I had this rash for a year or more, both under the arm and in the groin. Tried several prescription ointments that helped for a time. The most effective was Nizoral. Eventually I wound up with lymphoma, which required chemo and radiation therapy. The lymphoma returned after four years, and I had to go thru six additional chemo therapy sessions. At present I am in remission from the lymphoma. If you have not already done so, have a biopsy of the rash at once! I still use Nizoral ointment today, whenever any rash appears.