**Core Concept**
Specificity in medical diagnostics or clinical trials is a measure of a test's accuracy in identifying true negatives. It is calculated by dividing the number of true negatives by the sum of true negatives and false positives.

**Why the Correct Answer is Right**
The denominator in specificity is the sum of true negatives and false positives. This is because specificity estimates the proportion of actual negatives that are correctly identified as such by the test. In other words, it measures the test's ability to avoid false alarms. This calculation is essential in evaluating the performance of diagnostic tests, particularly in high-stakes clinical settings.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because sensitivity, not specificity, uses true positives as the numerator.
**Option B:** This option is incorrect because the total number of patients, not just those with the disease, is not the correct denominator for specificity.
**Option C:** This option is incorrect because the number of false negatives, not the sum of true negatives and false positives, is not the correct denominator for specificity.

**Clinical Pearl / High-Yield Fact**
Remember the mnemonic "SPAD" to recall the correct sequence for specificity: **S**um of true negatives and false positives, **P**roportion of actual negatives correctly identified, **A**ccuracy in avoiding false alarms, and **D**enominator in specificity calculation.

**Correct Answer:** D. Sum of true negatives and false positives.