Erythema nodosum is a lumpy, tender rash due to inflammation of the tissues just beneath the skin. It usually affects the lower legs.

Your son's doctor may be able to make the diagnosis by examining the boy, but a biopsy is often needed when the diagnosis is not entirely clear.

By itself, erythema nodosum is not a serious disorder. Sometimes, it appears during pregnancy or with the use of certain drugs (such as penicillin or sulfa antibiotics). It goes away after delivery or the drug is no longer used.

However, it can be associated with a number of diseases, including:

• Infections, such as Strep. throat or tuberculosis
• Sarcoidosis
• Inflammatory bowel disease, such as Crohn's disease or ulcerative colitis

Some of these conditions can be serious. For example, sarcoidosis can cause lung damage and require drugs that suppress the immune system.

Many cases of erythema nodosum have no clear cause and are not associated with any illness. In these cases, the skin inflammation may require treatment but it tends to subside over time without significant health effects.