A study compares the incidence of myocardial infarction (MI) in patients taking low-dose aspirin versus those not taking it. Patients already taking aspirin for an existing cardiovascular condition have a higher baseline risk. The resulting spurious apparent harm from aspirin is an example of:
- A Detection bias
- B Confounding by indication ✓
- C Recall bias
- D Lead-time bias
Correct answer: B. Confounding by indication
Explanation
Confounding by indication (channelling bias) occurs when the drug is prescribed because of the very condition it is being evaluated against, making it appear harmful when the underlying disease is the true cause of the worse outcome. This is common in pharmacoepidemiological observational studies. Detection bias relates to differential ascertainment of outcomes. Recall bias affects case-control studies where past exposure is self-reported. Lead-time bias is specific to screening studies.
Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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