Patients who find a purple patch on the leg may be caused by purpura, or it may be triggered by a ruptured blood vessel wall, or it may be caused by a bump or mosquito bite. When this symptom appears, the first thing to do is to identify the cause of the onset, and then you can do symptomatic treatment. If this condition occurs frequently, go to the hospital as soon as possible for examination so that you can prevent some diseases. If the bruises on the legs are caused by mosquito bites, you can apply some local alkaline drugs to neutralize the acidic venom of mosquitoes.