A bruise on the leg may be caused by trauma, thrombocytopenic purpura, aplastic anemia and other diseases. 1. Trauma: If the legs are hit by external force, it may lead to the rupture of tiny blood vessels under the skin, which may result in the appearance of bruises on the legs. 2. Thrombocytopenic purpura: often associated with autoimmune system, heredity, primary diseases and other factors, the clinical manifestations of the skin, mucous membranes will appear petechiae ecchymosis. 3. Aplastic anemia: this disease may be related to heredity, radiation, viral infection and other factors. Patients mainly show symptoms such as aggravation of anemia, bleeding and petechiae on the skin or mucous membranes. There are many reasons that lead to bruises and purple patches on the legs, and it is recommended that patients go to regular hospitals for examination to clarify the causes of the disease.