Crust

• A hardened layer that results when serum, blood, or purulent exudate dries on the skin or mucosal surface.
• Crust is a secondary surface change.
• Crusts are yellow-brown when formed from serum, green or yellow-green when formed from purulent exudate, or red-black when formed by blood.

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  Crust and exudative weeping in nonbullous impetigo.

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  Lip mucosal hemorrhagic crust in herpes simplex virus.

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  Thick brown crusted plaques and erosions in a child with impetiginized atopic dermatitis.

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  Purulence and crusting of impetigo on the earlobe.