Which of the following vaccines is a live attenuated virus?

The answer is correct because the Measles, Mumps, and Rubella (MMR) vaccine contains live attenuated viruses. Live attenuated vaccines are created from living pathogens that have been weakened so they do not cause disease in healthy individuals. This type of vaccine elicits a strong immune response, typically providing long-lasting immunity. In the case of MMR, the vaccine includes weakened strains of the measles virus, mumps virus, and rubella virus, allowing the body to build immunity without causing the full-blown diseases.

In contrast, the other vaccines listed employ different mechanisms. The Hepatitis B vaccine is a subunit vaccine, which means it contains pieces of the virus (specifically, the surface protein) rather than live viral components. The Adenovirus vaccine can be considered a live vaccine but generally functions differently from traditional live attenuated vaccines like MMR. Anthrax vaccine is an inactivated vaccine, meaning it contains killed bacteria or their components and does not involve live organisms. Understanding these distinctions is crucial for recognizing how various vaccines stimulate immune responses.