**Core Concept**
Heparin treatment is monitored by assessing its anticoagulant effect to prevent excessive bleeding or thrombosis. This is crucial in managing patients receiving heparin therapy for conditions like atrial fibrillation or deep vein thrombosis.

**Why the Correct Answer is Right**
The correct answer is monitoring of the activated partial thromboplastin time (aPTT). Heparin exerts its anticoagulant effect by enhancing the activity of antithrombin III, which inhibits the coagulation cascade, particularly the conversion of prothrombin to thrombin. The aPTT is a sensitive indicator of the anticoagulant effect of heparin and is used to adjust the heparin dose to achieve a therapeutic effect while minimizing the risk of bleeding.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the prothrombin time (PT) is used to monitor warfarin therapy, not heparin. Warfarin inhibits vitamin K-dependent clotting factors, which affects the PT.
**Option B:** Activated clotting time (ACT) is used to monitor heparin therapy in specific settings, such as during cardiopulmonary bypass surgery. However, it is not the standard method for monitoring heparin therapy in most clinical situations.
**Option C:** The international normalized ratio (INR) is used to monitor warfarin therapy, not heparin. The INR is a standardized measure of the PT that is used to adjust warfarin doses.

**Clinical Pearl / High-Yield Fact**
Remember that the aPTT is sensitive to the intensity of heparin therapy, and small changes in heparin dose can significantly affect the aPTT. This is why regular monitoring of the aPTT is essential to prevent both under- and over-anticoagulation.

**Correct Answer:** C.