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
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 ? 
Select and explain. 
A. 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
C: Its negative predictive value has increased
Answer: C: Its negative predictive value has increased 
Explanation: PPV and NPV of a screening test depends on:


Sensitivity
Specificity


Prevalence of disease in the population
In 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
But predictive value of a test (PPV and NPV) depends on prevalence of a disease in a population
PPV is directly proportional to prevalence of disease in the population

PPV α Prevalence of disease

As the prevalence of a disease increases in a population, PPV increases for the screening test
NPV is inversely proportional to Prevalence of disease in a population

NPV α 1/ Prevalence of disease

As 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.