Psoriasis is a chronic frequently relapsing disease of the skin, characterized by more or less numerous, small or large masses of white or yellowish dry overlapping scales loosely adherent to circumscribed red patches of epidermis. The redness nearly always extends a little distance beyond the scales. The primary lesions are pinhead-sized, slightly elevated papules. By peripheral growth they develop into pea-sized, glistening white scaly lesions that look like drops of mortar on the skin. When the scales are completely removed, a smooth red surface is exposed upon which several minute bleedng points may be seen, but often only with the aid of a lens.
Further growth and coalescence of the lesions give rise to coin-sized plaques and larger irregular patches. Occasionally lesions may curl inward at the center; in this way ring-formed, segmental, and serpentine figures are formed. The efflorescences vary greatly in number and are rarely of uniform size. Now and then there may be large red plaques covered with only a single translucent, wrinkled film. Only in old lesions is there any infiltration of the skin, but in extreme case the thickening may be sufficient to cause fissuring about the joints. The evolution of a lesion may occupy a few weeks or many months.