When the patient presents with a rash of two or more parts of the body with onset over days, the morphology is key. Is it urticarial? eczematous? Maculopapular? Pustular?

Morphology Typical Causes Reference Page
Maculopapular Drug Eruption, Measles
Urticarial Urticaria Urticarial
Papulosquamous Guttate Psoriasis, Secondary Syphilis, Pityriasis Rosea Papulosquamous
Pustular Bacterial folliculitis, Hot Tub Folliculitis, Agep Pustules Follicular, Non-follicular
Eczematous Allergic Contact Dermatitis Eczematous
Vesicular Insect Bites , Chickenpox Vesicles
Papular, Inflammatory Insect Bites , Viral exanthem Papules, Red, Acute
Mucous Membrane Inv. Stevens Johnson Syndrome, TEN
Purpuric Vasculitis
Targetoid Erythema Multiforme

Normal Adult

Viral exanthem--Small, diffuse inflammatory papules.
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Drug eruption--Classic macuopapular. Good history of a new drug.
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Urticaria The key question here is do the lesions move from location to location in less than 24 hours?--yes for urticaria.
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Hypersensitivity Reaction--hives vs drug eruption. This 16 year old boy had just started minocycline. The urticarial reaction started within 2 hours of taking the first pill.
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Stevens Johnson Syndrome
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Erythema Multiforme True target lesions as shown here are uncommon.
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Vasculitis Purpuric lesions imply a vasculitis.
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Guttate psoriasis
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Pityriasis rosea Note the herald patch on the left shoulder.
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Secondary Syphilis Any involvement of the palms and soles should make one consider secondary syphilis.
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Chemotherapy patient, acute maculopapular rash

The above, plus

Graft versus host disease
Lymphocyte recovery syndrome
Neutrophilic eccrine hidradenitis/chemotherapy induced eccrine squamous syringometaplasia

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