The main diagnoses to consider are.
Tinea capitis: erythema on the inner thigh near the area close to the anus, gradual peripheral spread, well-defined borders, localised mossy skin after scratching, positive fungal test, aggravated by topical application of creams containing glucocorticoids.
Eczema: a multi-linear, symmetrical, recurrent, itchy rash that is effectively relieved by the use of glucocorticoid creams.
Urticaria: manifests as an oedematous erythema, with a similar rash on other parts of the body except the inner thighs; the rash fades within a few hours and leaves no trace after fading.