Antigenic variation is/are seen in-a) Treponema pallidumb) Neisseriac) Corynebacteriumd) Borrelia recurrentis

## **Core Concept**
Antigenic variation refers to the ability of certain pathogens to change their surface antigens, allowing them to evade the host's immune system. This mechanism is crucial for the survival and persistence of these pathogens within a host.

## **Why the Correct Answer is Right**
The correct answer involves identifying which of the listed pathogens is known for antigenic variation. **Borrelia recurrentis**, the causative agent of relapsing fever, is a prime example. It changes its variable surface antigens (VSAs), specifically the variable major proteins (Vmps), to evade the host's immune response, leading to recurrent episodes of fever.

## **Why Each Wrong Option is Incorrect**
- **Option A: Treponema pallidum** - While *Treponema pallidum* has mechanisms to evade the immune system, it is not primarily known for antigenic variation in the same way as *Borrelia recurrentis*. Its pathogenesis involves immune evasion but through different strategies.
- **Option B: Neisseria** - *Neisseria* species, such as *Neisseria gonorrhoeae*, do exhibit antigenic variation, particularly in their pili proteins, which helps in evading the immune system. However, the question seems to focus on a more direct and notable example.
- **Option C: Corynebacterium** - *Corynebacterium* species are not typically highlighted for antigenic variation as a key pathogenic mechanism. Some species may have surface proteins that can vary, but this is not a hallmark feature.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Borrelia recurrentis** causes relapsing fever, characterized by recurring episodes of fever, and this is directly related to its ability to undergo antigenic variation. This feature makes it a classic example in the study of immune evasion strategies by pathogens.

## **Correct Answer:** D. Borrelia recurrentis