Erythema rigidum is a painful nodule or plaque that often occurs in young and middle-aged women, usually symmetrically distributed on the lower legs, especially on the lower and middle part of the trunk. The nodules can form ulcers, and there are two types of erythema, a hematogenous form of cutaneous tuberculosis in which the ulcers heal with depressed scars and a strong positive tuberculin test. Histopathology has tuberculosis-like structures, but tubercle bacilli are not always detected, and it is still considered a form of cutaneous tuberculosis. The other is subcutaneous arteriovenous vasculitis can cause fat necrosis, nodules do not destroy, anti-tuberculosis treatment is ineffective, histopathology into the changes of vasculitis, but there is no specific hard erythema with nodular erythema, skin variant nodular vasculitis, gangrenous pyoderma, syphilis, etc., need to distinguish nodular erythema damage often occurs in bilateral calf extensor, bright red nodules without ulcers occur, nodules have spontaneous pain and tenderness The duration of the disease is relatively short, often accompanied by other symptoms such as arthralgia and fever. Skin variant nodular vasculitis occurs on the extensor side of the calf, with recurrent episodes of chronic painful nodules, often accompanied by low-grade fever, fatigue, joint pain, and loss of appetite. The above questions are for reference only, please refer to the doctor’s guidance for specific medication and treatment.