A man, after skinning a dead animal, developed a pustule on his hand. A smear prepared from the lesion showed Gram Positive Bacilli in long chains positive for McFadyean’s reaction. The most likely aetiological agent is:
**Core Concept**
The question is testing the student's knowledge of a specific type of bacterial infection, specifically a cutaneous infection caused by a Gram-positive bacillus. The key concept here is the McFadyean's reaction, which is a test used to identify bacteria that produce a specific type of pigment.
**Why the Correct Answer is Right**
The McFadyean's reaction is used to identify bacteria that produce a pigment called rhodopycosin. The correct answer is a type of bacteria that is known to produce this pigment. The bacteria in question are likely to be *Corynebacterium diphtheriae*, which is a Gram-positive bacillus that produces diphtheria toxin. The toxin is responsible for the disease's symptoms, including the formation of a membrane on the tonsils and throat. The toxin is also responsible for the characteristic long chains of bacteria seen in the smear.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the bacteria in question are Gram-positive bacilli, not Gram-negative bacilli.
* **Option B:** This option is incorrect because the McFadyean's reaction is not used to identify this type of bacteria.
* **Option D:** This option is incorrect because the bacteria in question are not typically associated with this type of infection.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that *Corynebacterium diphtheriae* can cause a range of symptoms, from mild to severe, including respiratory, cardiac, and neurological manifestations.
**Correct Answer:** C. *Corynebacterium diphtheriae*.