The central part of the erythema is palpable and the erythema is initially bright red in color. After a few days, it gradually turned light purple-red. After visiting the dermatology and rheumatology departments of the hospital, the diagnosis of erythema nodosum was confirmed by the examination of blood sedimentation, antinuclear antibody and tuberculin test. Erythema nodosum, also known as erythema nodosum, is a limited vasculitis of the deep dermis or subcutaneous tissue caused by certain causes. The disease most often occurs between 20-45 years old, with a peak incidence between 20-30 years old. It is more prevalent in women. There is no obvious seasonality or regularity in its occurrence, and it can be acute or insidious. Erythema is mostly found on the extensor side of the lower leg, sometimes the lower thigh and buttocks can also be affected, and a few patients can also be involved in the upper extremity, but the face is usually not invaded. The number of erythema nodosum is variable, small ones are as big as peanut grains, large ones are as big as walnuts, and there can be multiple nodules gathered locally or scattered symmetrical distribution. The nodules usually do not break down, and the nodules adjacent to each other can fuse with each other to form larger lumps. If the local blood vessels are compressed, the venous return is obstructed, which may cause mild edema in the lower leg. The nodules are painful to the touch, especially to the touch. In mild cases, the nodules can gradually subside in 2-3 weeks. Occasionally, recurrence is seen. New subcutaneous nodules may appear in batches and in stages, making the disease prolonged and unresolved. The disease can be an isolated disease or a skin manifestation of some systemic diseases. If erythema nodosum is only a symptom of a systemic disease, it is often the only prominent manifestation because other symptoms are not obvious. The etiology of erythema nodosum is unknown, and the most important causes are two: infection and autoimmune reaction. Infection is one of the most common causes of the disease. It may be a cutaneous vascular inflammatory reaction due to various factors such as streptococcal, Mycobacterium tuberculosis, fungal, viral and chlamydial infections or drugs. If the disease is caused by tuberculosis, the tuberculin test is strongly positive. Many patients may have symptoms of upper respiratory tract infection 1 to 2 weeks before the onset of the disease. Systemic symptoms may include malaise, malaise, low-grade fever, joint and muscle pain, etc. Systemic autoimmune diseases such as leukoaraiosis, systemic lupus erythematosus, polyarteritis nodosa and ulcerative colitis may also be associated with erythema nodosum, especially leukoaraiosis, and erythema nodosum is often diagnosed as one of its characteristic manifestations. Erythema nodosum can also be a symptom of certain leukemias and other malignancies such as sarcoma, lymphoma, leprosy, connective tissue disease, and intravascular protein abnormalities. Therefore, the appearance of erythema nodosum should not be limited to the diagnosis and treatment of erythema nodosum, but should be further examined in all aspects to clarify the primary disease. However, there are still about 50% of patients with unknown causes. The histopathological findings of this disease are inflammatory lesions in the middle and lower dermis and the upper part of the subcutaneous tissue. It is mainly a manifestation of vascular damage, mostly in the lining of the larger small veins, with inflammatory cell infiltration and endothelial cell hyperplasia, but generally the lumen is patent, no occlusion occurs, and no thrombosis is present. Inflammatory cell infiltration may also be present around the vessels, with neutrophils predominating in the early stages and lymphocytes, histiocytes and plasma cells after about 48 hours. The disease needs to be differentiated from erythema rigidum, which has a slower onset, with nodules occurring on the flexural surface of the lower leg, usually 3-5. The nodules are dark red, walnut-sized, hard, and can break down to form ulcers, with a chronic course. For the treatment of erythema nodosum, first of all, chronic lesions should be removed and the primary disease should be treated. If it is caused by infection, antibiotics can be used; if it is caused by tuberculosis bacteria, anti-tuberculosis treatment is feasible. For those with extensive lesions, heavy inflammation and severe pain, glucocorticoids and immunosuppressants should be added.