The commonest cause of U/L mucopurulent rhinorrhoea in a child is -
"A unilateral nasal discharge is nearly always due to a foreign body and if the discharge has an unpleasant smell, it is pathognomonic". — Logan Turner 10th/e p. 63
"If a child presents with unilateral, foul smelling nasal discharge, the foreign body must be excluded". — Dhingra 5th/e p. 176
Foreign body
Adenoids which are blocking the airways
Deviated nasal septum
Inadequately treated acute frontal sinusitis