A 65-year-old man undergoes a technically difficult abdominoperineal resection for a rectal cancer during which he receives three units of packed red blood cells. Four hours later in the intensive care unit he is bleeding heavily from his perineal wound. Emergency coagulation studies reveal normal prothrombin, partial thromboplastin, and bleeding times. The fibrin degradation products are not elevated but the serum fibrinogen content is depressed and the platelet count is 70,000/mL. The most likely cause of the bleeding is