**Diagnostic vs Screening Tests**

**Core Concept**
Diagnostic tests are used to confirm or rule out a specific disease or condition in an individual who has already shown symptoms or risk factors, whereas screening tests are used to identify individuals who may be at risk of a particular disease or condition in a large population.

**Why the Correct Answer is Right**
The key difference between diagnostic and screening tests lies in their application and purpose. Diagnostic tests are typically more specific and sensitive, aimed at confirming a specific diagnosis in an individual with a high pre-test probability of the disease. Screening tests, on the other hand, are less specific and more sensitive, used to identify individuals with a low pre-test probability of the disease but a high prevalence in the population.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option may refer to the cost-effectiveness of diagnostic tests, which is not necessarily true. While diagnostic tests may be more expensive, they are often essential for confirming a diagnosis and guiding treatment.
* **Option B:** This option may refer to the frequency of testing, which is not a characteristic that distinguishes diagnostic from screening tests. Both types of tests may be repeated over time to monitor disease progression or response to treatment.
* **Option C:** This option may refer to the patient population, which is not a key difference between diagnostic and screening tests. Both types of tests may be applied to individuals with symptoms or risk factors, as well as to asymptomatic individuals in the general population.

**Clinical Pearl / High-Yield Fact**
When selecting a diagnostic test, clinicians should consider the pre-test probability of the disease, the sensitivity and specificity of the test, and the potential consequences of a false positive or false negative result.

**Correct Answer:** C.