Psoriatic arthritis (PsA) is a type of arthritis that occurs in people with psoriasis. Psoriasis is a skin disease where itchy, scaly red patches appear on the skin and scalp. According to The National Psoriasis Foundation (NPF), about two percent of the United States population has psoriasis. (NPF) Between 10 and 30 percent of psoriasis patients develop PsA.