## **Core Concept**
Tetanospasmin is a toxin produced by the bacterium *Clostridium tetani*, which causes tetanus. The toxin is encoded by a gene located on a plasmid. Understanding the genetic basis of toxin production is crucial for developing treatments and preventive measures.

## **Why the Correct Answer is Right**
The correct answer, , indicates that the tetanospasmin encoding gene is located on a plasmid. *Clostridium tetani* produces tetanospasmin, which is responsible for the symptoms of tetanus. This toxin is encoded by a gene carried on a plasmid, which is a small DNA molecule within a cell that is physically separated from chromosomal DNA and can replicate independently.

## **Why Each Wrong Option is Incorrect**
* **Option A:** - This option is incorrect because chromosomal DNA typically carries essential genes for bacterial survival and function, not usually toxin genes that can be transferred independently.
* **Option B:** - This option might seem plausible but is incorrect because while bacteriophages can carry genes, tetanospasmin encoding genes are specifically known to be on a plasmid.
* **Option D:** - This option is incorrect because, similar to option A, the primary chromosomal DNA is not where toxin genes like tetanospasmin are typically located.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that *Clostridium tetani* spores are commonly found in soil and the gastrointestinal tracts of animals. Tetanus toxin acts by blocking neurotransmitter release, leading to muscle stiffness and spasms. The disease can be prevented by vaccination with tetanus toxoid, which induces immunity against the toxin.

## **Correct Answer:** . Plasmid.