A common chronic, squamous dermatosis, marked by exacerbations and remissions and having a polygenic inheritance pattern. The most distinctive histological findings in well-developed psoriasis are Munro microabscesses and spongiform pustules. It is characterised clinically by the presence of rounded, circumscribed, erythematous, dry scaling patches of various sizes, covered by greyish white or silvery white, umbilicated, and lamellar scales, which have a predilection for the extensor surfaces, nails, scalp, genitalia, and lumbosacral region. Central clearing and coalescence of the lesions produce a wide variety of clinical configurations, including annular or circinate, discoid or nummular, figurate, and gyrate arrangements. Called also p. vulgaris