A 72-year-old woman who is planning to undergo ventral hernia repair is on warfarin for atrial fibrillation. She is advised to cease her warfarin several days before her surgery and is hospitalized preoperatively for heparinization. During her hospital stay, she complains of severe abdominal and flank pain. Her prothrombin time (PT) is normal, but her activated paial thromboplastin time (aPTT) is elevated. An abdominal CT scan demonstrates a large retroperitoneal hematoma. Which of the following should be administered to reverse the effects of the heparin?