## **Core Concept**
Live attenuated vaccines are made from weakened forms of the pathogen they aim to protect against. This weakening reduces the virulence of the pathogen, preventing the disease it causes while still triggering an immune response. Examples of live attenuated vaccines include those for measles, mumps, rubella, and varicella.

## **Why the Correct Answer is Right**
The Hepatitis A vaccine is an inactivated vaccine, not a live attenuated one. Inactivated vaccines contain a killed or inactivated form of the pathogen. This type of vaccine cannot cause the disease itself but still induces an immune response. The Hepatitis A vaccine is produced by inactivating the virus with formaldehyde, making it safe for use in individuals with weakened immune systems.

## **Why Each Wrong Option is Incorrect**
- **Option A:** BCG (Bacillus Calmette-Guérin) vaccine is a live attenuated vaccine used primarily against tuberculosis. It is made from a weakened form of the *Mycobacterium bovis* bacterium.
- **Option B:** The measles vaccine is a live attenuated vaccine. It is produced by growing the measles virus in a cell culture medium where it is weakened.
- **Option D:** The varicella vaccine is also a live attenuated vaccine, protecting against chickenpox. It is made from a weakened form of the varicella-zoster virus.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that live attenuated vaccines are generally contraindicated in individuals with severely compromised immune systems due to the risk of causing the disease itself. In contrast, inactivated vaccines like the Hepatitis A vaccine can be safely administered to such individuals.

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