Which of the following is the commonest cause of farmer’s lung?
**Core Concept**
Farmer's lung is a type of hypersensitivity pneumonitis, a lung disease caused by the inhalation of specific antigens. In this case, the disease is triggered by exposure to organic dusts found in agricultural environments.
**Why the Correct Answer is Right**
The correct answer, Thermophilic actinomycetes, is a type of bacteria that grows in composted hay, silage, and other agricultural products. When inhaled, these bacteria cause an immune-mediated response, leading to inflammation and scarring in the lungs. The presence of these bacteria in the environment is the primary risk factor for developing farmer's lung.
**Why Each Wrong Option is Incorrect**
**Option A:** Aspergillus fumigatus is a common cause of allergic bronchopulmonary aspergillosis (ABPA) and aspergillosis, but it is not the primary cause of farmer's lung.
**Option B:** Histoplasma capsulatum is a fungus that causes histoplasmosis, primarily affecting the lungs, but it is not associated with farmer's lung.
**Option C:** Cryptococcus neoformans is a fungus that causes cryptococcosis, primarily affecting immunocompromised individuals, but it is not related to farmer's lung.
**Clinical Pearl / High-Yield Fact**
Farmer's lung is a classic example of an occupational lung disease, highlighting the importance of respiratory protection and proper ventilation in agricultural settings.
**Correct Answer:** C. Thermophilic actinomycetes.