An itchy red rash at the back of the knee in the leg socket can be caused by insect bite dermatitis, contact dermatitis or eczema. It can be treated according to different situations. 1. Insect bite dermatitis: If the back of the knee and leg fossa area is bitten by mosquitoes, localized pimples and itchy bumps will appear. It is recommended to use local cool oil, glycerine lotion and other rapid itching. 2. Contact dermatitis: if contact with allergic substances, the knee area appears allergic symptoms, the local will appear red rash, papules, papules, with itching, scratching may appear after oozing, erosion. Without oozing and erosion of the skin lesions can be used locally glucocorticoid cream, such as mometasone furoate cream, hydrocortisone cream. 3. Eczema: When eczema is present in the back of the knee, there will be erythema, papules, flaking and oozing, accompanied by itching. It is recommended to keep the area clean and dry, and minimize contact with water. Topical glucocorticoid creams, such as hydrocortisone butyrate cream and trimethoprim cream, can be used, as well as oral antihistamine drugs, such as loratadine and cetirizine. If the above symptoms occur, it is recommended to go to the hospital in time for a clear diagnosis and early formal treatment.