SPEX Preventive Medicine 4 — Questions and Answers
Question 1: A new diagnostic test for diabetes has a sensitivity of 90% and specificity of 85%. In a population with 10% prevalence, what is the positive predictive value (PPV)?
- Approximately 40% (Correct answer)
- Approximately 60%
- Approximately 75%
- Approximately 90%
Correct answer: Approximately 40%
Using Bayes' theorem: PPV = (0.90×0.10)/[(0.90×0.10)+(0.15×0.90)] = 0.09/0.225 ≈ 40%.
Question 2: Which occupational lung disease is caused by inhalation of free crystalline silica and is associated with increased risk of tuberculosis?
- Asbestosis
- Berylliosis
- Silicosis (Correct answer)
- Coal workers' pneumoconiosis
Correct answer: Silicosis
Silicosis from silica dust exposure impairs pulmonary macrophage function, substantially increasing susceptibility to tuberculosis.
Question 3: The 'healthy worker effect' in occupational epidemiology refers to:
- Workers who comply better with safety protocols live longer
- Employed populations appear healthier than the general population because severely ill individuals are less likely to work (Correct answer)
- Health promotion programs in workplaces reduce mortality
- Workers in hazardous jobs develop immunity to certain toxins over time
Correct answer: Employed populations appear healthier than the general population because severely ill individuals are less likely to work
The healthy worker effect biases occupational studies toward lower observed mortality because the employed tend to be healthier than the general population.
Question 4: Which clinical preventive service is recommended by the USPSTF with a Grade B recommendation for all adults aged 35–70 who are overweight or obese?
- Annual fasting lipid panel
- Screening for prediabetes and type 2 diabetes (Correct answer)
- Annual abdominal ultrasound for aortic aneurysm
- Baseline colonoscopy
Correct answer: Screening for prediabetes and type 2 diabetes
USPSTF gives a Grade B recommendation for prediabetes/type 2 diabetes screening in overweight or obese adults aged 35–70.
Question 5: In communicable disease control, 'herd immunity threshold' is calculated as 1 − (1/R₀). For measles with an R₀ of 15, what percentage of the population must be immune?
- 85%
- 87%
- 93% (Correct answer)
- 75%
Correct answer: 93%
1 − (1/15) = 1 − 0.067 ≈ 0.933, meaning approximately 93% of the population must be immune.
Question 6: A chemical plant worker develops peripheral neuropathy after chronic low-level exposure to n-hexane. The primary toxic metabolite responsible is:
- Benzene diol epoxide
- 2,5-hexanedione (Correct answer)
- Trichloroacetic acid
- Methyl hippuric acid
Correct answer: 2,5-hexanedione
n-Hexane is metabolized to 2,5-hexanedione, which crosslinks neurofilament proteins causing the characteristic 'giant axon' neuropathy.
Question 7: When evaluating a screening program, which statistical measure indicates the number of people who need to be screened to prevent one adverse outcome?
- Attributable risk
- Number needed to screen (NNS) (Correct answer)
- Sensitivity
- Likelihood ratio
Correct answer: Number needed to screen (NNS)
Number needed to screen (NNS) is the reciprocal of the absolute risk reduction achieved by the screening program.
SPEX Preventive Medicine 4
A new diagnostic test for diabetes has a sensitivity of 90% and specificity of 85%.
In a population with 10% prevalence, what is the positive predictive value (PPV)?