Question category: Respiratory
A 48 year old farmer presents to his GP. He states that often in the evening following his morning duties of milking the cows and forking hay he feels unwell with a fever, cough, chest tightness and shortness of breath. This then resolves and he feels well again by the morning. On examination he has bibasal inspiratory crackles. A CXR shows diffuse micronodular interstitial shadowing. Which of the following is the most likely diagnosis?