## **Core Concept**
Subunit vaccines are a type of vaccine that uses only specific components of a pathogen, such as proteins or polysaccharides, rather than the entire pathogen. This approach helps to reduce the risk of adverse reactions while still inducing an immune response. Subunit vaccines can be made through various methods, including purification from the pathogen or recombinant DNA technology.

## **Why the Correct Answer is Right**
The correct answer, **B. Hepatitis B vaccine**, is a subunit vaccine produced through recombinant DNA technology. The Hepatitis B vaccine is made by inserting the gene for the Hepatitis B surface antigen (HBsAg) into yeast cells, which then produce the HBsAg protein. This protein is then purified and used as the vaccine. This type of vaccine is highly specific and has a good safety profile.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify a known subunit vaccine.
- **Option C:** This option is incorrect as it does not represent a subunit vaccine.
- **Option D:** This option is incorrect because it does not accurately describe a subunit vaccine.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that subunit vaccines, like the Hepatitis B vaccine, are particularly useful for individuals who are at high risk of severe disease but may not respond well to live or attenuated vaccines, such as immunocompromised patients.

## **Correct Answer:** . Hepatitis B vaccine