Erythema nodosum is an inflammatory skin disease that occurs in small and medium-sized blood vessels in the deep dermis, with an acute onset and basic lesions of red nodules and plaques, mostly involving the extensor side of the lower leg, thighs and forearms. The etiology is complex. It is generally believed to be a delayed allergic reaction to blood vessels caused by streptococcal infection, tuberculosis infection, Mycobacterium leprae, viral infection, fungal infection or drugs, and can also be a cutaneous manifestation of certain immune abnormalities (e.g., nodular disease, leukoaraiosis, ulcerative colitis, etc.). The nodular erythema of aneurysmal leprosy reaction is an immune complex vasculitis, so nodular erythema can be considered as a syndrome or as a specific response to various triggers (lipofuscinosis). Clinical manifestations It affects mostly young or middle-aged women, with a predilection for spring and autumn. Initially, there may be fever, myalgia and arthralgia, and after a few days, painful nodules occur symmetrically on the front of both shins, with gradual red elevation of the surface skin, 1-2 cm in diameter, medium firmness and pressure pain. The nodules can gradually increase to several to more than 10 on each side, and a few can occur on the flexor side of the calf, thigh, upper extremity, and even the trunk can have similar lesions. Individual lesions can fade on their own in 2-3 weeks without breaking down, leaving pigmentation, and the disease often recurs. In some patients, the nodules are persistent, the inflammation and pain are mild and last for 1-2 years, and they do not break down, which is called chronic erythema nodosum or prolonged erythema nodosum. Diagnosis is based on typical clinical manifestations, history of infection or drug use (sulfonamides, birth control pills, bromine and iodine, etc.) before the onset of the disease, and histopathology to confirm the diagnosis. Treatment Finding and removing the cause is the key to treatment and preventing recurrence, and sensitive antibiotics should be used if there is streptococcal infection. Bed rest during the acute phase.