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Itchy white spot on vagina is suggestive of fungal infection
It could be due to improper hygiene after sex,perspiration and scratching.
Take proper local care,keep that area dry,use medicated soap.
Apply local anti fungal,steroid
cream.Oral fluconazole-150 will be helpful.
Take oral levocetrizine for itching.Consult your physician for proper medical management.