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 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?
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McFadyean reaction with capsulated bacilli is a classic clue for Bacillus anthracis.
Updated On: May 14, 2026
- Clostridium perfringens
- Bacillus anthracis
- Yersinia pestis
- Francisella tularensis
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The Correct Option is B
Solution and Explanation
Concept:
Cutaneous anthrax presents with a painless necrotic ulcer or black eschar. Bacillus anthracis shows a capsule that gives a positive McFadyean reaction with polychrome methylene blue stain.
Step 1: Identify clinical clue.
A farmer with necrotic ulcerative skin lesion suggests: \[ \text{Cutaneous anthrax} \]
Step 2: Use staining clue.
Polychrome methylene blue stain showing McFadyean reaction is characteristic of: \[ \textit{Bacillus anthracis} \]
Step 3: Choose organism.
Therefore, the causative organism is: \[ \textit{Bacillus anthracis} \]
Step 1: Identify clinical clue.
A farmer with necrotic ulcerative skin lesion suggests: \[ \text{Cutaneous anthrax} \]
Step 2: Use staining clue.
Polychrome methylene blue stain showing McFadyean reaction is characteristic of: \[ \textit{Bacillus anthracis} \]
Step 3: Choose organism.
Therefore, the causative organism is: \[ \textit{Bacillus anthracis} \]
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