A man, after skinning a dead animal, developed a pustule on his hand. A smear prepared from the lesion showed the presence of gram-positive bacilli in long chains which were positive for McFadyean’s reaction. The most likely etiological agent is:
**Core Concept**
The question is testing the student's knowledge of a specific type of bacterial infection, which is caused by a gram-positive bacillus that exhibits a characteristic staining pattern and a specific biochemical reaction. This is a classic example of a zoonotic infection, where an animal-borne pathogen is transmitted to a human.
**Why the Correct Answer is Right**
The presence of gram-positive bacilli in long chains, along with a positive McFadyean's reaction, is highly suggestive of *Bacillus anthracis*, the causative agent of anthrax. The McFadyean's reaction, also known as the "hairy brush" appearance, is a characteristic feature of *B. anthracis* due to the presence of the capsule. The capsule is composed of a polysaccharide material that is produced by the bacterium and is responsible for its virulence. The capsule is also what gives the bacterium its ability to evade the host's immune system.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not match the description of the bacterium in the question. *Bacillus cereus* is a gram-positive bacillus, but it does not exhibit the same staining pattern or biochemical reactions as *B. anthracis*.
**Option B:** This option is incorrect because it is not a gram-positive bacterium. *Escherichia coli* is a gram-negative bacillus that is commonly found in the human gut.
**Option C:** This option is incorrect because it is not a typical bacterium that causes skin infections. *Streptococcus pyogenes* is a gram-positive coccus that is commonly associated with pharyngitis and skin infections, but it does not match the description in the question.
**Option D:** This option is incorrect because it is not a typical bacterium that causes skin infections. *Pseudomonas aeruginosa* is a gram-negative bacillus that is commonly associated with opportunistic infections, but it does not match the description in the question.
**Clinical Pearl / High-Yield Fact**
* *Bacillus anthracis* is a zoonotic pathogen that can be transmitted to humans through contact with infected animals or contaminated animal products. It is a highly virulent bacterium that can cause severe disease in humans, including cutaneous anthrax, inhalational anthrax, and gastrointestinal anthrax.
**Correct Answer: D. *Bacillus anthracis***