In some cases of psoriasis there are prominent features of seborrheic dermatitis. Though the lesions occur on the usual sites of psoriasis, they also involve the typical seborrheic sites, such the nasolabial crease, midchest, axillae, perineum, flexures, palms and soles. The scales tend to be greasy and soft rather than dry and micaceous.

In other cases of psoriasis there is a strong tendency to lichenffication and many of the individual papules and small patches may have the characteristics of lichen planus. The areas affected tend to be those favored by lichen planus, i.e., the inner thighs, both flexor and extensor surfaces  of the upper extremities, and the lower legs.

Diagnosis. Psoriasis must be differentiated from seborrheic dermatitis, pityriasis rosea, lichen planus, eczema, the psoriasiform syphilid, and lupus erythematosus. The distribution in psoriasis is on the extensor surfaces, especially of the elbows and knees, and on the scalp, whereas in seborrheie denndtitis, although the kip is involved, there is 3 predilection for the eyebrows, ndsibidi angle, the ears, the sternal region, and the kaires, The scales in psoildsis are dry, iite, and sky, whereas those in seborrheie derindtitis are gredsy and lusteiless, On removal of the scales in psoriasis there is an oozing of blood from the cdpiildri es, whereas this does not occur in seideie dennitis, In pitiridsis rosed the eruption is iocated

on the upper arms, trunk, and thighs, and the duration is a matter of weeks9 There are oval, fawn-colored patches which centrally show a crinkling of the epidermis and an almost imperceptible scaling. The onset with the herald patch and the tendency of the subsequent lesions to arrange themselves so that their long diameters are parallel with the direction of the ribs usually facilitate the distinction between pityriasis rosea and psoriasis. Lichen planus affects chiefly the flexor surfaces of the forearms and wrists, and the shins and ankles. The patches are pruritic and thick-ened. Often the vialaceous color is pronounced, but at other times the patches are a dirty brown color and are then only distinguished from psoriasis by close examination, which reveals that the scaling is not at all micaceous and that there are lichen papules at the edge of the patch,

In eczema the distribution is usually not on the extensor surfaces of the elbows and knees, and exudation and a slight grayish scaling, accompanied. By some itching, are present. The psoriasiform syphilid affects the palms and soles, where psoriasis is very rare, as well as the trunk. The lesions are infiltrated patches composed of dark-red or copper-colored papules which are often arranged in a configurate manner. The scales are brownish and sparse. serologic tests are positive; a general adenopathy, and often mucous patches, condylomas and other symptoms of late secondary syphilis are present. In lupus erythematosus, the lesions are discrete plaques usually on the face and scalp, associated with atrophy, scaling, and alopecia. Only infrequently is the face affected by psoriasis. The scales of lupus erythematosus are grayish and adherent. Upon removal of the scale the under surface is seen to be papil lose due to the projecting follicular plugs.