Does psoriasis cause arthritis?

Sometimes. Psoriasis is an immunological condition that affects the skin and joints. However, not everyone have manifestations of both, and many have only skin or only joint involvement. We are learning that psoriasis also affects the cardiovascular system, increasing the risk for heart disease.
Sometimes. One type of arthritis is psoriatic arthritis. That does not mean that everyone that has skin psoriasis will have psoriatic arthritis.