I have skin rash groin age 17. dermatologist visit, biopsy revealed nothing significant?

PAS or giemsa stain. it's either a dermatophyte (fungal infection) or erythrasma (caused by coryneform bacteria), most likely. Could be other things like Hailey-Hailey. Itchy? Are you a diabetic? Sometimes, pretreated fungal infections reveal no fungi.