Okay, let's tackle this question about Corynebacterium diphtheriae. The user wants to know which statement is not true. First, I need to recall the key facts about this organism.

Corynebacterium diphtheriae is a Gram-positive, non-spore-forming, non-motile, aerobic bacillus. It's the causative agent of diphtheria. The bacteria produce a toxin that's responsible for the severe symptoms, like the pseudomembrane in the throat. The toxin inhibits protein synthesis by ADP-ribosylating elongation factor 2 (EF-2).

Now, the options are missing, so I have to think of common misconceptions or incorrect facts about this bacterium. Let's consider possible wrong options. For example, someone might incorrectly say it's Gram-negative, which is a common mistake. Or maybe that it's motile, but it's not. Another possible wrong fact could be that it's a spore-former. Also, the toxin's mechanism is crucial here. If an option states that the toxin causes cell lysis directly, that's incorrect because it inhibits protein synthesis instead.

Let me structure the explanation. The core concept here is the basic biology of C. diphtheriae and the pathogenesis of diphtheria. The correct answer would be the statement that contradicts these facts. For the incorrect options, I need to explain why each is wrong. For example, if an option claims it's Gram-negative, that's false because it's Gram-positive. If another says it's motile, that's incorrect as it's non-motile. Then the clinical pearl would emphasize the toxin's mechanism and the importance of the vaccine.

**Core Concept**
*Corynebacterium diphtheriae* is a Gram-positive, non-motile, non-spore-forming aerobic bacillus. It causes diphtheria via a toxin that ADP-ribosylates elongation factor 2 (EF-2), inhibiting protein synthesis in host cells. The pseudomembrane in the throat is a hallmark of infection.

**Why the Correct Answer is Right**
The false statement is likely that *C. diphtheriae* is Gram-negative. This organism is definitively Gram-positive, appearing as club-shaped bacilli with a characteristic "Chinese letter" arrangement. Its toxin, produced by beta-hemolytic strains via phage-mediated genes, is critical for pathogenesis. Other false claims might involve motility or spore formation, which are inconsistent with its biology.

**Why Each Wrong Option is Incorrect**
**Option A:** If claiming *C. diphtheriae* is Gram-negative, this is incorrect because it stains Gram-positive.
**Option B:** If stating the toxin causes cell lysis directly, this is wrong; it inhibits protein synthesis.
**Option C:** If asserting it is motile, this is false; *Corynebacterium* species are non-motile.
**Option D:** If suggesting it forms spores, this is incorrect; spore formation is absent in this genus.

**Clinical Pearl / High-Yield Fact**
Diphtheria toxoid vaccination (DTaP) prevents disease by neutralizing the toxin. Remember: **"Diphtheria toxin = EF-2 ADP

✓ Correct Answer: D. Toxigenicity is mediated by chromosomal change