A medical research team is investigating a potential link between maternal exposure to a s… — USMLE Step 2 CK

A medical research team is investigating a potential link between maternal exposure to a specific class of organic solvents during early pregnancy and the development of neural tube defects in offspring. The investigators review regional hospital registries from the past decade. They identify $100$ infants born with neural tube defects and select $300$ healthy infants without congenital anomalies from the same geographic area to serve as a comparison group.

Subsequent review of occupational and medical records reveals that $60$ mothers of the infants with neural tube defects had documented occupational exposure to the organic solvents during their first trimester. In contrast, $100$ mothers of the healthy infants had similar documented exposure.

Based on the design of this investigation, which of the following represents the most appropriate measure of association and its correctly calculated value?

The measure is the odds ratio, and the calculated value is $3.0$ Answer
B
The measure is the relative risk, and the calculated value is $2.25$
C
The measure is the relative risk, and the calculated value is $1.8$
D
The measure is the odds ratio, and the calculated value is $0.33$
E
The measure is the attributable risk, and the calculated value is $0.21$

Answer

The measure is the odds ratio, and the calculated value is

3.0

The study described selects participants based on the presence or absence of a disease (neural tube defects) and then looks back in time to assess for a risk factor (organic solvent exposure). This is the exact definition of a case-control study. Because the proportion of cases to controls is artificially set by the researchers (100 to 300 in this scenario), the true incidence of the disease in the underlying population cannot be calculated. Therefore, absolute risk and relative risk cannot be determined. The correct measure of association is the odds ratio, which compares the odds of exposure in the cases to the odds of exposure in the controls. The cross-product formula is (a*d)/(b*c). Using the data provided: (60 exposed cases * 200 unexposed controls) / (100 exposed controls * 40 unexposed cases) = 12000 / 4000 = 3.0.

Step-by-Step Solution

Identify the study design based on how participants were selected.

Participants were selected based on the presence of the outcome (100 infants with neural tube defects) or absence of the outcome (300 healthy infants). This is a case-control study.

Identifying the study architecture dictates which statistical measures are mathematically valid.

Determine the appropriate measure of association for the identified study design.

The odds ratio (OR) is the appropriate measure for a case-control study. Relative risk (RR) cannot be calculated because incidence rates are not available.

Case-control studies use artificial ratios of cases to controls, invalidating calculations of absolute risk or relative risk.

Construct a 2x2 contingency table using the provided data.

Cases (Neural Tube Defects): 60 exposed, 40 unexposed (Total 100). Controls (Healthy): 100 exposed, 200 unexposed (Total 300).

Organizing the data ensures the correct values are placed into the odds ratio cross-product formula.

Calculate the odds ratio using the cross-product formula: (Exposed Cases * Unexposed Controls) / (Unexposed Cases * Exposed Controls).

OR = (60 * 200) / (40 * 100) = 12000 / 4000 = 3.0.

This calculation yields the odds of exposure among cases relative to the odds of exposure among controls.

Key Concept

In a case-control study, participants are selected based on disease status, making the odds ratio the only valid measure of association. Relative risk requires calculating disease incidence, which is impossible when the ratio of cases to controls is artificially determined by the investigator.

Estimated Time:1m 30s

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