Okay, let's tackle this question about Corynebacterium diphtheriae. First, I need to recall the key points about this bacterium. Corynebacterium diphtheriae is the causative agent of diphtheria. The question is asking which statement is true about it. Since the options aren't provided, I'll have to think about the common true facts associated with this organism.

Core Concept: The core concept here is the characteristics of Corynebacterium diphtheriae, including its morphology, pathogenesis, and clinical features. The bacterium is a Gram-positive, non-spore-forming, non-motile bacillus. It's known for producing diphtheria toxin, which is essential for the disease's pathogenesis.

Why the Correct Answer is Right: The correct answer would be related to the toxin production. The diphtheria toxin is an exotoxin that inhibits protein synthesis by ADP-ribosylating elongation factor 2 (EF-2). This leads to cell death and the formation of a pseudomembrane in the throat. The toxin is encoded by a prophage gene, so only lysogenic strains produce it. This is a key point because not all strains produce the toxin, which differentiates pathogenic from non-pathogenic strains.

Now, for the incorrect options. Let's think about common distractors. One might confuse C. diphtheriae with other Corynebacterium species. For example, C. diphtheriae is a bacillus, not a coccus. Another wrong option could be about its Gram staining; it's Gram-positive, not negative. Also, some might think it's a spore-former, but it's not. Another possible wrong fact is about its motility; C. diphtheriae is non-motile. Additionally, the pseudomembrane formation is a hallmark, but if the option incorrectly states it's a true membrane, that's wrong.

Clinical Pearl: Remember that the diphtheria toxin is crucial for virulence. The presence of the toxin differentiates pathogenic from non-pathogenic strains. Also, the disease is prevented by the diphtheria toxoid vaccine, which is part of the DTaP or Tdap vaccines.

Putting this all together, the correct answer would be about toxin production or the pseudomembrane. Common true facts include the role of the toxin, the pseudomembrane, and the fact that it's a Gram-positive bacillus. Incorrect options would involve Gram-negative, spore-forming, motile, or not producing the toxin. The clinical pearl emphasizes the importance of the toxin in pathogenesis and the vaccine's role in prevention.

**Core Concept**
Corynebacterium diphtheriae is a Gram-positive, non-spore-forming, non-motile bacillus causing diphtheria. Its hallmark is production of a potent exotoxin (diphtheria toxin) via a prophage-encoded gene, essential for pseudomembrane formation and systemic toxicity.

**Why the Correct Answer is Right**
The diphtheria toxin inhibits protein synthesis by ADP-ribosylating elongation factor 2 (EF-2), leading to cell death. Toxin-producing strains (lysogenic for β-corynephage) cause disease; non-tox

✓ Correct Answer: B. Toxin production is dependent upon critical concentration of iron