**Question:** True about Corynebacterium diphtheriae?

A. It causes diphtheria
B. It produces diphtheria toxin
C. It only infects the respiratory tract
D. It can cause pneumonia and meningitis

**Core Concept:**
Corynebacterium diphtheriae is a gram-positive, aerobic, rod-shaped bacterium that is well-known for causing diphtheria, an infectious disease affecting humans. Diphtheria is primarily characterized by membrane formation in the respiratory tract, leading to respiratory distress and potential complications.

**Why the Correct Answer is Right:**
A: Diphtheria is indeed caused by Corynebacterium diphtheriae, making option A true.
B: The bacterium produces a toxin called diphtheria toxin, which contributes to the clinical manifestations of the disease. This is the reason for choosing option B as the correct answer.
C: While C. diphtheriae predominantly affects the respiratory tract, it can also cause infection in other body systems. Therefore, option C is partially true, but option D is more comprehensive, as it includes the involvement of other body systems.

**Why Each Wrong Option is Incorrect:**
Option A is partially correct, as diphtheria is indeed caused by C. diphtheriae. However, it does not address the toxin production aspect, which is crucial in understanding diphtheria pathogenesis.
Option C only mentions respiratory tract infection, which is true but narrower than option D, which covers respiratory, cardiovascular, and neurological complications.
Option D covers various complications associated with diphtheria, including respiratory, cardiovascular, and neurological issues, making it the most comprehensive incorrect option.

**Clinical Pearl:**
Diphtheria is a severe infectious disease primarily caused by Corynebacterium diphtheriae. The bacterium produces a toxin (diphtheria toxin) that leads to the characteristic membrane formation in the respiratory tract, causing severe respiratory distress. The correct answer (option D) highlights the various complications associated with diphtheria, which include respiratory, cardiovascular, and neurological issues. Understanding this bacterium's role in causing diphtheria, along with the toxin production, is crucial for understanding the disease and its management.