**Core Concept:** Warfarin is an anticoagulant drug that inhibits vitamin K-dependent clotting factors (II, VII, IX, X, protein C, and protein S), thereby reducing the formation of blood clots. The International Normalized Ratio (INR) is a standardized measure of clotting time, used to monitor the effectiveness of anticoagulant therapy.

**Why the Correct Answer is Right:** The target INR in a patient on warfarin therapy depends on the patient's specific condition and risk of bleeding. Generally, the target INR range is between 2.0 and 3.0 for patients with atrial fibrillation, deep vein thrombosis, or pulmonary embolism. For patients with mechanical heart valves, the target INR range is 2.5-3.5.

**Why Each Wrong Option is Incorrect:**

A. A target INR of 1.5 is too low, increasing the risk of thrombosis.
B. A target INR of 4.0 is too high and can lead to excessive bleeding.
C. A target INR of 6.0 is too high and can lead to excessive bleeding.
D. A target INR of 7.0 is too high and can lead to excessive bleeding.

**Clinical Pearl:** Regular monitoring of INR is essential in patients on warfarin therapy as it helps to adjust the dose appropriately and maintain the therapeutic INR range, minimizing the risk of both thrombosis and bleeding complications. In clinical practice, a moderate INR range (2.0-3.0) is typically chosen, depending on the patient's condition and risk factors.