A 55-year-old male with atrial fibrillation is prescribed warfarin for stroke prevention. The physician orders baseline INR and plans therapeutic drug monitoring. The structure marked **D** in the plasma concentration-time curve represents the area under the curve (AUC), which reflects total drug exposure and bioavailability. Which of the following best explains why therapeutic drug monitoring is essential for warfarin, and how AUC relates to its clinical use?