for staining the capsule of Bacillus anthracis, it is not specifically mentioned in the question, making option D the correct answer.

**Clinical Pearl:** The Warthin-Starry stain is essential for identification and differentiation of Bacillus anthracis from other similar bacteria, ensuring appropriate diagnosis and treatment of anthrax infections. In clinical practice, this stain aids in confirming the presence of anthrax in biological samples like sputum, blood, or tissue samples.

**Correct Answer Explanation:**

D. Silver stain (Warthin-Starry stain) is the correct answer as it highlights the polysaccharide capsule of Bacillus anthracis, facilitating the identification and differentiation of this bacterium from other closely related species. This stain plays a crucial role in clinical microbiology and helps in confirming anthrax infections in various biological samples.