This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Analytical Epidemiology, Subject name: Social & Preventive Medicine
Prevalance of cataract at one point of time can be determined by: 
Select and explain. 
A. Longitudinal study, B. Cross-sectional study, C. Surveillance, D. Cohort study
B: Cross-sectional study
Answer: B: Cross-sectional study 
Explanation: Ans is 'b' i.e. Cross-sectional study RepeatCross-sectional or Prevalence study - in cross-sectional studies, we 'survey' several individuals at one point in time and collect information on health status, health-related behaviour, and other exposure factors. In this way we can estimate the burden of disease in a group of individuals and try to estimate whether a particular exposure is linked to a disease or health outcome.