USMLE Step 1 Question 308

Question:

A field researcher is conducting a study of HIV transmission in South Africa. She has already obtained access to a cohort of HIV- positive patients and a cohort of age-matched HIV-negative patients. She now plans to undertake a case-control study using the data set. Which of the following would be a feasible outcome measure for this study?

Correct Answer: B.  The frequency difference of past unprotected sex between cohorts

B is the correct answer because it is the only risk factor in the listed choices which may be obtained from a case-control study. A case-control study specifically collects data about risk factors based on presence or absence of disease. A cohort study collects data on the development of disease based on the presence of absence of risk factors. Answers A, C and E would be feasible in a cohort study. Answer D is an example of an outcome feasible when an intervention is given to the groups to gauge efficacy or side-effects of the intervention. This is specific to clinical trial studies.