The correct answer is Warfarin.

- Warfarin is a vitamin K antagonist that has been used for decades as an oral anticoagulant.
- One of the critical aspects of warfarin therapy is the need for routine monitoring of the International Normalized Ratio (INR) to ensure the drug remains within the therapeutic range.
- This is essential because warfarin’s anticoagulant effect can be influenced by various factors, including dietary vitamin K intake, drug interactions, and individual metabolism variability.
- Maintaining the INR within the target range helps balance the risk of thrombosis versus bleeding complications.

- In contrast, the newer direct oral anticoagulants (DOACs), such as Dabigatran, Rivaroxaban, and Apixaban, have predictable pharmacokinetics and pharmacodynamics and do not require routine INR monitoring.
- These agents are generally preferred for their fixed dosing and fewer dietary restrictions but may need dose adjustment in certain clinical situations such as renal impairment.

Key points:
- Warfarin requires regular INR monitoring to guide dosing.
- INR monitoring prevents complications related to under- or over-anticoagulation.
- DOACs (Dabigatran, Rivaroxaban, Apixaban) do not require routine INR monitoring.

Reference: Harrison's Principles of Internal Medicine, 20th Edition, Volume 2, Chapter 198: Anticoagulant Therapy, Page 1656-1658.