**Core Concept**
Clostridium tetani is a species of bacteria that belongs to the genus Clostridium, a group of gram-positive, rod-shaped bacteria. These bacteria are known for their role in producing tetanus toxin, which causes tetanus, a potentially life-threatening disease.

**Why the Correct Answer is Right**
Clostridium tetani bacteria are obligate anaerobes, meaning they grow in the absence of oxygen. This characteristic is crucial for their survival and pathogenesis, as they form spores that can germinate in low-oxygen environments, such as wounds or the gut. The bacteria produce a potent neurotoxin, tetanospasmin, which blocks neurotransmitter release and leads to the characteristic muscle spasms and rigidity associated with tetanus.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while Clostridium tetani is a gram-positive rod, it is not a gram-negative bacterium.
**Option B:** This option is incorrect because Clostridium tetani does not have a unique cell wall structure; it is simply a gram-positive rod.
**Option C:** This option is incorrect because Clostridium tetani is not a mycobacterium; it is a distinct genus within the family Clostridiaceae.

**Clinical Pearl / High-Yield Fact**
It's essential to note that Clostridium tetani spores are ubiquitous in the environment and can be found in soil, dust, and the gastrointestinal tract of animals. This widespread distribution emphasizes the importance of proper wound care and hygiene in preventing tetanus infections.

**Correct Answer: D. Obligate anaerobes.**