The main diagnoses to consider are
Ringworm: erythema on the inner thighs 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 itself as an oedematous erythema, with similar rashes on other areas except the inner thighs, which fades within a few hours and leaves no trace after fading.