Psoriasis is a common skin disease that affects more than 7 million people in the United States. Patients usually have red, scaly, small to large raised areas of skin (called plaques). These commonly affect the scalp, trunk, elbows, knees, and genital areas but can affect any part of the body, including the nails. About a quarter of patients develop pain, stiffness, and swelling in their joints (psoriatic arthritis). Patients with severe psoriasis may have an increased risk of obesity, diabetes, high cholesterol, and cardiovascular disease. Psoriasis also increases risk of depression.