Most virulent variety of shigellosis is caused by –
**Core Concept**
Shigellosis is an infectious disease caused by the bacterium Shigella, characterized by severe diarrhea, abdominal cramps, and fever. The virulence of Shigella strains varies, with some being more pathogenic than others.
**Why the Correct Answer is Right**
The most virulent variety of shigellosis is caused by Shigella dysenteriae type 1, which produces the Shiga toxin. This toxin causes damage to the intestinal lining, leading to severe diarrhea, hemorrhage, and even death. The Shiga toxin is produced by the bacterium through the action of the chromosomal gene _shgA_, which encodes the toxin subunit A. The toxin is then secreted by the bacterium through a type III secretion system.
**Why Each Wrong Option is Incorrect**
**Option A:** Shigella flexneri is another common cause of shigellosis, but it is not as virulent as Shigella dysenteriae type 1. While it can still cause severe disease, it does not produce the Shiga toxin.
**Option B:** Shigella sonnei is a less virulent strain that typically causes milder disease. It does not produce the Shiga toxin and is more commonly found in developed countries.
**Option C:** Shigella boydii is another less virulent strain that can cause disease, but it is not as virulent as Shigella dysenteriae type 1.
**Clinical Pearl / High-Yield Fact**
The Shiga toxin is responsible for the severe damage caused by Shigella dysenteriae type 1, and its production is the key factor in determining the virulence of this bacterium. Remember the Shiga toxin as the main culprit behind the severe disease caused by Shigella dysenteriae type 1.
**Correct Answer:** C. Shigella dysenteriae type 1