Which statement best defines a cohort study and distinguishes prospective from retrospective designs?

• A. A cohort study follows a defined group over time to compare outcomes by exposure status; prospective cohorts collect exposure data before outcomes occur, while retrospective cohorts use existing records to reconstruct timelines.
• B. A cohort study randomly assigns participants to exposure groups to measure outcomes.
• C. A cohort study examines multiple outcomes at a single point in time.
• D. A cohort study is always prospective and relies on active surveillance for events.

Answer: (A)

A cohort study follows a defined group over time to compare outcomes by exposure status, using exposure to partition participants and then watching to see who develops the outcome. In a prospective cohort, exposure is assessed before any outcome occurs and participants are followed forward to observe future events. In a retrospective cohort, both exposure and outcome data already exist in records, so the timeline is reconstructed after the fact to analyze associations. This setup shows temporality—exposure precedes outcome—which is the hallmark of cohort designs. The other descriptions fit different study types: random assignment describes an experimental design, examining multiple outcomes at one point in time aligns with a cross-sectional study, and saying a cohort is always prospective with active surveillance would be inaccurate since cohorts can be retrospective and rely on existing data rather than ongoing monitoring.