Residents of three villages were asked to participate in a survey to identify carriers of a disease. The proper classification is:

• A. Cross-sectional study
• B. Case-control study
• C. Concurrent cohort study
• D. Nonconcurrent cohort study

Answer: (A)

This item tests understanding of study design based on when data on exposure and outcome are collected. The scenario describes surveying residents at a single point in time to identify who is a carrier, which provides a snapshot of how common the carrier state is in the population. There is no follow-up to observe who becomes a carrier or who develops disease, nor is there a comparison based on prior exposures or disease status. That makes it a cross-sectional study, which is used to measure prevalence at one moment rather than incidence over time.

In contrast, a case-control study would begin with individuals who have the disease and compare their past exposures to those without the disease, which isn’t described here. A concurrent (prospective) or nonconcurrent (retrospective) cohort study follows defined groups over time to observe outcomes, which again involves a time element and progression that isn’t part of this single-point survey.