Condition	Key clinical features	Tests
Atopic dermatitis	Dry skin; pruritus; erythema; erythematous papules; excoriations; scaling; lichenification; accentuation of skin lines; keys to diagnosis are pruritus, eczematous appearance of lesions, and personal or family history of atopy12	Skin biopsy is nonspecific and not often done*
Contact dermatitis	Erythema; edema; vesicles; bullae in linear or geometric pattern; common causes include cosmetics, topical medications, metal, latex, poison ivy, textiles, dyes, sunscreens, cement, food, benzocaine, neomycin13; keys to diagnosis are linear or geometric pattern and distribution of lesions	Skin biopsy is nonspecific and not often done,* but it can help exclude other conditions
Drug eruption†	Many patterns, but most commonly maculopapular (95% of cases)14; common in patients taking allopurinol (Zyloprim), beta-lactam antibiotics, sulfonamides, anticonvulsants, angiotensin-converting enzyme inhibitors, nonsteroidal anti-inflammatory drugs, hypoglycemics, and thiazide diuretics, but can occur with almost any drug14; usually appears within 1 to 4 weeks of initiating drug; key to diagnosis is timing of rash appearance in relation to drug use14	Skin biopsy is usually nonspecific and not often done*15
Erythema multiforme	Round, dusky red lesions that evolve into target (iris) lesions over 48 hours; starts on backs of hands and feet and on extensor surfaces of arms and legs; symmetric; may involve palms, soles, oral mucous membranes, or lips; key to diagnosis is presence of target lesions	Skin biopsy is generally diagnostic and occasionally done; biopsy should be taken from the erythematous (not blistered) portion of the target16
Fifth disease (i.e., erythema infectiosum)†	“Slapped cheek” appearance with sparing of periorbital areas and nasal bridge; unique fishnet pattern; erythema on extremities, trunk, and buttocks; keys to diagnosis in children are slapped cheek appearance and net-like rash, and in adults are arthralgias and history of exposure to affected child	Parvovirus B19 serology; skin biopsy is nonspecific and rarely done*
Folliculitis	Multiple small pustules localized to hair follicles on any body surface; key to diagnosis is hair follicle at center of each lesion	Skin biopsy is often diagnostic but not often done*
Guttate psoriasis	Pinpoint to 1-cm scaling papules and plaques on trunk and extremities; often preceded by streptococcal pharyngitis 1 to 2 weeks before eruption17; keys to diagnosis are scaling and history of streptococcal pharyngitis17	Throat culture; antistreptolysin O titer; early skin biopsy may not be diagnostic and is not often done*
Insect bites	Urticarial papules and plaques; keys to diagnosis are outdoor exposure (usually) and distribution of lesions where insects are likely to bite	Skin biopsy is nonspecific and not often done*
Keratosis pilaris	Pinpoint follicular papules and pustules on posterolateral upper arms, cheeks, anterior thighs, or buttocks18; keys to diagnosis are upper arm distribution, absence of comedones, and tiny palpable lesions	Skin biopsy can be diagnostic but is not often done*
Lichen planus	Violaceous flat-topped papules and plaques; commonly on ankles and wrists; 5 P's (pruritic, planar, polygonal, purple plaques); Wickham striae (reticular pattern of white lines on surface of lesions)19; lacy white buccal mucosal lesions; Koebner phenomenon (development of typical lesions at the site of trauma); keys to diagnosis are purple color and distribution of lesions20	Skin biopsy is diagnostic and often done
Miliaria rubra (i.e., prickly heat, heat rash)	Erythematous nonfollicular papules associated with heat exposure or fever; lesions on back, trunk, neck, or occluded areas; keys to diagnosis are history of heat exposure and distribution of lesions	Skin biopsy can be diagnostic but is not often done*
Nummular eczema	Sharply defined, 2- to 10-cm, coin-shaped, erythematous, scaled plaques; lesions on dorsal hands and feet, extensor surfaces of arms and legs, flanks, and hips; key to diagnosis is sharply defined, round, erythematous, scaled lesions	Skin biopsy is nonspecific and not often done,* but it may help exclude other diagnoses
Pityriasis rosea	Discrete, round to oval, salmon pink, 5- to 10-mm lesions; “Christmas tree” pattern on back; often (17 to 50%) preceded by solitary 2- to 10-cm oval, pink, scaly herald patch21; keys to diagnosis are oval shape, orientation with skin lines, and distinctive scale	Skin biopsy is nonspecific and not often done,* but it may help exclude other diagnoses; rapid plasma reagin testing is optional to rule out secondary syphilis
Psoriasis (plaque psoriasis)	Thick, sharply demarcated, round or oval, erythematous plaques with thick silvery white scale; lesions on extensor surfaces, elbows, knees, scalp, central trunk, umbilicus, genitalia, lower back, or gluteal cleft; positive Auspitz sign (removal of scale produces bleeding points); Koebner phenomenon; keys to diagnosis are distinctive scale and distribution of lesions22	Skin biopsy can be diagnostic but is not often done*
Roseola (i.e., exanthem subitum, sixth disease)	Sudden onset of high fever without rash or other symptoms in a child younger than 3 years; as fever subsides, pink, discrete, 2- to 3-mm blanching macules and papules suddenly appear on trunk and spread to neck and extremities; key to diagnosis is high fever followed by sudden appearance of rash as fever abruptly resolves23	Skin biopsy is nonspecific and not often done*
Scabies	Discrete, small burrows, vesicles, papules, and pinpoint erosions on fingers, finger webs, wrists, elbows, knees, groin, buttocks, penis, scrotum, axillae, belt line, ankles, and feet; keys to diagnosis are distribution of lesions, intense pruritus, and positive mineral oil mount	Mineral oil mount is routinely done to identify mites or eggs; skin biopsy is usually nonspecific and not often done*
Seborrheic dermatitis	Erythematous patches with greasy scale; lesions behind ears or on scalp and scalp margins, external ear canals, base of eyelashes, eyebrows, nasolabial folds, central chest, axillae, inframammary folds, groin, and umbilicus; keys to diagnosis are greasy scale and distribution of lesions	Skin biopsy is nonspecific and not often done*
Tinea corporis	Flat, red, scaly lesions progressing to annular lesions with central clearing or brown discoloration; keys to diagnosis are annular lesions with central clearing and positive KOH preparation	KOH preparation is routinely done; skin biopsy can be diagnostic24 but is not often done*
Urticaria (i.e., hives)	Discrete and confluent, raised, edematous, round or oval, waxing and waning lesions with large variation in size; may have erythematous border (flare) and pale center (wheal); patient may have history of drug, food, or substance exposure; key to diagnosis is distinctive appearance of edematous lesions	Skin biopsy is nonspecific and not often done*
Varicella†	Vesicles on erythematous papules (“dewdrop on rose petal” appearance); all stages (papules, vesicles, pustules, crusts) are present at the same time and in close proximity; keys to diagnosis are crops of lesions in different stages, systemic illness, and exposure to persons with the infection	Diagnosis is usually clinical, but real-time polymerase chain reaction assay of skin lesion or direct fluorescent antibody testing of skin scrapings could be done25; skin biopsy is often diagnostic but cannot distinguish herpes zoster or herpes simplex, and is not often done*
Viral exanthem, nonspecific	Blanchable, red, sometimes confluent macules and papules; may be indistinguishable from drug eruptions26; keys to diagnosis are nonspecific generalized maculopapular rash in a child with systemic symptoms (fever, diarrhea, headache, fatigue)	Skin biopsy is nonspecific and not often done*