Pappu 2 yrs old boy presents in the causality with H/O sudden onset of respiratory difficulty & stridor on auscultation, ↓ed breath sound & wheeze on the RT side. The X-ray shows RT opaque hemithorax. What will be diagnosis –
Foreign body aspiration should always be considered as a potential cause of stridor and airway obstruction in children".  

Opaque hemithorax in this child is due to atelectasis which can occur after foreign body aspiration.
So, you should keep in mind that foreign body aspiration can cause opaque hemithorax as well as hyperinflation.
Pneumothorax
Acute epiglottitis
Massive pleural effusion
Foreign body aspiration