A researcher conducts a cross-sectional study of hypertension prevalence and notices that patients with hypertension also have higher rates of a certain genetic variant. When age is controlled for, the association disappears because older age independently causes both hypertension and the variant to increase. This is an example of:
- A Confounding ✓
- B Effect modification
- C Mediation
- D Collider bias
Correct answer: A. Confounding
Explanation
Confounding occurs when a third variable (here, age) is independently associated with both the exposure (genetic variant) and outcome (hypertension), and is not on the causal pathway. When the confounder is controlled, the spurious association disappears — a hallmark of confounding. Effect modification occurs when the effect of exposure differs across strata of the third variable. Mediation means the third variable is on the causal path. Collider bias occurs when conditioning on a common effect of exposure and outcome.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
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