David Smith, MD, a dermatologist with The Everett Clinic discusses psoriasis. Psoriasis is a very common condition that affects about 3% of Americans and usually affects elbows, knees and scalps. It is often itchy and can be painful. Psoriasis can also cause arthritis in the joints. People with Psoriasis have higher rates of diabetes, high blood pressure and cardiovascular disease. Treatments include topical steroids and medication based on vitamin D.