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What is In a child with a pre-existing cardio valvular disease, antibiotic prophylaxis should be administered before a dental extraction so as to prevent: and explain in detail?
Premedication with a systemic background

Patients with Subacute Bacterial Endocarditis - antibiotic prophylaxis is needed.
Patients with abscess - antibiotic therapy.

In a child with a pre-existing cardio valvular disease, antibiotic prophylaxis should be  administered before a dental extraction so as to prevent bacterial endocarditis.
Reference: Pediatric Dentistry SECOND EDITION, Shobha Tandon; page no 163