**Core Concept**
Tetanus toxin is a potent neurotoxin produced by the bacterium *Clostridium tetani*. Toxigenicity refers to the ability of a bacterium to produce a toxin. In the case of tetanus, the toxin is responsible for the characteristic muscle spasms and rigidity associated with the disease.

**Why the Correct Answer is Right**
The toxigenicity of tetanus is determined by the presence of the tetanus toxin gene, which is encoded by the tetanus toxin gene (tetX). This gene is responsible for encoding the tetanus toxin, which is then synthesized and secreted by the bacterium. The tetanus toxin works by binding to gangliosides on the surface of neurons, which leads to the inhibition of neurotransmitter release, resulting in muscle spasms and rigidity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the toxigenicity of tetanus is not determined by the presence of a specific enzyme, but rather by the presence of the tetanus toxin gene.
**Option B:** This option is incorrect because the toxigenicity of tetanus is not determined by the bacterium's ability to adhere to a specific receptor, but rather by the presence of the tetanus toxin gene.
**Option C:** This option is incorrect because the toxigenicity of tetanus is not determined by the bacterium's ability to produce a specific protein, but rather by the presence of the tetanus toxin gene.

**Clinical Pearl / High-Yield Fact**
The tetanus toxin is a potent neurotoxin that can cause significant morbidity and mortality if not treated promptly. The toxin works by binding to gangliosides on the surface of neurons, leading to the inhibition of neurotransmitter release and resulting in muscle spasms and rigidity.

**Correct Answer: C. Blood agar plate. Tetanus toxin production can be detected by a mouse bioassay or by using a toxin detection kit on a blood agar plate.**