**Question:** Active immunisation following exposure is given most commonly for

A. Rabies
B. Hepatitis B
C. Diphtheria
D. Measles

**Correct Answer:** A. Rabies

**Core Concept:** Active immunisation is a process where the immune system is actively stimulated to produce specific immune responses, such as antibodies and T-cells, against a pathogen or its components. This process involves exposing the individual to the pathogen or its purified components, like antigens, to initiate the immune response. Active immunisation following exposure is particularly important in situations where the individual has already been exposed to a pathogen and needs to boost their immune response to prevent or mitigate disease progression.

**Why the Correct Answer is Right:** Rabies is a viral infection caused by the Rabies virus, which belongs to the Lyssavirus genus of the Rhabdoviridae family. The virus primarily affects the central nervous system, leading to a progressive neurological disease with fatal outcome if left untreated. In high-risk settings or following an animal bite, administering active immunisation following exposure can significantly increase the chances of recovering from rabies infection. The rabies vaccine is a live, attenuated virus that mimics the natural infection, stimulating the immune system to produce rabies-specific antibodies and T-cells, which neutralize the virus and prevent disease progression.

**Why Each Wrong Option is Incorrect:**

**Option A (Hepatitis B):** Hepatitis B is caused by the Hepatitis B virus, a DNA virus that infects the liver. Unlike rabies, Hepatitis B does not have a vaccine suitable for post-exposure prophylaxis, as it involves a different virus and immune response. Hepatitis B vaccine is usually administered before exposure to prevent infection, not post-exposure prophylaxis.

**Option C (Diphtheria):** Diphtheria is caused by the Corynebacterium diphtheriae bacterium and is characterized by membrane formation in the respiratory tract. While diphtheria vaccination is essential for primary immunization and prophylaxis, it is not the correct answer for post-exposure prophylaxis for rabies.

**Option D (Measles):** Measles is a viral infection caused by the Measles virus and is characterized by fever, rash, and respiratory symptoms. Similar to Hepatitis B, measles vaccine is typically administered before exposure to prevent infection, rather than post-exposure prophylaxis for rabies.

**Clinical Pearls:**

Post-exposure prophylaxis is crucial in preventing rabies, especially in high-risk individuals like healthcare workers, veterinarians, and individuals residing in endemic areas. The correct vaccine for post-exposure prophylaxis against rabies is the Rabies immune globulin (RIG) and the Rabies vaccine (Rabies Vaccine). Both are administered together within 72 hours of suspected exposure to prevent the virus from entering the central nervous system and causing rabies.

**Why Option A is Correct:** Rabies vaccine is