**Question:** Live viral vaccine is

A. attenuated
B. weakened
C. killed
D. inactivated

**Correct Answer:** A. attenuated

**Core Concept:** A live viral vaccine is a form of immunization that uses a weakened form of a virus that is still capable of multiplying in the host but does not cause the disease. This leads to the production of specific immune responses, including the activation of both cellular and humoral immunity.

**Why the Correct Answer is Right:** A live viral vaccine is considered right because it mimics the natural infection by exposing the host to a virus that can replicate but does not cause disease. This allows the immune system to recognize and respond to the viral antigens, resulting in the development of immunity.

**Why Each Wrong Option is Incorrect:**

B. Weakened (Option B) refers to a vaccine where the virus is weakened but still retains its ability to replicate in the host. Although this may induce immunity, it is less preferred compared to attenuated vaccines as there is a higher risk of causing the disease in immunocompromised individuals.

C. Killed (Option C) vaccines involve the inactivation of the virus, rendering it unable to replicate or cause disease. While these vaccines are safe for immunocompromised individuals, the immune response is usually weaker compared to live viral vaccines, leading to lower efficacy.

D. Inactivated (Option D) vaccines are similar to killed vaccines, where the virus is inactivated or destroyed, preventing replication and disease. However, inactivated vaccines generally provide weaker immune responses compared to live viral vaccines or killed vaccines.

**Clinical Pearl:** Live viral vaccines are generally preferred because they provide a stronger and more durable immune response compared to killed or inactivated vaccines. However, they must be used cautiously in immunocompromised individuals to avoid causing the disease. Inactivated vaccines, on the other hand, are safer but provide a weaker immune response.