Psoriasis is usually asymptomatic, although many patients experience some pruritus. The disease affects 1% of the population of the United States and approximately 3% worldwide. It usually begins by the age of 10, although patients may present any time later. The cardinal features of psoriasis are sharply circumscribed, thick plaques of erythematous skin covered with silvery scales (Fig. 20-1). Nail involvement is common and can be the key to confirming a diagnosis. Look for pitting and nail plate dystrophy (Fig. 20-2). Several types of psoriasis exist.

Plaque psoriasis presents as thick, fixed plaques on the extensor elbows, knees, scalp, lower back, sacral area, and scalp. The demarcation between normal and psoriatic skin is sharp. Any area of the body can be affected, but the face is usually spared. One variant, called inverse psoriasis, affects intertriginous areas of the groin, intergluteal cleft, axillae, and inframammary areas.

Guttate psoriasis appears as multiple 0.5- to 1.0-cm psoriatic papules, which develop abruptly on the trunk and extremities, frequently associated with a recent streptococcal pharyngitis (Fig. 20-3).

In pustular psoriasis, multiple superficial pustules that may coalesce into pustular lakes develop on previously normal skin or on top of typical, preexisting psoriatic plaques. Pustules are usually all over the body, but a variant of pustular psoriasis is limited to the palms and soles.