This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: ENT
The commonest cause of U/L mucopurulent rhinorrhoea in a child is - 
Select and explain. 
A. Foreign body, B. Adenoids which are blocking the airways, C. Deviated nasal septum, D. Inadequately treated acute frontal sinusitis
A: Foreign body
Answer: A: Foreign body 
Explanation: "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