The nurse is assessing a postoperative patient who recently underwent a total knee replacement. During the assessment, the nurse notes that the patient's vital signs are stable, but there is a noticeable swelling in the operated knee. The nurse understands that one potential complication of this type of surgery is the development of a blood clot. To assess for this risk, the nurse should monitor for signs of deep vein thrombosis (DVT). One classic sign to watch for is ___________.