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What is The most common site of nosebleed in a child - and explain in detail?
Epistaxis is relatively common in children.
The most common cause of epistaxis is the mucosal maceration caused by upper respiratory infection and nose picking (trauma).

The usual site of bleeding in the anterior nasal septum, i.e. Kiesselbach's area (Little's area) “Most common cause of epistaxis in children is nose picking in the little’s area " — Pediatric emergency.