**Core Concept**
Tetanus is a bacterial infection caused by Clostridium tetani, characterized by muscle rigidity and spasms. The spores of the bacteria are commonly found in soil and can enter the body through wounds.

**Why the Correct Answer is Right**
The pathogenesis of tetanus involves the production of tetanospasmin, a potent neurotoxin that blocks the release of neurotransmitters, such as glycine and GABA, which inhibit muscle contraction. This leads to the characteristic muscle rigidity and spasms. The toxin affects the central nervous system, specifically the spinal cord and brainstem, causing the symptoms of tetanus.

**Why Each Wrong Option is Incorrect**
**Option A:** Tetanus is caused by a protozoan, which is incorrect because tetanus is caused by the bacterium Clostridium tetani.
**Option B:** Tetanus is transmitted through the air, which is incorrect because tetanus is typically acquired through wounds contaminated with the spores of C. tetani.
**Option C:** Tetanus is treated with antivenom, which is incorrect because tetanus is treated with tetanus immunoglobulin and wound debridement.

**Clinical Pearl / High-Yield Fact**
Tetanus is a vaccine-preventable disease, and immunization is crucial to prevent tetanus infections, especially in individuals with wounds or injuries.

**Correct Answer:** A. Tetanus is caused by a protozoan.