A researcher studies association between arsenic exposure and bladder cancer using stored blood samples from a cohort enrolled 10 years ago. This study design is best described as:

• A Prospective cohort study
• B Ambidirectional cohort study ✓
• C Case-control study nested within a cohort
• D Retrospective (historical) cohort study

Explanation

An ambidirectional cohort study has both retrospective and prospective components. The cohort was enrolled in the past (retrospective component — using stored samples to determine past exposure) while also following the same individuals forward in time. This distinguishes it from a purely retrospective cohort (historical) where all data collection is from past records, and from a nested case-control which selects cases and controls within the cohort rather than following all participants.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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