What causes sore welts behind knees?

Sore welts behind the knees can be caused by several factors, including allergic reactions, insect bites, or skin conditions like eczema or psoriasis. They may also result from friction or irritation due to clothing or physical activity. In some cases, underlying medical conditions such as dermatitis or infections could be responsible. If the welts persist or worsen, it's advisable to consult a healthcare professional for a proper diagnosis and treatment.