Condition	Similar rashes (distinguishing features)
Atopic dermatitis	Contact dermatitis (not associated with dry skin)
	Keratosis pilaris (nonpruritic, involves posterolateral upper arms)
	Mycosis fungoides (lesion borders sharper, fixed size and shape)
	Psoriasis (well-defined plaques, silvery white scale, involves extensor surfaces)
	Scabies (involves genitalia, axillae, finger webs)
	Seborrheic dermatitis (nonpruritic, greasy scale, characteristic distribution)
Contact dermatitis	Atopic dermatitis (symmetric distribution, history of hay fever or asthma, flexural areas, hyperlinear palms, family history, not limited to area of exposure, dry skin and itching precede skin lesions rather than follow them)
	Dermatitis herpetiformis (vesicles on extensor surfaces, enteropathy, burning pain)
	Psoriasis (patches on knees, elbows, scalp, and gluteal cleft; pitted nails)
	Seborrheic dermatitis (greasy scale on eyebrows, nasolabial folds, or scalp)
Drug eruption (morbilliform)	Erythema multiforme (target lesions)
	Viral exanthem (more common in children, less intense erythema and pruritus, less likely to be dusky red, more focal systemic symptoms, less likely to be polymorphic, less likely to be associated with eosinophilia)8,26
Pityriasis rosea	Drug eruption (no scale, lesions coalesce)
	Erythema multiforme (target lesions)
	Guttate psoriasis (thicker scale, history of streptococcal pharyngitis)
	Lichen planus (violaceous, involves wrists and ankles)
	Nummular eczema (larger round [not oval] lesions, do not follow skin lines)
	Psoriasis (thick white scale, involves extensor surfaces)
	Secondary syphilis (positive serology; involves palms and soles)
	Tinea corporis (positive KOH preparation, scale at peripheral border of lesions rather than inside border)
	Viral exanthem (no scale, lesions coalesce)
Psoriasis	Atopic dermatitis (atopic features, flexural areas, lichenification)
	Lichen planus (violaceous, minimal scale, involves wrists and ankles)
	Mycosis fungoides (lesion borders less distinct)
	Pityriasis rubra pilaris (islands of normal skin)
	Seborrheic dermatitis (greasy scale, involves anterior face)
	Secondary syphilis (red-brown lesions on palms and soles)
	Tinea corporis (thinner peripheral scale, positive KOH preparation)
Seborrheic dermatitis	Atopic dermatitis (nongreasy scale, atopic history, pruritic)
	Psoriasis (silver scale, sharply demarcated lesions on extensor surfaces of extremities; involvement of scalp commonly extends onto forehead, whereas seborrheic dermatitis of scalp stops at scalp margin)