Epidemiology & Prevention USMLE Step 1 Practice Question
A randomized controlled trial comparing two antihypertensive agents (amlodipine vs. lisinopril) enrolls 300 patients with Stage 2 hypertension. Patients are randomized 1:1 using a computer-generated sequence with stratification by age and baseline systolic BP. Baseline characteristics show appropriate balance in age (mean 58 years), sex (55% male), and BP (156/94 mmHg in both groups). However, at the end-of-study analysis, the amlodipine group has a significantly higher prevalence of diabetes (45% vs. 25%, p=0.03) and elevated HbA1c (7.8% vs. 6.9%, p=0.02), which were not documented as baseline differences. The amlodipine group also has numerically higher diastolic BP reduction (12 mmHg vs. 8 mmHg), but the difference is not statistically significant (p=0.08). Which of the following best explains why the observed difference in diabetes prevalence limits the internal validity of the trial?
Answer choices
- ADifferential loss to follow-up between treatment groups compromised the randomization process
- BUncontrolled confounding by diabetes status may bias the estimate of treatment effect on blood pressure outcomesCorrect answer
- CThe investigators failed to measure HbA1c and diabetes status at baseline, resulting in detection bias
- DMeasurement error in fasting glucose levels was more common in the amlodipine group due to protocol violations
- ERegression to the mean in the amlodipine group led to spurious diabetes classification
- FSelection bias in enrollment enriched the amlodipine arm for patients with metabolic syndrome
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