**Core Concept**
Warfarin is a vitamin K antagonist used as an oral anticoagulant. It acts by inhibiting the synthesis of vitamin K-dependent clotting factors in the liver. The effectiveness of warfarin can be influenced by various factors, including other medications that either enhance or decrease its anticoagulant effect.

**Why the Correct Answer is Right**
Cholestyramine is a bile acid sequestrant used to lower cholesterol levels. It can decrease the absorption of warfarin by binding to it in the gastrointestinal tract, thereby reducing its anticoagulant effect. This interaction can lead to a decrease in the international normalized ratio (INR), which is used to monitor the effectiveness of warfarin therapy.

**Why Each Wrong Option is Incorrect**
**Option A:**
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**Clinical Pearl / High-Yield Fact**
It is essential to monitor INR levels closely when starting or adjusting warfarin therapy, especially in patients taking other medications that may interact with it. Regular monitoring of INR can help prevent thrombotic events while minimizing the risk of bleeding.

**Correct Answer:**