Erythema multiforme, also known as exudative erythema multiforme, is an acute inflammatory skin disease with a complex etiology. The rash is polymorphic, often with mucosal damage, and the characteristic lesion is iris-like erythema. The disease has a high incidence in spring and autumn and is prone to recurrence. 10-30 years old has the highest incidence. (1) Infection is a common cause, the most common of which is herpes simplex virus infection, some bacterial, fungal, mycoplasma and protozoan infections can also be triggered. (2) Drugs and certain foods such as sulfonamide, barbitur, antipyrine, vaccines and other drugs, and the consumption of spoiled fish and meat. (3) Physical factors such as cold, sunlight, radiation, etc. (4) Some diseases such as malignant tumor, connective tissue disease and pregnancy, menstruation, etc. Pathogenesis: It is generally believed that the disease is caused by epidermal cell damage due to specific cytotoxic reactions of the body stimulated by foreign antigens. Clinical manifestations] The first symptoms may include chills and fever, general malaise and sore throat. The rash occurs within 24 hours and is symmetrically distributed, usually around the mouth and nose, the back of the hands and feet, the forearms and the extensor side of the lower legs, and may also be seen on the trunk. The damage is erythematous, papular, maculopapular, nodular, blistering, or in severe cases, maculopapular, hemorrhagic or purpura. It is often accompanied by pruritus, pain or burning sensation. The mucous membranes of the mouth, nose, eyes, and external genitalia may be involved, showing erythema, blistering, and erosion. Clinically, one type of lesion is often predominant, while other types of lesions are also present. According to the characteristics of the lesions, there are three clinical types. (1) maculopapular- papular type This type is the most common, initially as a round edematous erythema or flat papule, lentil to coin size, bright red color, clear boundary. The rash expands telecentrically and becomes dark purple in the center after 1-2 days, or purpura, blisters or even blood blisters appear, forming the so-called iris-like or target-shaped damage, which is the characteristic damage of the disease. The lesions are symmetrically distributed, with the back of the hands, forearms, and ankles as the preferred sites. This type of mucosal damage is less frequent, and systemic symptoms are mild. (2) Blister-herpetic type The damage is mainly clustered or scattered blisters and herpes. Blisters can occur on the basis of erythema, when the central part of the lesion is a blister, blister or blood blister, surrounded by a dark red halo, iris-like. This type is often accompanied by mucosal blisters and erosions in the mouth, nose and genitals. Systemic symptoms such as arthralgia, fever, proteinuria and increased blood sedimentation may occur. (3) Severe type, also known as Stevens-Johnson syndrome. The onset is rapid, often with high fever, headache, sore throat, arthralgia and general discomfort. The lesions are bright red or dark red iris-like erythema or petechiae with blisters, macules or hemorrhagic blisters, and may be positive for Ney’s sign. The lesions may fuse into large patches and are widely distributed. The mouth, nose, eyes, genitalia and other parts of the mucosa are severely involved. The oral and nasal mucous membranes may blister and erode, while the eyes may develop conjunctivitis, keratitis, corneal ulcers, and in severe cases, total uveitis and blindness. Pneumonia, myocarditis, arthritis, gastrointestinal ulcers, liver and kidney damage can also be complicated, with associated clinical symptoms and signs. The skin lesions may become infected and even septic. This type may cause death due to untimely resuscitation. Third, [histopathology] can see partial necrosis of keratinocytes, liquefaction and degeneration of basal cells to form subepidermal blisters, intracellular edema and sponge formation. There are significant vasculitis changes in the upper part of the dermis, with lymphocyte infiltration around the vessels, mixed with neutrophils and eosinophils. 4. [Diagnosis and Differential Diagnosis] According to the characteristics of the lesions, combined with the prevalent sites, the diagnosis is generally not difficult. Attention should be paid to the careful questioning of medical history and the search for causative factors. It should be differentiated from the following diseases: (1) Frostbite occurs in winter. The lesions are deep red or purplish-red edematous erythema, or in severe cases, blistering and erosion, but there is usually no iris-like damage and no mucosal damage. (2) Drug rash Erythema multiforme type drug rash may be similar to erythema multiforme, but with a clear history of drug use, no seasonality and certain predilection sites. (3) Toxic epidermolysis bullosa (Lyell’s disease) should be distinguished from severe erythema multiforme. The onset of the disease is rapid, rapid, the rash is initially on the face, neck and chest, quickly spread to the whole body, the lesions are second-degree scald-like large loosening necrosis, the skin is dark since the red, Nee’s sign is positive. V. [Prevention and control] (1) Find and remove the causative factors: such as removing the foci of infection in the body, stop using the suspected allergenic drugs. (2) Light cases are generally given symptomatic treatment, such as antihistamines, calcium, vitamin C, etc., and topical application of glyburide lotion or corticosteroid cream. (3) In severe cases, adequate amount of corticosteroids should be given promptly. The infection-induced erythema multiforme should be selected with suitable antibiotics, and those with serious skin mucous membrane erosion should also be selected with antibiotics to prevent and control infection, but care should be taken to avoid drugs that may cause allergy. At the same time, various supportive therapies should be given according to the condition, to maintain the balance of water and electro-medium, and to improve the nutrition status of the whole body. To pay attention to the care of the skin mucosa, keep the mouth clean, use 3% hydrogen peroxide or chlorhexidine mouthwash, and for those who have painful oral mucosa erosion and affect eating, use 1% lidocaine gargle before eating. To avoid or reduce ocular sequelae, clean up the secretions in time, alternate eye dots with antibiotics and cortisone eye drops, and use erythromycin eye ointment at night. For large skin blisters, herpes fluid should be extracted, and when vesicles exude a lot, use 3% boric acid or 1/8000 potassium permanganate solution for wet compresses.