Which study design begins with individuals who have a disease and looks back to determine prior exposures?

• A. Case-control
• B. Cohort
• C. Cross-sectional
• D. Randomized trial

Answer: (A)

This question hinges on the type of study that starts with who already has the disease and then looks back in time to determine prior exposures. That approach is a case-control study. In this design, you identify cases (people with the disease) and controls (those without the disease) and compare their histories to see if prior exposures or risk factors are more common among the cases. It’s particularly efficient for studying rare diseases or diseases with long latency, because you don’t have to follow a large cohort over time. The main measure derived is the odds ratio, reflecting how strongly a past exposure is associated with the disease. In contrast, cohort studies begin with exposure status and follow people forward to see who develops the disease, cross-sectional studies assess exposure and disease at a single time point, and randomized trials assign exposure to participants to evaluate outcomes.