Which description matches a cohort study?

• A. A study describing a single patient in depth
• B. A study using two populations: one that had contact with the agent causing the disease and another that had not had contact
• C. A study that randomizes participants into control and experimental groups
• D. A study using two populations: one that had contact with the agent causing the disease and another that had not had contact

Answer: (D)

Cohort studies follow groups defined by exposure to something and then look forward in time to see who develops the outcome. The key idea is starting with people based on whether they were exposed to the agent causing the disease (exposed vs unexposed) and tracking them to observe incidence of disease. This setup lets you compare risk between the two groups and calculate measures like relative risk, whether the study is prospective or retrospective.

That’s why the description with two populations split by exposure status and then monitoring for disease fits best. It’s not a single-patient description (that would be a case report), nor is it an experimental design where participants are randomly assigned to groups (that would be a randomized controlled trial). A snapshot at one time point without following outcomes over time would describe a cross-sectional study, not a cohort study.