Those that know the channels of elimination could tell you what organ is related to what area of the skin, I do not know. Probably chinese type information would give the best guesses.
The one thing we know, where the rash appears is the weakest spot of the organ we call skin. In all cases, you stop the constipation of the bowel and the skin will clear up. To do this takes habit change and learning about foods, BOOK I is such an educational book.