**Question:** Diphtheria is pathogenic only when infected with

A. Corynebacterium diphtheriae
B. Streptococcus pyogenes
C. Pseudomonas aeruginosa
D. Bacillus anthracis

**Correct Answer:** A. Corynebacterium diphtheriae

**Core Concept:**
Diphtheria is a serious infectious disease caused by the bacterium Corynebacterium diphtheriae. It primarily affects the respiratory system, leading to inflammation and obstruction, which can be life-threatening due to respiratory distress and heart complications.

**Why the Correct Answer is Right:**
Diphtheria is pathogenic when infected with Corynebacterium diphtheriae, a gram-positive bacterium belonging to the Actinomycetales order and Corynebacteriaceae family. This bacterium produces a toxin that is responsible for the disease's characteristic symptoms and complications.

**Why Each Wrong Option is Incorrect:**
A. Streptococcus pyogenes (Group A Streptococcus) is a different bacterium causing streptococcal infections like pharyngitis (strep throat) and impetigo. It is not associated with diphtheria.
B. Pseudomonas aeruginosa is a gram-negative bacterium causing severe infections in immunocompromised patients and those with pre-existing lung diseases. It is not responsible for diphtheria.
C. Bacillus anthracis is another gram-positive bacterium causing anthrax, a zoonotic disease affecting animals and humans. It is not involved in diphtheria.

**Clinical Pearl:**
Understanding the causative agent of diphtheria is crucial for recognizing and managing this potentially life-threatening illness. Prompt antibiotic treatment and vaccination are essential preventive measures against diphtheria.