**Question:** All are true regarding Corynebacterium diphtheriae except

A. It is a Gram-positive bacillus
B. It is commonly found in soil and dust
C. It produces a toxin that causes diphtheria
D. It is non-spore-forming

**Correct Answer:** .

**Core Concept:** Corynebacterium diphtheriae is a Gram-positive bacterial species known for its ability to produce a toxin that causes the disease diphtheria. It is a non-spore-forming bacillus and is commonly found in soil and dust.

**Why the Correct Answer is Right:** Corynebacterium diphtheriae is indeed a Gram-positive bacillus, and it is responsible for causing diphtheria through its toxin. The bacterium is commonly found in soil and dust, which explains its presence in these environments.

**Why Each Wrong Option is Incorrect:**

A. Corynebacterium diphtheriae is a Gram-positive bacillus, not a Gram-negative one as stated in option A.

B. While C. diphtheriae can be found in soil and dust, option B is not entirely correct because the bacterium can also be found in human respiratory secretions, causing the disease diphtheria in humans.

C. The toxin produced by C. diphtheriae is responsible for the pathogenesis of diphtheria, not the bacterium itself. This toxin is responsible for the characteristic membrane formation in the throat and respiratory tract, leading to respiratory obstruction and complications.

D. Corynebacterium diphtheriae is a non-spore-forming bacterium, meaning it does not form spores.

**Clinical Pearl:** A thorough understanding of the characteristics of pathogens like Corynebacterium diphtheriae and their roles in disease pathogenesis is essential for medical students and healthcare professionals. This knowledge helps in proper diagnosis, treatment, and prevention of infections.