<prompt> A 3-month-old child has moderate fever and non productive cough and mild dyspnea. After a course of antibiotic, the condition of the child improved transiently but he again develops high fever, productive cough and increased respiratory distress. CXR shows hyperlucency and PFT shows obstructive pattern. Most probable diagnosis is:Alveolar microlithiasisPostviral syndromeFollicular bronchitisBronchiolitis obliterans  <response>: d. Bronchiolitis obliteransRef: Nelsons 20e p 2044-2047, Ghai 8e p 381-382Bronchiolitis obliterans BOWhat is it?A chronic obstructive lung disease of bronchioles and smaller airways, that results from an insult to the lower respiratory tract leading to fibrosis of small airwaysPathogenesis After the initial insult, inflammation affecting terminal bronchioles, respiratory bronchioles and alveolar ducts can result in the obliteration of the airway lumen Epithelial damage resulting in abnormal repair is characteristic of BO Complete or partial obstruction of the airway lumen can result in air trapping or atelectasisClinical features Cough, fever, cyanosis, dyspnea, chest pain and respiratory distress followed by initial improvement It is easily confused with pneumonia, bronchitis, or bronchiolitis Progression of disease with increasing dyspnea, chronic cough, sputum production and wheezing seenDiagnosis Chest X-ray may be normal or can demonstrate hyperlucency and patchy infiltrates Occasionally, a Swyer-James syndrome unilateral hyperlucent lung develops PFT shows obstructive pattern 