**Core Concept**
Positive predictive value (PPV) is a measure of the probability that a patient with a positive test result actually has the disease. It is an essential concept in evidence-based medicine, particularly in the context of screening and diagnostic tests. In the context of PPV, the denominator represents the total number of individuals who test positive for the disease.

**Why the Correct Answer is Right**
The correct answer is **C. Number of true positives + number of false positives** because the denominator of PPV includes all individuals who test positive, regardless of their true disease status. This is in contrast to the numerator, which includes only true positives. The PPV is calculated as the number of true positives divided by the total number of positive results, which includes both true positives and false positives. By including false positives in the denominator, PPV takes into account the possibility of test false positives, which is a critical consideration in the interpretation of diagnostic test results.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the denominator of PPV does not include true negatives, which are individuals who test negative for the disease.

**Option B:** This option is incorrect because the denominator of PPV does not include true negatives, which are individuals who test negative for the disease.

**Option D:** This option is incorrect because the denominator of PPV should include both true positives and false positives, not just true positives and false negatives.

**Clinical Pearl / High-Yield Fact**
When interpreting diagnostic test results, it is essential to consider both the sensitivity and specificity of the test, as well as the prevalence of the disease in the population being tested. By taking these factors into account, clinicians can better estimate the PPV and make more informed decisions about patient care.

**✓ Correct Answer: C. Number of true positives + number of false positives**