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The food he eats could be acidic after digestion and can also be a sign of lactose intolerance
manifested by redness
of the skin and erosions of the skin near the anus. Does he get diarrhoea
after drinking milk or milk products. Give him more water to dilute the contents. Give him more fresh fruits and green leafy vegetables to make the contents more alkaline. You can apply antibiotic ointment and Petroleum jelly to that area to give relief. Consult your Paediatrician if your child isn't gaining weight, irritable, the rash
is not resolving.