**Core Concept**
Prothrombin Time (PT) is a measure of the time it takes for blood to clot, reflecting the efficiency of the extrinsic coagulation pathway. The International Normalized Ratio (INR) is a standardized measure of PT, allowing for comparison of coagulation status across different laboratories. The INR is calculated using the PT ratio and the sensitivity index of the thromboplastin reagent used.

**Why the Correct Answer is Right**
The INR is calculated by multiplying the PT ratio (patient's PT divided by the control PT) by the sensitivity index of the thromboplastin reagent. The sensitivity index is a correction factor that accounts for the variability in thromboplastin reagents. In this case, the sensitivity index is 1.41, and the PT ratio is 26/13. To calculate the INR, we multiply the PT ratio by the sensitivity index: (26/13)1.41. This adjustment ensures that the INR is a standardized measure of coagulation status, allowing for accurate comparison between patients and laboratories.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not account for the sensitivity index of the thromboplastin reagent. The INR calculation requires multiplication of the PT ratio by the sensitivity index, not simple division.

**Option B:** This option is incorrect because it incorrectly combines the PT ratio and the sensitivity index using a multiplication sign. The correct operation is exponentiation, not multiplication.

**Option C:** This option is incorrect because it incorrectly adds the PT ratio and the sensitivity index. The INR calculation requires multiplication of the PT ratio by the sensitivity index, not addition.

**Clinical Pearl / High-Yield Fact**
When interpreting INR results, it's essential to remember that the INR is a ratio, not an absolute value. A patient's INR may be affected by various factors, including the type of thromboplastin reagent used and the patient's underlying coagulation status.

**✓ Correct Answer:** D. (26/13)1.41