Erythema nodosum

What it is

Erythema nodosum is inflammation of subcutaneous fat. It typically causes tender, erythematous, nodular lesions, and it usually occurs over the shins, though it may also occur elsewhere, e.g. forearms, thighs. The case that carries it is a painful elbow and shin lesion, and the slide carries two views of the shins, with red nodules on them.

The causes, held under NODOSUM

The causes are held under the word NODOSUM, written on the page as:

• NO: No Cause (idiopathic)
• D: Drug
• O: OCP
• S: sarcodosis / Streptococcus
• U: Ulcerative Colitis
• M: Materinity / Mycobederium

The first entry is that there is no cause at all: idiopathic erythema nodosum is itself one of the causes listed, and is not a gap in the list.

The same causes set out in full

• Infection: streptococci, TB, brucellosis
• Systemic disease: sarcoidosis, inflammatory bowel disease, Behcet's
• Malignancy and lymphoma
• Drugs: penicillins, sulphonamides, combined oral contraceptive pill
• Pregnancy

Read against the mnemonic, the S covers sarcoidosis and streptococcus, the U covers the inflammatory bowel disease entry, the D and the O cover the drug list and the oral contraceptive pill in it, and the M covers pregnancy and TB. Malignancy and lymphoma, brucellosis and Behcet's are on the full list and not in the mnemonic.

A second list of the other common causes runs: IBD, that is Crohn's and UC; tuberculosis; drugs, that is OCPs and sulfonamides; and pregnancy. And where the case asks for two underlying causes, the two given as the answer are streptococcal throat infection and sarcoidosis - so the S of the mnemonic carries both of those answers, and the throat is where the streptococcal infection is looked for.