**Question:** In a case control study, the most characteristic feature is -

A. Random selection of participants
B. Longitudinal study design
C. Comparing cases with controls
D. Prospective study design

**Core Concept:** A case-control study is a type of observational study where the researcher examines the characteristics of cases (individuals with the disease or outcome of interest) and controls (individuals without the disease or outcome of interest). The aim is to determine if there is an association between a particular exposure (risk factor) and the disease or outcome.

**Why the Correct Answer is Right:** The correct answer is **C. Comparing cases with controls**. In a case-control study, the most distinctive feature is comparing the characteristics, risk factors, or exposures of cases with those of controls to identify potential risk factors associated with the development of the disease or outcome of interest.

**Why Each Wrong Option is Incorrect:**

A. **Random selection of participants (Random sampling)** is important but not the most characteristic feature. Random sampling ensures that the study's results are representative of the target population. However, the focus in a case-control study is on comparing cases and controls.

B. **Longitudinal study design** involves following individuals over time to observe changes in their health status or exposures. A case-control study, on the other hand, compares cases and controls at a single time point, not following individuals over time.

D. **Prospective study design** involves recruiting participants before the development of the disease or outcome and following them prospectively. In contrast, a case-control study compares cases and controls retrospectively, once the disease or outcome has occurred.

**Clinical Pearl:** The key aspect of a case-control study is comparing cases with controls to identify potential risk factors associated with the development of a disease or outcome. Random sampling is essential for representativeness, but focusing on the comparison between cases and controls is the defining feature of a case-control study.