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I have gone through your query and can understand your concern.
Red Itchy rashes on thighs and underside of arms can be due to
1.Rashes due to friction between body parts- This is most probable reason of itchy rashes in you. It is common in hot weather & on underside of arms & inner side of thighs as in you. Maintain hydration by using cream, oils on affected area. Use loose cotton cloth to wear
2.Rashes due to contact with allergic or poisonous agent found in herbs like
poison oak or ivy, or present in the insect’s sting or animal’s bite. Anti allergic medicament can be tried such as
benedryl,
cetrizine.
3.
Psoriasis,
impetigo and eczema are few of the diseases that are known to produce rashes or skin eruption. In these cases rashes are more generalised & not confined to underarms & inner thighs.
4. Fungal Infection- Antifungal cream can be given in case of fungal infection.
Maintain hygiene as this is very important in preventing skin eruption. Wash affected area in antiseptic solution. try not touch it
If problem persist visit a dermatologist
Hope your query is solved
Get well soon