We've did a skin biopsy due to a rash. results were negative for any skin problems. but cortisone shot cleared them up for 3 wks. what would be cause?

Different reasons: Sometimes with the biopsy it is hard to determine the cause of a non specific rash When cortisone works it usually indicates an inflammatory process and the cortisone in some people only lasts a few weeks If it comes back and it is worse then get a second opinion and another biopsy Sometimes if you tell provider the shot worked they will recommend a steroid cream if the biopsy is negative