**Core Concept**
_Cornebacterium diphtheriae_ is a Gram-positive, facultative anaerobic bacterium that causes diphtheria, a highly contagious and potentially life-threatening disease. The bacterium produces a potent toxin that leads to the characteristic symptoms of diphtheria.

**Why the Correct Answer is Right**
The diphtheria toxin is produced by the bacterium through a complex process involving the diphtheria toxin gene (tox gene). This gene is encoded on a lysogenic bacteriophage (a virus that infects bacteria) called the corynephage β. The toxin is then secreted by the bacterium and causes damage to the respiratory tract by inhibiting protein synthesis. This leads to the characteristic membrane formation and eventual tissue necrosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because _Cornebacterium diphtheriae_ is actually a Gram-positive bacterium, not Gram-negative. The Gram stain is a critical diagnostic tool in identifying bacteria, and this distinction is essential in differentiating _C. diphtheriae_ from other bacteria.
* **Option B:** This option is incorrect because the diphtheria toxin is not produced by the bacterium's cell wall. Instead, it is produced through a complex process involving the diphtheria toxin gene and the corynephage β bacteriophage.
* **Option C:** This option is incorrect because the diphtheria toxin does not cause damage to the respiratory tract through an allergic reaction. Instead, it causes damage through the inhibition of protein synthesis, leading to tissue necrosis.

**Clinical Pearl / High-Yield Fact**
_Cornebacterium diphtheriae_ is a classic example of a bacterium that produces a potent toxin, highlighting the importance of understanding the relationship between bacteria, toxin production, and disease. This concept is crucial in the diagnosis and treatment of various bacterial infections.

**Correct Answer:** D.