Differential diagnosis of pediatric diseases with rash

1. Big picture

A rash in a child is not diagnosed by appearance alone. In pediatrics, the safe exam approach is:

Rash in child
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Is the child toxic or stable?
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Is there fever?
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What is the rash morphology?
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What is the distribution and progression?
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Are there mucosal signs, palms/soles involvement, petechiae, purpura, desquamation, or systemic signs?
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Think: viral exanthem, bacterial toxin disease, meningococcemia/sepsis, vasculitis, allergic/drug reaction, autoimmune disease, or dermatologic infection

The most important rule:

Fever + non-blanching petechiae/purpura = meningococcemia/sepsis until proven otherwise. Treat first, investigate second.