If a rash has you stumped, you could try using Google Lens (or Google reverse image search) – open the Google app and click the little multi-colored camera, then with your patient’s permission train your camera on the rash and snap the shutter. Visual matches from websites and suggested diagnoses will appear below the photo. This is a screenshot of me searching a rash of my own (it was not hives, the first suggested diagnosis, because it wasn’t transient, so obviously any suggested diagnoses are just jumping off points for further research.