Which of the following is a structural reason why diabetes and heart disease would be associated in the DAG above?

Question

BlackTom AI · Accepted Answer

To tackle this question, I’ll evaluate what each option implies about the relationship between physical activity, diabetes, and heart disease.

Option a: 'Diabetes is a cause of heart disease' — This reverses the typical directional assumption in a DAG about risk factors. While diabetes increases risk for heart disease, the statement claims diabetes causes heart disease directly as a structural path, which oversimplifies and misrepresents causal direction in many DAGs. It ignores the possibility that both conditions share common causes and that diabetes may mediate or confound relationships rather than act as an ultimate cause of heart disease in the given structure.

Option b: 'Physical activity is a common cause of both diabetes and heart disease' — This describes a structural scenario where a single upstream variable (physical activity) influences both downstream outcomes (diabetes and heart disease). In DAG terminology, this is a confounding or common cause structure: physical activity -> diabetes and physical activity -> heart disease. Such a configuration would generate an association between diabetes and heart disease even if there is no direct causal path between them, simply because they share a parent node. This is a plausible and standard structural reason for the observed association across the two diseases in the diagram.

Option c: 'Both diabetes and heart disease affect mortality and the analysis is restricted to only those who are alive at the time of the study' — This describes selection bias or conditioning on a collider/outcome (survival bias). If the DAG includes mortality, and the study conditions on survivors, this could induce or distort associations between diabetes and heart disease. However, the statement focuses on mortality affecting the association rather than describing a structural reason for a direct connection between the two diseases. It speaks to a bias mechanism, not a fundamental structural link between the diseases.

Option d: 'All of the above' — Since option a is not a correct structural reason and option c describes a bias mechanism rather than a direct structural link, this blanket choice would be inaccurate if we are focusing specifically on a structural reason for the association.

In summary, option b reflects a valid structural explanation: a common cause such as physical activity can create an association between diabetes and heart disease in a DAG, independent of a direct causal path from one disease to the other. The other options either misstate causal directions, describe bias mechanisms rather than structural paths, or incorrectly bundle multiple conclusions that are not all correct in the same structural context.

Which of the following is a structural reason why diabetes and heart disease would be associated in the DAG above?

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