**Core Concept**
_Clostridium tetani_ (C.tetani) is a gram-positive, anaerobic bacterium that produces the powerful neurotoxin tetanospasmin, causing tetanus infection. This bacterium is known for its characteristic drumstick-shaped spores, which play a crucial role in its survival and pathogenicity.

**Why the Correct Answer is Right**
C.tetani is an anaerobic bacterium, meaning it thrives in environments devoid of oxygen. This is in contrast to aerobic bacteria, which require oxygen to grow. The correct answer also states that C.tetani is gram-positive, indicating that it retains the crystal violet stain used in gram staining, whereas gram-negative bacteria do not. Furthermore, C.tetani is a motile bacterium, characterized by its ability to move using flagella. This motility is essential for its survival and spread within the host.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because C.tetani is anaerobic, not aerobic. Aerobic bacteria require oxygen to grow, which is not the case for C.tetani.
**Option B:** Incorrect because C.tetani is gram-positive, not gram-negative. The gram-positive cell wall retains the crystal violet stain, whereas gram-negative bacteria do not.
**Option C:** Incorrect because C.tetani is motile, not nonmotile. Motility is a key characteristic of C.tetani, allowing it to move and spread within the host.

**Clinical Pearl / High-Yield Fact**
It's essential to note that C.tetani spores can survive in the environment for extended periods, making tetanus a significant concern in wounds contaminated with soil or feces. The tetanus toxin produced by C.tetani causes muscle rigidity, spasms, and rigidity, highlighting the importance of proper wound care and tetanus vaccination.

**✓ Correct Answer:** D. An anaerobic, gram-positive, motile bacillus