Investigators at a state department of health are investigating a sudden increase in cases… — USMLE Step 2 CK

Investigators at a state department of health are investigating a sudden increase in cases of acute kidney injury (AKI) among young adults in a specific county. They identify $50$ patients who were hospitalized with idiopathic AKI over the past month. To evaluate potential risk factors, they randomly select $150$ healthy young adults from the same county to serve as a comparison group. A detailed survey regarding recent dietary and lifestyle exposures is administered to all participants.

The survey results reveal that $40$ of the patients with AKI and $30$ of the healthy individuals report regular consumption of a newly introduced, heavily marketed energy drink.

Which of the following represents the most appropriate measure of association for this study design and its calculated value?

A
Odds ratio of $4.0$
B
Relative risk of $16.0$
Odds ratio of $16.0$ Answer
D
Relative risk of $7.4$
E
Relative risk of $4.0$

Answer

Odds ratio of

16.0

This study is a case-control design because participants were selected based on their disease status (AKI cases versus healthy controls) and then surveyed for a past exposure (energy drink consumption). In case-control studies, incidence cannot be measured, so Relative Risk (RR) cannot be directly calculated. The appropriate measure of association is the Odds Ratio (OR). The odds of exposure in cases is

40/10 = 4.0

. The odds of exposure in controls is

30/120 = 0.25

. Therefore, the

\text{OR} = \frac{4.0}{0.25} = 16.0

Step-by-Step Solution

Identify the study design.

The study selects participants based on outcome status (

50

AKI cases and

150

healthy controls) and looks backward at prior exposures. This is a case-control study.

Determining the study design is essential to select the correct measure of association.

Select the appropriate measure of association.

The Odds Ratio (OR) must be used. Relative Risk (RR) cannot be calculated because the study does not measure disease incidence in populations at risk.

Case-control studies cannot determine absolute risk or incidence rates.

Calculate the odds of exposure for cases and controls.

Cases:

40

exposed,

10

unexposed; Odds =

40/10 = 4.0

. Controls:

30

exposed,

120

unexposed; Odds =

30/120 = 0.25

The OR requires comparing the odds of exposure between the two groups.

Calculate the Odds Ratio.

\text{OR} = \frac{4.0}{0.25} = 16.0

This provides the final measure of association between the exposure and outcome.

Key Concept

Case-control studies use the Odds Ratio (OR) to estimate the association between an exposure and an outcome, as they cannot directly measure disease incidence to calculate Relative Risk (RR).

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