## Core Concept
Bornholm's disease, also known as epidemic pleurodynia, is a condition characterized by sharp pains in the chest. It is caused by viral infections. Understanding the causative agent helps in diagnosis and management.

## Why the Correct Answer is Right
The correct answer, **Coxsackie B virus**, is a type of enterovirus known to cause a variety of diseases, including Bornholm's disease. This virus affects the muscles and the pleura, leading to the characteristic chest pain and sometimes fever. The mechanism involves the virus directly infecting muscle cells and the pleura, leading to inflammation.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because, although some bacteria can cause similar symptoms, Bornholm's disease is specifically known to be caused by a viral agent, not bacteria.
* **Option B:** This option is incorrect as there is no commonly known pathogen by this designation related to Bornholm's disease.
* **Option D:** This option is incorrect because, while Mycoplasma pneumoniae can cause respiratory infections, it is not associated with Bornholm's disease.

## Clinical Pearl / High-Yield Fact
A key point to remember is that Bornholm's disease often presents with pleuritic chest pain and can be associated with fever and muscle weakness. It is diagnosed based on clinical presentation and sometimes confirmed by serological tests for Coxsackie B virus.

## Correct Answer: C. Coxsackie B virus.