A seven and a half years old child presents with non productive cough and mild stridor for 6 months. On oral antibiotics, patient is improving but suddenly develops wheezing, productive cough and mild fever. X-ray shows hyperlucency and PFT shows obstructive curve. Most probable diagnosis is

Bronchiolitis obliterans causes permanent obstruction following an insult (often viral infection). The progression from mild stridor to obstructive PFT with hyperlucency on imaging, along with initial improvement on antibiotics then worsening with wheezing, fits BO perfectly. Post-viral syndrome is too vague, and other options don't match.