A 65-year-old male patient presents to the hospital with the complaint of increasing shortness of breath. His condition has been progressively worsening over the past six months. He is a smoker and a bird keeper. He has one son and one daughter with asthma. On examination, there is a wheeze and coarse end-inspiratory crackles in the chest. A chest radiograph reveals diffuse non-specific changes consistent with lung disease. What is the most appropriate investigation in this case?

1.Bronchoalveolar lavage
2.Serum precipitins
3.Spirometry with reversibility
4.CT chest

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