**Question:** Diphtheria toxin is produced only by those strains of C. diphtheriae that are

A. non-toxigenic
B. toxigenic
C. intermediate
D. non-invasive

**Correct Answer:** B. toxigenic

**Core Concept:**
Diphtheria is a severe bacterial infectious disease caused by the bacterium Corynebacterium diphtheriae. The disease is characterized by the production of diphtheria toxin, which primarily affects the respiratory system and leads to severe complications if not properly treated. Diphtheria toxin is responsible for the disease's symptoms and severity.

**Why the Correct Answer is Right:**
Diphtheria toxin is produced exclusively by two types of C. diphtheriae strains:

1. **Toxigenic strains:** These strains possess the tox gene (dt gene) on their genome, enabling them to produce diphtheria toxin. This is the correct option. The toxin plays a pivotal role in the pathogenesis of diphtheria, causing inflammation and tissue damage, leading to the symptoms associated with the disease.

2. **Non-toxigenic strains:** These strains lack the tox gene and therefore do not produce diphtheria toxin. They are harmless and do not cause diphtheria.

**Why Each Wrong Option is Incorrect:**
3. **Intermediate strains:** Intermediate strains possess the tox gene but produce the toxin at a reduced level. However, they are still considered toxigenic and can cause diphtheria. This option is incorrect as intermediate strains are not distinct from toxigenic strains.

4. **Non-toxigenic strains:** As mentioned previously, these strains lack the tox gene and are unable to produce diphtheria toxin. They are harmless and do not cause diphtheria. This option is incorrect as it does not align with the disease's pathophysiology.

**Clinical Pearl:**
The distinction between toxigenic and non-toxigenic strains of C. diphtheriae is crucial for understanding the severity and clinical manifestations of diphtheria. Toxigenic strains produce the toxin, leading to the disease, while non-toxigenic strains do not. Timely diagnosis and appropriate treatment with antibiotics and antitoxin are essential for managing diphtheria and preventing complications.