I am probably still a "rookie" compared to many contributors here on the board, as I have been in the business since May 2002.
I have seen many a rash in my day. I have had customers with pseudomonas.
It is very difficult to know exactly the cause of a rash. All we can do is trial and error or elimination processes to isolate causes. Doctors are really in a hard position too. They often say stop using the hot tub, or start writing prescriptions until something works.
At the end of the day the most common "cure" for customers with rashes has been a sanitizer switch. Often when this is recommended, I suggest to stop using any and all "accessory" products (stain & scale, enzymes, classifiers) and re-add them to the routine one at a time. This has "cured" 90% of the rashes.
Other common causes seem to be:
eczema (this may not have been present before hot tub ownership)
chloramines / bromamines - I may have missed it, but I was surprised no one mentioned this. If they did, sorry i missed it. MANY do not understand shocking/oxidizing. I know some of you will disagree with me, but to super chlorinate/brominate the tub you really need to leave the cover off for 45-60 minutes.
Lastly but not least, pseudomonas can cause rashes.