I don't know I assume it would tell you put some cortisone cream on it or to seek a dermatologist. 99% of internet advice boils down to that I don't think chatgpt/gpt 4 would be much different in that regard. It definitely can bullshit but for really generalized stuff like that I doubt it.
It's not like your asking a singular person the question like it is when you ask an accountant but an abstraction of written human thought and interactions.
I'm an accountant, and when someone asks me about the rash on their skin I say, 'I'm not a doctor, I'm an accountant'.
When you ask ChatGPT something that is not it's job it just bullshits its way through like a cheap salesman.