## **Core Concept**
The question tests knowledge of **hypersensitivity pneumonitis**, a category of lung diseases that result from an immune reaction to the inhalation of specific antigens. Farmer's lung is a classic example of hypersensitivity pneumonitis caused by exposure to certain microorganisms in agricultural environments.

## **Why the Correct Answer is Right**
The correct answer, **Saccharopolyspora rectivirgula (formerly known as Micropolyspora faeni)**, is a type of thermophilic actinomycete. This bacterium is commonly found in moldy hay and grains. When farmers inhale the spores of this organism, they can trigger an immune response leading to inflammation in the lungs, characteristic of hypersensitivity pneumonitis. The pathogenesis involves both immune complex-mediated and cell-mediated hypersensitivity reactions.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While certain species of *Aspergillus* can cause lung diseases (like aspergillosis), they are not the primary cause of Farmer's lung.
- **Option B:** *Histoplasma capsulatum* causes histoplasmosis, a different type of lung infection, primarily associated with bird or bat droppings.
- **Option D:** *Mycobacterium tuberculosis* causes tuberculosis, a bacterial infection that primarily affects the lungs but is distinct from hypersensitivity pneumonitis.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that Farmer's lung is a preventable condition through avoidance of exposure to moldy hay and use of protective gear like masks. Early recognition and removal from exposure can prevent progression to chronic lung disease.

## **Correct Answer:** C. Saccharopolyspora rectivirgula.