**Core Concept**
Clostridium tetani is a Gram-positive, anaerobic, spore-forming bacterium that produces tetanospasmin, a potent neurotoxin responsible for tetanus infection. This bacterium is commonly found in soil and can enter the body through wounds.

**Why the Correct Answer is Right**
The spores of Clostridium tetani are highly resistant to environmental stress and can survive for extended periods. When a spore enters the body, it germinates into the vegetative form, which then produces tetanospasmin. This neurotoxin acts by binding to presynaptic neurons, releasing the neurotransmitter GABA, and blocking the release of inhibitory neurotransmitters, leading to muscle rigidity and spasms.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the characteristics of Clostridium tetani.
**Option B:** Clostridium difficile is a different species that causes pseudomembranous colitis, not tetanus.
**Option C:** Clostridium botulinum is another species that produces a different neurotoxin, botulinum toxin, causing botulism.

**Clinical Pearl / High-Yield Fact**
Remember that tetanus infection is characterized by muscle rigidity and spasms, and can be prevented by vaccination with tetanus toxoid.

**Correct Answer: D. Clostridium tetani**