**Core Concept**
The odds ratio (OR) is a statistical measure used to compare the strength and direction of association between an exposure and an outcome in case-control studies. It represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure.

**Why the Correct Answer is Right**
The odds ratio estimates the relative risk of disease or outcome in the exposed group compared to the unexposed group. It is used to quantify the strength and direction of association between a risk factor and a disease. The OR is calculated as the ratio of the odds of exposure among cases to the odds of exposure among controls. This helps researchers to identify potential risk factors and to estimate the magnitude of the association.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the odds ratio is not a measure of correlation. While correlation coefficients can indicate the strength and direction of association, the odds ratio is a more specific measure that accounts for the effect of the exposure on the outcome.

**Option B:** This option is incorrect because the odds ratio is not a measure of sensitivity or specificity. While these metrics are important in diagnostic testing, the odds ratio is used to quantify the association between an exposure and an outcome in observational studies.

**Option C:** This option is incorrect because the odds ratio is not a measure of effect size. While effect size can be estimated using various metrics, the odds ratio is a specific measure that represents the ratio of odds between exposed and unexposed groups.

**Clinical Pearl / High-Yield Fact**
When interpreting odds ratios, remember that an OR of 1 indicates no association between the exposure and the outcome. An OR greater than 1 indicates a positive association, while an OR less than 1 indicates a negative association.

**Correct Answer:** C. Odds ratio.