Erythema nodosum is the name given to extremely tender lumps on the legs and sometimes the arms. They are caused by inflammation of the fat beneath the skin. The bumps look like bruises. They change color in the same way, from pink to blue to brown.

Erythema Nodosum
Erythema  Nodusum
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Erythema nodosum is thought to be a type of allergic reaction. It may be caused by: