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These patches are related to vitamin B deficiency and gastric discomfort. Sometimes, they also have superadded infection of bacterial or fungal origin. I would advise you to start a multivitamin syrup, deworm the child and apply a mouth paint (Candid mouth paint) for 4 to 5 days.
Syrup A to Z (multivitamin) one teaspoon once daily for 30 days, syrup Zentel (Albendazole) 2 teaspoons single stat dose, Enterogermina suspension (probiotic) one teaspoon twice daily for three days. Candid mouth paint local application for 4 to 5 days.