All Clostridia cause myonecrosis except-
**Core Concept**
The question is based on the pathogenic characteristics of **Clostridia**, a genus of gram-positive, anaerobic bacteria. **Myonecrosis**, or the death of muscle tissue, is a hallmark of infection by certain species. Understanding the differences in virulence and disease presentation among **Clostridia** species is crucial.
**Why the Correct Answer is Right**
The correct answer is an exception because it does not produce the toxins typically responsible for **myonecrosis**. Most **Clostridia** that cause myonecrosis do so through the action of powerful toxins, such as **alpha-toxin** (a phospholipase C) in **Clostridium perfringens**. The exception does not have this capability.
**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it is a well-known cause of **myonecrosis**.
**Option B:** Similarly, this option is also a cause of **myonecrosis** due to its toxin production.
**Option D:** This is another species that can lead to **myonecrosis**, making it an incorrect choice.
**Clinical Pearl / High-Yield Fact**
Remember, **Clostridium perfringens** is the most common cause of **gas gangrene**, a severe form of **myonecrosis**. Understanding the exceptions among **Clostridia** can help in diagnosing and managing these severe infections.
**Correct Answer:** D. Clostridium tetani.