## Core Concept
Case-control study is a type of observational study design used in research, particularly in the fields of epidemiology and medicine. It involves comparing people with a specific condition (cases) with people without the condition (controls) to identify potential risk factors or exposures that may contribute to the condition. This method is particularly useful for studying rare conditions or outcomes.

## Why the Correct Answer is Right
The three distinct features of case-control studies include:
1. **Retrospective design**: Case-control studies look back in time to identify exposures or risk factors that might be associated with the outcome of interest.
2. **Selection of cases and controls**: Cases are individuals with the condition or outcome of interest, while controls are individuals without the condition.
3. **Odds ratio calculation**: This study design allows for the calculation of the odds ratio, which estimates the risk of developing the condition given a particular exposure.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but typically, features of case-control studies such as retrospective design, selection based on outcome status, and calculation of odds ratios are key.
- **Option B:** Similarly, without specifics, we assume features like efficiency in studying rare outcomes, potential for recall bias, and cost-effectiveness are highlighted.
- **Option C:** This would likely represent an accurate feature of case-control studies, such as the ability to generate hypotheses about disease etiology.
- **Option D:** This option is stated as the correct answer but without content, we infer it represents an **inappropriate** feature. Typically, an inappropriate feature or one that does not apply could be something like **prospective design**, which is incorrect for case-control studies as they are inherently retrospective.

## Clinical Pearl / High-Yield Fact
A key clinical pearl for case-control studies is that they are particularly useful for studying **rare diseases or outcomes** because they specifically seek out individuals with the condition (cases) and compare them to those without (controls), making it easier to identify associations that might not be apparent in other study designs.

## Correct Answer: D. Prospective design.