A child with acute respiratory distress showing hyperinflation of the unilateral lung in X-ray is due to –
Sudden onset of respiratory distress with hyperinflation of lung suggest the diagnosis of foreign body aspiration.

"Foreign body aspiration should always be considered as a potential cause of stridor and airway obstruction in children".

Congenital lobar emphysema can also cause respiratory distress with hyperinflation of lung. But congenital lobar emphysema produces respiratory distress immediately after birth i.e. in neonatal periods.
Staphylococcal bronchopneumonia
Aspiration pneumonia
Congenital lobar emphysema
Foreign body aspiration