A 65-year-old man undergoes a technically difficult abdominal-perineal resection for a rectal cancer during which he receives 3 units of packed red blood cells. Four hours later, in the intensive care unit (ICU), he is bleeding heavily from his perineal wound. Emergency coagulation studies reveal normal prothrombin, paial 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. Which of the following is the most likely cause of his bleeding?