**Core Concept**
A case-control study is a type of observational study that involves comparing individuals with a specific outcome or disease (cases) with individuals who do not have the outcome or disease (controls), with the goal of identifying potential risk factors or associations.

**Why the Correct Answer is Right**
Option B, "Risk factor is present but not outcome," is incorrect because in a case-control study, the outcome (disease or condition) has already occurred in the case group. The study then retrospectively investigates the presence of potential risk factors among cases and controls. This means that both the outcome and risk factors have already occurred, making option C a characteristic of case-control studies. Option B is incorrect because it suggests that only risk factors are present without the outcome, which is not a defining feature of case-control studies.

**Why Each Wrong Option is Incorrect**
**Option A:** A case-control study can be retrospective (looking back in time) or prospective (looking forward in time), but the term "retrospective study" is more general and applies to various study designs, including case-control studies.
**Option C:** This option is actually a characteristic of case-control studies, as both the outcome and risk factors have already occurred.
**Option D:** While case-control studies are generally considered to be safe for participants, as they do not involve interventions or randomization, this option is too vague and does not accurately capture the unique features of case-control studies.

**Clinical Pearl / High-Yield Fact**
When designing a case-control study, it's essential to ensure that the selection of cases and controls is unbiased and representative of the population being studied. This can be achieved by using well-defined inclusion and exclusion criteria and ensuring that the study is adequately powered to detect the desired associations.

✓ Correct Answer: B. Risk factor is present but not outcome