Farmer&;s lung is caused by
**Core Concept**
Farmer's lung is a type of hypersensitivity pneumonitis, a lung disease caused by an allergic reaction to inhaled particles. This condition is characterized by an abnormal immune response to specific antigens, leading to inflammation and damage in the lungs.
**Why the Correct Answer is Right**
The correct answer is related to the presence of thermophilic actinomycetes, specifically *Micropolyspora faeni*, in the lung tissue of affected individuals. These microorganisms are commonly found in moldy hay and other agricultural products. When inhaled, the antigens from these microorganisms trigger an immune response, leading to the characteristic symptoms of farmer's lung. The condition is often associated with exposure to moldy hay in agricultural settings.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not directly related to the cause of farmer's lung. While aspergillus species can cause other types of lung infections, they are not the primary cause of farmer's lung.
**Option B:** Histoplasma capsulatum is a fungus that causes histoplasmosis, a different type of lung infection. It is not associated with farmer's lung.
**Option C:** This option is not a known causative agent of farmer's lung. The correct answer is a specific type of thermophilic actinomycete.
**Clinical Pearl / High-Yield Fact**
Farmer's lung is a classic example of an occupational lung disease, highlighting the importance of proper ventilation and protective measures in agricultural settings to prevent exposure to inhaled antigens.
**Correct Answer: D. *Micropolyspora faeni***