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Erythema nodosum is a disorder that causes a rash with painful lumps under the skin. Often, the cause is unknown. Sometimes, it may be caused by strep or other infections.

Symptoms include a rash with flat, firm, hot, painful lumps under the skin. It often shows up on the shins. Later, they may fade and look more like a bruise and then clear up on their own.

The underlying infection or condition needs treatment. Medicines can treat the rash.