This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Social & Preventive Medicine
[{'content': "Due to an effective prevention program, the prevalence of an infectious disease in a community has been reduced by 90%. A physician continues to use the same diagnostic test for the disease that she has always used. How have the test's characteristics changed ? \nSelect and explain. \nA. Its sensitivity has increased, B. Its positive predictive value has increased, C. Its negative predictive value has increased, D. The test's characteristics have not changed", 'role': 'user'}, {'content': 'Answer: C \nExplanation: PPV and NPV of a screening test depends on:\n\n\nSensitivity\nSpecificity\n\n\nPrevalence of disease in the population\nIn this question, since a physician continues to use the same diagnostic test for the disease that she has always used, sensitivity and specificity of the test will remain same\nBut predictive value of a test (PPV and NPV) depends on prevalence of a disease in a population\nPPV is directly proportional to prevalence of disease in the population\n\nPPV α Prevalence of disease\n\nAs the prevalence of a disease increases in a population, PPV increases for the screening test\nNPV is inversely proportional to Prevalence of disease in a population\n\nNPV α 1/ Prevalence of disease\n\nAs the prevalence of a disease increases in a population, NPV decreases for the screening test. Therefore, since the prevalence of an infectious disease in a community has been reduced by 90%, its PPV will reduce and its NPV will increase.', 'role': 'assistant'}]