Psoriasis is a chronic, inflammatory skin disease. Knees, elbows, scalp, trunk, and nails are the most commonly affected areas. There are several types of psoriasis:

• Plaque—inflamed patches of skin topped with silvery, white scales (most common type)
• Guttate—small dot-like lesions
• Pustular—weeping lesions and intense scaling
• Inverse—in body folds (armpits, groin, under breasts)
• Erythrodermic—intense sloughing and inflammation of the skin

The cause of psoriasis is unclear. Signals from a defect in the immune system may result in an overgrowth of skin cells. Because the cells grow faster than they can be shed, they "pile up" on the skin's surface. The excess skin cells are thought to cause the silvery white scales that are characteristic of plaque-type psoriasis.

A risk factor is something that increases your chance of getting a disease or condition. Risk factors for this condition include:

• Family history of psoriasis
• Cold climates
• Suppression of the immune system, including AIDS
• Certain bacterial infections
• Certain medications, such as beta blockers and lithium
• Smoking

The red, thickened, and rough patches of psoriasis may occur anywhere, but are commonly found on the scalp, elbows, knees, palms, and soles. Other symptoms include:

• Silvery white scales
• Pitted or dented fingernails and/or toenails
• Red lesion in the folds of the buttock
• Joint pain suggesting arthritis

The doctor will ask about your symptoms and medical history, and perform a physical exam. Your skin and nails will be examined. There are no specific blood tests or diagnostic procedures for psoriasis. Sometimes a skin biopsy will be done to confirm the diagnosis.

Treatment is based on:

• The severity of the disease
• The extent and location of the areas involved
• Responsiveness to the treatment

Many patients respond very well to treatments applied directly to the skin. Topical treatments include:

• Corticosteroid creams and ointments (most common treatment)
• Synthetic forms of vitamin D and retinoids ( calcipotriene ointment)
• Retinoids (tazarotene gel 0.05 and 0.1%)
• Coal tar preparations (Denorex, Pentrax, Tegrin, Zetar)
• Bath solutions and moisturizers
• Tacrolimus and pimecrolimus (especially for inverse psoriasis)

If psoriasis covers more than 30% of the body it is difficult to treat with topical medications alone. Daily, short, nonburning exposure to sunlight clears or improves psoriasis in many people. Sunlight is often included among initial treatments. A more controlled form of artificial light treatment (UVB phototherapy) is often used in more widespread cases. Alternatively, psoriasis can be treated with ultraviolet A (UVA light) and psoralen. Psoralen is an oral or topical medication that makes the body more sensitive to light. This treatment is known as PUVA.

Phototherapy can be very effective in controlling psoriasis but it requires frequent treatments. It may cause side effects such as nausea, headache, fatigue, burning, and itching. Both UVB and PUVA may increase the person's risk for squamous cell and, possibly, melanoma skin cancers . It is unclear whether UVB increases the risk of skin cancer.

For more severe types of psoriasis, doctors may prescribe a number of other powerful medications, which can be effective, but are associated with more serious side effects. These include:

• Methotrexate (Rheumatrex, Trexall)—a type of systemic medicine that affects the whole immune system; should not be taken by pregnant women, women planning to become pregnant, or by their male partners
• Cyclosporine (Gengraf, Neoral)—another type of systemic medicine that suppresses the immune system to slow the turnover of skin cells; should not be taken by pregnant or breastfeeding women
• Hydroxyurea (Hydrea)—less toxic than methotrexate or cyclosporine, but may be less effective
• Systemic retinoids—Compounds with vitamin A-like properties taken internally may be prescribed in severe cases. Retinoids can cause birth defects, and women must diligently protect themselves from pregnancy for several years after completing treatment. Systemic retinoids are often combined with phototherapy for increased effectiveness and for their property of being protective against squamous skin cancer.

Newer medicines include biologic agents, which affect a part of the body's immune response. For example, ustekinumab (Stelara) is a recently approved drug that targets certain cells in the immune system that cause inflammation.