A 79-year-old woman is admitted with ongoing lower gastrointestinal bleeding and a haemoglobin concentration of 72 g/L (reference range 115–155). She is receiving red-cell transfusion, has been confined to bed and has no previous venous thromboembolism. Anticoagulant prophylaxis is being considered, but fresh blood continues to appear on repeated rectal examinations. Which one of the following is the most appropriate venous thromboembolism prevention?