A nurse is caring for a client who is receiving heparin for deep vein thrombosis. Which of the following laboratory values should the nurse monitor?

The correct answer is B: aPTT. The nurse should monitor the client's aPTT levels because heparin affects the intrinsic pathway of the coagulation cascade. Monitoring aPTT helps ensure that the client is receiving an appropriate dose of heparin to prevent clot formation without increasing the risk of bleeding. INR (choice A) is used to monitor warfarin therapy, not heparin. Platelet count (choice C) is important but does not directly monitor heparin therapy. Potassium level (choice D) is not typically affected by heparin therapy.