## Core Concept
Farmer's lung is a type of hypersensitivity pneumonitis, an immune system disorder that occurs in response to the inhalation of specific antigens. This condition is commonly seen in individuals who work with organic materials such as hay, grains, or animal products. The antigens responsible are usually thermophilic actinomycetes.

## Why the Correct Answer is Right
The correct answer, **D. Thermophilic actinomycetes**, is right because these microorganisms are commonly found in moldy hay, grains, and other organic materials that farmers handle. When these antigens are inhaled, they can trigger an immune response leading to inflammation in the lungs, characteristic of hypersensitivity pneumonitis or farmer's lung.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a known cause of farmer's lung.
- **Option B:** This option is incorrect as it does not relate to the known causes of farmer's lung.
- **Option C:** This option is incorrect because, similar to options A and B, it does not accurately represent a cause of farmer's lung.

## Clinical Pearl / High-Yield Fact
A key point to remember is that farmer's lung is a form of occupational lung disease. Prevention involves reducing exposure to moldy hay and using protective gear. Early recognition and removal from exposure can significantly improve outcomes.

## Correct Answer: D. Thermophilic actinomycetes