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Big red spots on thigh ,seems to be a fungal infection
.And it is also associated with itching .The second possibility I would say would be a allergic reaction
(less likely) .
You can first start with using anti fungal creams and tablets for first 2 weeks and then decide whether to continue it or not.The usual course is of 4 weeks .
If you don't respond in first 2 weeks then switch over to a steroid
cream.Along with this you can take any anti histaminic tablet like cetrizine
to relieve itching.
But as these medicines are prescription based ,so you should consult a dermatologist
and should not try it yourself.
I'll be available to guide your further if needed.