**Core Concept**
Sensitivity is a crucial concept in the evaluation of screening tests, which is a fundamental aspect of social and preventive medicine. It refers to the ability of a screening test to detect individuals with the disease, i.e., true positives.

**Why the Correct Answer is Right**
Sensitivity measures the proportion of actual positives correctly identified by the test. In other words, it is the ratio of true positives to the sum of true positives and false negatives. A highly sensitive test is capable of detecting most of the individuals with the disease, thereby reducing the number of false negatives. For instance, a screening test for breast cancer that detects 90% of actual cases is considered highly sensitive.

**Why Each Wrong Option is Incorrect**
**Option B:** Specificity is the ability of a screening test to detect individuals without the disease, i.e., true negatives. It is not related to detecting true positives.
**Option C:** Positive predictive value (PPV) is the probability that an individual with a positive test result actually has the disease. While PPV is an important measure, it is not the same as sensitivity.
**Option D:** Negative predictive value (NPV) is the probability that an individual with a negative test result does not have the disease. Like PPV, NPV is an important measure, but it is not related to detecting true positives.

**Clinical Pearl / High-Yield Fact**
Remember that sensitivity and specificity are not mutually exclusive, and a test can be both sensitive and specific. However, it is rare to find a test that is both highly sensitive and highly specific.

**Correct Answer Line**
✓ Correct Answer: A. Sensitivity