A primary care physician is evaluating the safety profile of a recently approved weight-lo… — USMLE Step 2 CK

A primary care physician is evaluating the safety profile of a recently approved weight-loss medication following multiple post-marketing reports of acute pancreatitis. To better understand this potential risk, she reviews a publication in a prominent medical journal. The authors of the paper identified $200$ patients diagnosed with acute pancreatitis and matched them with $400$ healthy patients without pancreatitis from a national health registry. A review of their pharmacy records reveals that $80$ of the patients with pancreatitis had been prescribed the weight-loss medication, whereas $40$ of the healthy patients had been prescribed the same medication. Based on the design of this study, which of the following is the most appropriate measure of association and its calculated value?

Odds ratio of $6.0$ Answer
B
Relative risk of $6.0$
C
Relative risk of $2.7$
D
Odds ratio of $2.7$
E
Odds ratio of $2.0$

Answer

The most appropriate measure is the Odds ratio, and its calculated value is 6.0.

The study groups patients by their outcome (acute pancreatitis vs. no pancreatitis) and retroactively reviews their exposure status (weight-loss medication). This is the hallmark of a case-control study. In case-control studies, true disease incidence cannot be determined because the researchers set the number of cases and controls. Therefore, Relative Risk (RR) cannot be calculated. The appropriate measure is the Odds Ratio (OR), which compares the odds of exposure among cases to the odds of exposure among controls. The formula is

\text{OR} = \frac{a \times d}{b \times c}

. Here,

a = 80

(exposed cases),

c = 120

(unexposed cases),

b = 40

(exposed controls), and

d = 360

(unexposed controls). Calculating this yields

(80 \times 360) / (40 \times 120) = 28800 / 4800 = 6.0

Step-by-Step Solution

Identify the study design based on the scenario.

The study design is a case-control study.

Patients were selected based on their disease outcome (pancreatitis vs. healthy), and then their past exposure (weight-loss medication) was assessed.

Determine the appropriate measure of association.

The appropriate measure is the Odds Ratio (OR).

Relative Risk (RR) requires incidence data, which can only be obtained from cohort studies or clinical trials. Case-control studies must use the OR to estimate association.

Set up a 2x2 contingency table using the provided data.

Cases (Pancreatitis) = 200 total (80 exposed, 120 unexposed). Controls (Healthy) = 400 total (40 exposed, 360 unexposed).

Accurate tabulation of exposed (

a, b

) and unexposed (

c, d

) groups is required for the formula.

Calculate the Odds Ratio.

OR = (Odds of exposure in cases) / (Odds of exposure in controls) = (80 / 120) / (40 / 360) = 6.0.

Using the cross-product formula

(a \times d) / (b \times c) = (80 \times 360) / (40 \times 120) = 28800 / 4800 = 6.0

Key Concept

Case-control studies select patients based on disease status and use the Odds Ratio (OR) as the primary measure of association. Relative Risk (RR) cannot be directly calculated because incidence is not measured.

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