**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the formation of a pseudomembrane in the throat, which can lead to respiratory obstruction. The 'bull neck' appearance is a clinical manifestation of this condition.

**Why the Correct Answer is Right**
The 'bull neck' appearance is primarily due to the accumulation of edematous tissue and the pseudomembrane in the neck region, which occurs as a result of the bacterial infection. This swelling is caused by the release of toxins by the bacteria, including the exotoxin that damages the tissues and leads to the formation of the pseudomembrane. The toxin also causes the release of histamine, which contributes to the edema.

**Why Each Wrong Option is Incorrect**
**Option A:** This option does not directly relate to the 'bull neck' appearance in Diphtheria. While C. diphtheriae infection can cause systemic symptoms, the 'bull neck' is a localized manifestation.
**Option B:** This option is incorrect as the 'bull neck' appearance is not primarily due to the formation of a lymph node. While lymphadenopathy can occur in Diphtheria, it is not the cause of the 'bull neck' appearance.
**Option C:** This option does not accurately describe the 'bull neck' appearance in Diphtheria. While the pseudomembrane can cause respiratory obstruction, it is not the primary cause of the swelling in the neck region.

**Clinical Pearl / High-Yield Fact**
The 'bull neck' appearance in Diphtheria is a classic clinical sign that requires prompt recognition and treatment to prevent complications. It is essential to remember that Diphtheria can cause both local and systemic symptoms, and prompt treatment with antibiotics and diphtheria antitoxin is crucial to prevent respiratory failure and other complications.

**Correct Answer:** D.