The diagnosis of atopic dermatitis depends on excluding other conditions, such as the following (1): Seborrheic dermatitis Contact dermatitis (irritant or allergic) Psoriasis Nummular dermatitis (nonatopic) Scabies Ichthyoses Cutaneous T-cell lymphoma Photosensitivity dermatoses Immune deficiency diseases Erythroderma of other causes Although a personal and / or family history of atopy and the distribution of lesions are helpful in making the diagnosis of atopic dermatits, the following distribution patterns can help with differentiation: Psoriasis is typically easy to recognize by its sharply demarcated, thick, erythematous, and scaly plaques.