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A common multifactorial inherited condition characterized by the eruption of circumscribed, discrete and confluent, reddish, silvery-scaled maculopapules; the lesions occur predominantly on the elbows, knees, scalp, and trunk, and microscopically show characteristic parakeratosis and elongation of rete ridges with shortening of epidermal keratinocyte transit time due to decreased cyclic guanosine monophosphate. [G. p., fr. psora, the itch]

- p. annularis, p. annulata SYN: p. circinata.

- p. arthropica p. associated with severe arthritis resembling rheumatoid arthritis, although serum rheumatoid factor is absent.

- p. circinata p. in which healing is taking place at the center of the lesion while the process continues at the periphery, producing a ring-shaped or annular lesion. SYN: p. annularis, p. annulata.

- p. diffusa, diffused p. a form of p. with extensive coalescence of the lesions.

- exfoliative p. exfoliative dermatitis developing from chronic p., sometimes resulting from overtreatment of p..

- flexural p. p. involving intertriginous folds, e.g., axillary and inguinal skin, which may resemble seborrheic dermatitis.

- generalized pustular p. of Zambusch SYN: pustular p. (1).

- p. geographica p. gyrata in which the lesions suggest the coast outline on a map.

- p. guttata p. occurring abruptly in round patches of small size; seen in young persons following streptococcal infections.

- p. gyrata p. circinata in which there is a coalescence of the rings giving rise to figures of various outlines.

- p. nummularis p. in which the lesions are discrete and discoid.

- palmar p. patchy, hyperkeratotic p. affecting contact points of the volar surface of fingers and palms, alone or with mild p. elsewhere; believed to be an isomorphic response, it may affect one palm involved in a sport or occupation.

- p. punctata p. in which the individual lesions are papules, each red in color and tipped with a single white scale.

- pustular p. 1. an extensive exacerbation of p., with pustule formation in the normal and psoriatic skin, fever, and granulocytosis; sometimes precipitated by oral steroids; SYN: generalized pustular p. of Zambusch. 2. a local pustular eruption of the palms and soles, occurring most commonly in a patient with p.; difficult to distinguish from acrodermatitis continua.

pso·ri·a·sis sə-'rī-ə-səs n, pl -a·ses -.sēz a chronic skin disease characterized by circumscribed red patches covered with white scales

a chronic skin disease in which scaly pink patches form on the elbows, knees, scalp, and other parts of the body. Psoriasis is one of the commonest skin diseases in Britain, affecting about 2% of the population, but its cause is not known. The disorder often runs in families, the commonest time of onset being in adolescence. It sometimes occurs in association with arthritis (see psoriatic arthritis). Occasionally the disease may be very severe, affecting much of the skin and causing considerable disability in the patient. While psychological stress may cause an exacerbation of psoriasis, the only significant event that precipitates the disease is a preceding streptococcal infection. Drugs, such as lithium or beta blockers, may occasionally be responsible.

Although there is as yet no cure, treatment of psoriasis has improved greatly in recent years. Tar and dithranol may still be used and topical corticosteroids remain popular. The vitamin D analogues, e.g. calcipotriol, are highly effective, while PUVA is an effective treatment for moderately severe disease. Systemic therapy, such as methotrexate, retinoid, or ciclosporin, is reserved for the worst cases.