I have a purplish reddish rash on my arms and legs and a bump filled with pus on my knee. what could this be?" "i have a purplish reddish rash on my arms and legs and a bump filled with pus on my knee. what could this be?"

This : This is quite challenging without a clinical examination. I would recommend you see a dermatologist for diagnosis and treatment options. Could be many conditions, including folliculitis, eczema (with secondary infection), psoriasis, lichen planus, a drug rash, or other possibilities. Read more about rashes and other possibilities at the link below. I hope that helps a little.

Can't say: This rash would likely need to be examined and more history obtained to be able to provide you with useful input about the diagnosis or possible treatment.