A research team investigates the relationship between regular consumption of a novel plant… — USMLE Step 3

A research team investigates the relationship between regular consumption of a novel plant-based protein drink and the incidence of nephrolithiasis. In the initial analysis of a large prospective cohort, the crude relative risk (RR) of developing nephrolithiasis for daily consumers compared to non-consumers is $2.5$ ( $95\%$ CI: $1.8-3.4$ ). Suspecting the influence of underlying participant characteristics, the investigators perform a stratified analysis based on daily water intake (adequate vs. inadequate). The stratum-specific RR for participants with adequate water intake is $1.0$ ( $95\%$ CI: $0.8-1.2$ ), and the RR for those with inadequate water intake is $1.1$ ( $95\%$ CI: $0.9-1.4$ ). Which of the following best explains the discrepancy between the crude and stratified relative risk calculations in this study?

Confounding by daily water intakeAnswer
B
Effect modification by daily water intake
C
Cognitive bias during data interpretation
D
Incorrect selection of inferential statistical tests
E
Distortion due to the underlying disease prevalence

Answer

Confounding by daily water intake

The correct answer is confounding by daily water intake. Confounding occurs when an extraneous variable is associated with both the exposure (protein drink) and the outcome (nephrolithiasis), creating a false association. The classic method to identify confounding during the analysis phase of a study is stratification. If the crude (unadjusted) relative risk shows a significant association, but the stratum-specific relative risks both shift toward the null value (

1.0

), it indicates that the initial association was driven entirely by the confounding variable.

Step-by-Step Solution

Analyze the initial finding.

The crude relative risk is

2.5

, suggesting a significant association between the protein drink and nephrolithiasis.

Establishing the baseline unadjusted relationship is the first step in epidemiological analysis.

Evaluate the results after stratification.

When separated into subgroups based on water intake, the relative risks drop to

1.0

and

1.1

, meaning the association effectively disappears.

Stratification isolates the impact of the third variable to reveal the true underlying relationship.

Differentiate between confounding and effect modification.

Because the risk became statistically insignificant in both strata rather than diverging (e.g., high in one, low in the other), the third variable is a confounder.

This is the classic textbook pattern used to identify confounding in observational studies.

Key Concept

Confounding

Question

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Step-by-Step Solution

Key Concept

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