important ones Linear lesions can also be seen with allergic contact dermatitis and inoculation sites for warts or molluscum The distribution of skin lesions may also provide useful diagnostic clues Plaque-type psoriasis favors the extensor surfaces of the extremities like the elbows and knees, as well as the scalp, ears, and intertriginous areas While seborrheic dermatitis also tends to involve intertriginous areas, it has a predilection for the perinasal areas, eyebrows, and external auditory canals PR usually manifests predominantly on the torso, and skin lesions are characteristically distributed along skin tension lines (the lines of Langer) producing the so-called “fir tree” or “pine tree” pattern on the back Concomitant involvement of the diaper area, acral areas, and the face would raise the possibility of seborrheic dermatitis, acquired zinc deficiency, or acrodermatitis enteropathica Examination of the mucous membranes and skin appendages can provide corroborating evidence for a particular diagnosis Geographic tongue may be a normal variant but is often a feature of psoriasis Oral erosions or lacy white patterns in the mouth suggest lichen planus Genital involvement is also a typical feature of psoriasis, lichen planus, lichen nitidus, and syphilis Nail pitting is frequently seen in psoriasis, but nails are generally normal in PRP The degree of pruritus is a helpful differentiating symptom for papulosquamous disorders Many of the papulosquamous disorders are not particularly pruritic When intense, itching suggests lichen planus or lichenoid drug eruptions ( Table 70.1 ) Psoriasis Psoriasis is a relatively common, chronic papulosquamous disease that makes up approximately 4% of all skin disorders encountered in children There is a predisposition for involvement of the scalp, perineum (particularly in infants), and the extensor surfaces of the body, particularly the elbows and knees Psoriatic arthritis occurs in a minority of patients, but arthritis is more common among pediatric patients One-third of adults with psoriasis experiences onset of disease in childhood or adolescence Among children with psoriasis, the majority develop the condition during adolescence although the condition arises in 12% before the age of 10 years