A 45-year-old man with a 20-year smoking history presents to the emergency department with acute anterior wall myocardial infarction. His initial vital signs are BP 148/92 mmHg, HR 102/min, RR 18/min, and SpO2 94% on room air. Troponin I is 2.8 ng/mL. During admission, his serum total cholesterol is measured at 245 mg/dL. Epidemiologists studying a large cohort of patients with acute MI note a strong positive association between serum cholesterol level and risk of myocardial infarction in the overall population (p < 0.001). However, when the cohort is stratified by smoking status, the association between cholesterol and MI risk substantially weakens in both smokers and non-smokers (p = 0.15 in each stratum). Which of the following epidemiologic phenomena best explains the difference between the crude and stratum-specific associations?

1. A)Effect modification, in which smoking status alters the strength of the relationship between cholesterol and MI risk
2. B)Selection bias, as differential enrollment of high-cholesterol smokers versus non-smokers artificially strengthens the crude association
3. C)Information bias, due to differential misclassification of smoking status based on cholesterol levels
4. D)Confounding, in which smoking status is associated with both cholesterol and MI risk, distorting their crude relationshipGABARITO
5. E)Regression to the mean, as extremely elevated cholesterol values in the overall group regress toward baseline upon stratification