**Core Concept**
The diagnostic power of a test refers to its ability to accurately identify those with a particular disease or condition. This is quantitatively measured by various statistical parameters that assess the test's sensitivity, specificity, and predictive values. One such parameter is the likelihood ratio, which helps clinicians interpret the results of a diagnostic test in the context of pre-test probabilities.

**Why the Correct Answer is Right**
The correct answer is **Positive Likelihood Ratio (PLR)**. The PLR represents the ratio of the probability of a positive test result in a patient with the disease to the probability of a positive test result in a patient without the disease. This helps clinicians determine how much the test result changes the pre-test probability of a patient having the disease, thereby aiding in decision-making. The PLR is a valuable tool for evaluating the diagnostic power of a test, especially in situations where the pre-test probability of the disease is uncertain.

**Why Each Wrong Option is Incorrect**
**Option A:** **Negative Likelihood Ratio (NLR)**. While the NLR is also a useful parameter, it reflects the ratio of the probability of a negative test result in a patient with the disease to the probability of a negative test result in a patient without the disease. It is useful for evaluating the test's ability to rule out a disease, but not its diagnostic power.

**Option B:** **Sensitivity**. Sensitivity is a measure of a test's ability to correctly identify those with the disease, but it does not take into account the test's ability to correctly identify those without the disease. Therefore, it does not fully reflect the diagnostic power of a test.

**Option C:** **Specificity**. Specificity is a measure of a test's ability to correctly identify those without the disease, but it does not take into account the test's ability to correctly identify those with the disease. Like sensitivity, it does not fully reflect the diagnostic power of a test.

**Clinical Pearl / High-Yield Fact**
When interpreting the results of a diagnostic test, clinicians should always consider the pre-test probability of the disease, the test's sensitivity and specificity, and the likelihood ratio to accurately assess the diagnostic power of the test.

**Correct Answer: C. Specificity.**