Inexplicable bruising on legs may be related to impact of foreign objects, allergic purpura, hemophilia, hereditary factor VII deficiency and so on. 1. Impact of foreign objects: when a foreign object impacts the leg, it may lead to bruising under the skin, which is manifested as inexplicable bruising on the leg. 2. Anaphylactic purpura: the cause of the disease is still unclear, and may be related to infection, immune disorders, heredity and other factors. Patients mainly have skin purpura, joint swelling, activity limitation, nausea and vomiting, etc., which can be manifested as inexplicable bruises on the legs. 3. Hemophilia: hemophilia is mainly considered to be related to genetic abnormalities, which can cause inexplicable bruises on the legs of some patients. Some patients are also prone to limb movement disorder, joint blood accumulation, muscle hematoma and other symptoms. 4. Hereditary factor VII deficiency: mainly due to the mutation of coagulation factor VII gene, patients mainly have nosebleeds, ecchymosis of skin and mucous membranes, bleeding gums, joints, etc., which can also be manifested as inexplicable bruises on the legs. It is recommended that people with inexplicable bruises on the legs should consult a doctor as soon as possible for examination, and follow the doctor’s instructions for treatment after clarifying the cause of the disease.