**Core Concept**
A case-control study is a type of observational study that compares individuals with a specific outcome (cases) to those without the outcome (controls) to identify potential risk factors or associations.

**Why the Correct Answer is Right**
In a case-control study, the researcher retrospectively selects participants based on their outcome status, which is the opposite of a cohort study where participants are selected based on their exposure status. This design allows researchers to investigate rare outcomes or outcomes with long latency periods. The study involves identifying cases and controls, collecting data on potential risk factors, and then analyzing the data to determine if there is an association between the risk factors and the outcome. This design is particularly useful for investigating potential risk factors for diseases with long latency periods.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because a case-control study is not a type of experimental study, where the researcher actively intervenes to alter the outcome.
* **Option B:** This option is incorrect because a case-control study is not a type of cross-sectional study, where the researcher collects data on a population at a single point in time.
* **Option C:** This option is incorrect because a case-control study is not a type of randomized controlled trial, where participants are randomly assigned to different groups.

**Clinical Pearl / High-Yield Fact**
One key consideration when designing a case-control study is the potential for recall bias, where participants with the outcome may be more likely to remember exposures or risk factors than those without the outcome.

**Correct Answer: D. Case-control studies are a type of observational study.**