Question

A farmer presents with a necrotic ulcerative skin lesion. Microscopy using polychrome methylene blue stain reveals capsulated bacilli showing a positive McFadyean reaction. What is the most likely causative organism?

• A. Clostridium perfringens
• B. Bacillus anthracis
• C. Yersinia pestis
• D. Francisella tularensis

Hint:

McFadyean reaction with capsulated bacilli is a classic clue for Bacillus anthracis.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The combination of a farmer (occupational exposure to livestock), a "necrotic ulcerative skin lesion" (malignant pustule), and very specific laboratory findings (McFadyean reaction) is diagnostic.
Step 2: Detailed Explanation:

Bacillus anthracis: The causative agent of Anthrax. In cutaneous anthrax, a small papule develops into a painless ulcer with a characteristic black necrotic center (eschar), surrounded by edema.

McFadyean Reaction: This is a classic diagnostic test for B. anthracis. When a blood smear or tissue specimen is stained with polychrome methylene blue, the capsule of the bacilli appears as a disintegrated pink/purple amorphous material surrounding the blue-stained bacterial cell. This "pink capsule" effect is the McFadyean reaction.

Microscopy: B. anthracis appears as large, Gram-positive, non-motile, spore-forming bacilli in chains (bamboo stick appearance).

Why not others? Clostridium perfringens (Option A) causes gas gangrene, not a black eschar. Yersinia pestis (Option C) causes buboes (lymphadenopathy) and shows bipolar "safety pin" staining with Giemsa, not the McFadyean reaction.

Step 3: Final Answer:
The McFadyean reaction is specifically used to demonstrate the capsule of Bacillus anthracis.