Have you ever been frustrated with the treatment of tinea cruris? I have! Once I went through several rounds of topical anti-fungals and the stick'in rash still wouldn't go away! In my case, the reason was because the patient did not have tinea cruris. Rather, it was erythrasma which, honestly, can be extremely difficult to differentiate from tinea cruris. Doing a KOH slide would have helped I suppose! Live and learn! I've also heard that a wood's lamp will make erythrasma glow, but I think that some forms of tinea will do the same.
The treatment for erythrasma is simple: erythromycin 333 mg, one tablet by mouth 3x/day for 10 days.