## Core Concept
Swine flu, also known as swine influenza, is primarily caused by influenza viruses that are typically endemic in pigs. The most common cause of swine flu in humans is a subtype of the influenza A virus.

## Why the Correct Answer is Right
The correct answer, **Influenza A (H1N1)**, is right because the 2009 swine flu pandemic, which was a global outbreak, was caused by a novel **Influenza A (H1N1)** virus. This particular strain, also known as **H1N1pdm09**, was first identified in Mexico and the United States in March and April 2009. It is a type of influenza A virus that can infect both pigs and humans, making it a zoonotic agent.

## Why Each Wrong Option is Incorrect
* **Option A: Influenza B** - This type of influenza virus primarily causes seasonal epidemics in humans and is not commonly associated with swine flu.
* **Option C: Influenza C** - This type mainly infects pigs and some animal species but is not commonly linked to swine flu outbreaks in humans.
* **Option D: Respiratory Syncytial Virus (RSV)** - RSV is a major cause of lower respiratory tract infections in young children and older adults but is not associated with swine flu.

## Clinical Pearl / High-Yield Fact
A key point to remember is that swine flu, caused by **Influenza A (H1N1)**, presents similarly to seasonal influenza and can lead to severe complications, especially in high-risk groups like the elderly, young children, and those with certain chronic health conditions.

## Correct Answer Line
**Correct Answer: B. Influenza A (H1N1).**