## **Core Concept**
The problem of bias is a significant concern in epidemiological studies. Bias refers to any systematic error introduced into sampling or testing by selecting or encouraging one outcome or answer over others. Different study designs have varying levels of susceptibility to bias.

## **Why the Correct Answer is Right**
The correct answer, **C. Case-control study**, is particularly prone to bias, especially recall bias and selection bias. In case-control studies, individuals are selected based on whether they have the outcome of interest (cases) or not (controls), and then their past exposures are compared. This design can lead to differential recall of exposure histories, with cases possibly recalling their exposures more accurately or differently than controls, simply because they know they have the disease.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Cohort studies, while susceptible to bias (e.g., loss to follow-up), generally have a prospective design that helps mitigate recall bias. Participants are selected based on exposure status and followed over time to assess outcomes, reducing the likelihood of differential recall.
- **Option B:** Cross-sectional studies can suffer from prevalence bias and recall bias but are often conducted at a single point in time, which limits the ability to establish temporality between exposure and outcome.
- **Option D:** Randomized controlled trials (RCTs) are considered the gold standard for determining causality and are designed to minimize bias through randomization, which helps ensure comparability between groups.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while all study designs have limitations, case-control studies are particularly vulnerable to biases that can distort the apparent relationship between exposure and outcome. Understanding these biases is crucial for critically evaluating the medical literature.

## **Correct Answer: C. Case-control study**