To test whether heavy physical exertion triggers acute myocardial infarction, researchers enrol patients admitted with a first MI and ask each patient about exertion during the 2 hours before symptom onset and during matched 2-hour control periods from preceding days, comparing exposure levels across these windows within the same individual. This design is called:

• A Cross-sectional study
• B Case-crossover study ✓
• C Retrospective cohort study
• D Interrupted time series

Explanation

A case-crossover design uses each case as his or her own control, comparing transient exposure immediately before the event with exposure during earlier reference periods. It suits brief, intermittent exposures with acute outcomes, and self-matching automatically controls for stable personal characteristics. Cohort designs require unexposed comparison groups, cross-sectional studies measure simultaneous exposure and outcome, and interrupted time series evaluates population trends over time.

Reference: Modern Epidemiology (Rothman), 4th ed.

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