**Core Concept**
Positive predictive value (PPV) is a measure of the probability that a patient with a positive test result truly has the disease. It is calculated as the number of true positives divided by the sum of true positives and false positives.

**Why the Correct Answer is Right**
The denominator of PPV is the sum of true positives and false positives, which represents all patients with positive test results. This means that PPV takes into account the proportion of patients with positive test results who actually have the disease (true positives) and those who do not (false positives). The correct answer is the sum of true positives and false positives.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because PPV is calculated as the ratio of true positives to the sum of true positives and false positives, not just the number of true positives.

**Option B:** This option is incorrect because the numerator of PPV is the number of true positives, not the denominator.

**Option C:** This option is incorrect because the specificity of a test is a measure of the proportion of patients without the disease who test negative, not the denominator of PPV.

**Option D:** This option is incorrect because the sensitivity of a test is a measure of the proportion of patients with the disease who test positive, not the denominator of PPV.

**Clinical Pearl / High-Yield Fact**
To remember the denominator of PPV, recall that it includes both patients who truly have the disease (true positives) and those who do not (false positives), emphasizing the importance of considering both in the calculation of PPV.

**Correct Answer:** D. The sum of true positives and false positives.