**Question:** All the following are false regarding C. diphteriae toxin EXCEPT:

A. C. diphteriae toxin is an exotoxin produced by the bacterium diphtheria causing diphtheria.
B. Diphtheria toxin is produced by Bacillus anthracis, causing anthrax.
C. Diphtheria toxin is an enzyme that directly damages host cell proteins.
D. The diphtheria toxin has a high affinity for gangliosides on the surface of neurons.

**Correct Answer:** C. Diphtheria toxin is an enzyme that directly damages host cell proteins.

**Core Concept:**
C. Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae. The primary toxin produced by this bacterium is diphtheria toxin, which is a modified bacterial enzyme. Diphtheria toxin is a protein that directly inactivates host cell ribosomes, preventing protein synthesis, ultimately leading to cell death.

**Why the Correct Answer is Right:**
The correct answer is D - Diphtheria toxin is an enzyme that directly damages host cell proteins. The other options are incorrect for the following reasons:

A. C. diphteriae toxin is a protein, not an enzyme. The main role of the toxin is to inhibit protein synthesis in host cells, not to damage proteins directly.
B. This option refers to anthrax, caused by Bacillus anthracis and produces anthrax toxin, not diphtheria toxin.
C. Diphtheria toxin does not directly damage host cell proteins; rather, it inhibits ribosomes' function, leading to protein synthesis inhibition and cell death.

**Why Each Wrong Option is Incorrect:**
A. Diphtheria toxin is an enzyme, not a protein. It is responsible for inhibiting ribosomes in host cells, not directly damaging proteins.
B. This refers to anthrax, caused by Bacillus anthracis, which produces anthrax toxin, not diphtheria toxin.
C. Diphtheria toxin does not directly damage host cell proteins; instead, it inhibits ribosomes' function, causing protein synthesis inhibition and cell death.

**Clinical Pearls:**
1. Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, which produces diphtheria toxin. This toxin is responsible for causing the clinical manifestations of diphtheria, such as respiratory involvement, cardiovascular system impairment, and neuromuscular dysfunction.
2. Understanding the mode of action of diphtheria toxin is crucial for understanding the disease's pathophysiology and treatment strategies.
3. Diphtheria toxin is an ABT (Adenylate Binding Toxin) that binds to and inhibits the enzyme ADP-ribosylating enzyme (ADP-ribosylating enzymes are a class of enzymes that modify proteins by adding ADP-ribose groups to specific amino acids, changing protein function or structure, and leading to cellular dysfunction