**Core Concept**
A case-control study is a type of observational study where two groups, cases and controls, are compared based on their exposure to a potential risk factor or disease outcome. This study design is particularly useful for investigating rare diseases or outcomes where cohort studies may not be feasible.

**Why the Correct Answer is Right**
In a case-control study, the most characteristic feature is the retrospective design, where the outcome (disease) has already occurred, and the investigator looks back in time to identify potential risk factors or exposures that may have contributed to the development of the disease. This approach allows researchers to investigate rare outcomes and identify potential risk factors that may not be apparent in cohort studies. The study design involves selecting cases (individuals with the disease) and controls (individuals without the disease) and comparing their exposure histories to identify potential associations.

**Why Each Wrong Option is Incorrect**
**Option A:**
*This option is not applicable as it does not describe a characteristic feature of case-control studies.*

**Option B:**
*This option is incorrect as cohort studies are a different type of study design, where participants are followed over time to observe the development of outcomes.*

**Option C:**
*This option is incorrect as while case-control studies can be used to investigate genetic factors, it is not the most characteristic feature of this study design.*

**Clinical Pearl / High-Yield Fact**
When interpreting the results of a case-control study, it is essential to consider the potential for recall bias, where cases may be more likely to recall exposures or risk factors than controls.

**Correct Answer: D. Retrospective design.**