**Core Concept**
Diphtheria is a bacterial infection caused by Corynebacterium diphtheriae, characterized by the formation of a thick, gray membrane in the throat. The prognosis of diphtheria depends on the type of toxin produced by the bacteria.

**Why the Correct Answer is Right**
The toxin produced by Corynebacterium diphtheriae is responsible for the clinical manifestations of the disease. The toxin, also known as diphtheria toxin, is a potent exotoxin that inhibits protein synthesis in host cells. The membrane toxin, on the other hand, is responsible for the formation of the characteristic membrane in the throat. However, the toxin that suggests the worst prognosis is the **toxin-producing strain**, which produces the diphtheria toxin. This type of strain is associated with a higher mortality rate and more severe complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Non-toxin producing strain - This type of strain does not produce the diphtheria toxin and is associated with a milder course of the disease.

**Option B:** Membrane-producing strain - While this type of strain produces the characteristic membrane in the throat, it is not associated with the worst prognosis.

**Option C:** Pneumonic diphtheria - This is a complication of diphtheria, not a type of diphtheria.

**Clinical Pearl / High-Yield Fact**
Remember that the toxin-producing strain of Corynebacterium diphtheriae is associated with the worst prognosis, and vaccination is the most effective way to prevent diphtheria.

**Correct Answer:** D. Toxin-producing strain.