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If the surrounding skin near the inner region if thighs and groin affected similarly. Is there itching. Any fever
. It could be a fungal infection
of the skin. You should keep the area clean and dry. Take bath regularly. Change underwear daily. Avoid wearing tight underwear. Change underwear daily to avoid sweat accumulation. You can apply talcum powder for relief but proper anti-fungal treatment is required by a Dermatologist
. Do not delay.