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Probability diagnosis

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Varicella (chicken pox)

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Measles

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Rubella

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Erythema infectiosum (‘slapped cheek’ disease)

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Roseola infantum

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Other viral exanthema (e.g. enterovirus)

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Hand, foot and mouth disease

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Pityriasis rosea

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Herpes zoster (shingles)

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Drug reaction (see list)

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Impetigo

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Herpes simplex

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Allergic rash (incl. contact dermatitis)

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Serious disorders not to be missed

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Vascular:

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  • Henoch–Schönlein purpura

  • Stevens-Johnson syndrome

  • other vasculitides

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Infection:

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  • purpura of meningococcus

  • primary HIV infection

  • folliculitis (e.g. pseudomonas, staphylococcus)

  • secondary syphilis

  • scarlet fever

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Other:

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  • erythema nodosum

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Pitfalls (often missed)

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Guttate psoriasis

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Epstein--Barr virus (EBV) mononucleosis

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Arbovirus infection (e.g. dengue, Ross River fever,

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Barmah Forest virus, Japanese encephalitis)

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Scabies

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Kawasaki disease

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Eczema herpeticum

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Zoonoses (e.g. listeriosis, Q fever)

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Rarities:

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  • filovirus haemorrhagic diseases (e.g. Ebola, Marburg virus)

  • erythema multiforme

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Key history

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Should be adapted to patient’s age as viral exanthema common in children. Site and mode of onset of rash, mode of progression and past history (e.g. eczema). Constitutional disturbance (e.g. pyrexia, pruritus). Drug history and exposure to irritants. Diet including unaccustomed food. Herald patch (pityriasis rosea). Contact with infectious diseases including child care centres and school. Overseas travel. Bleeding or bruising tendency.

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Key examination

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Skin of whole body

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  • Nature and distribution of the rash including lesion characteristics

  • Nails and soles of feet

  • Scalp, mucous membranes and oropharynx

  • Conjunctivae and the lymphopoietic system (?lymphadenopathy, ?splenomegaly)

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Key investigations

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Many diagnoses are clinical. Consider:

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  • FBE/ESR/CRP

  • EBV test

  • HIV test

  • serology for rubella, parvovirus, syphilis and other suspected infections

  • viral and bacterial cultures.

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Diagnostic tips

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  • Be vigilant for the deadly meningococcal septicaemia, which may present as an erythematous rash initially prior to the development of purpura.

  • Prescribed drugs are a common cause of rash, especially toxic erythema. Examples are antibiotics, especially penicillin, thiazides, anti-epileptics, allopurinol, NSAIDs and other anti-arthritic agents.

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