SPEX Exam Preventive Medicine 6

Question 1

A screening program for colorectal cancer detects 40 true positives and 10 false negatives per 1,000 patients tested. The disease prevalence in the population is 5%. Using Bayes' theorem, which value BEST characterizes the post-test probability of disease given a negative result?

Accepted Answer

Approximately 1.1%, meaning roughly 1 in 91 negative tests harbors disease

Answer

With prevalence 5%, true disease = 50 per 1,000. The test misses 10 (false negatives), so 40 true positives and 10 false negatives. Negative tests = 950 true negatives + 10 false negatives = 960. The negative predictive value = 950/960 ≈ 98.96%, meaning the post-test probability of disease given a negative result ≈ 10/960 ≈ 1.04%, approximately 1.1%. This is reassuring but not zero — clinically important when prevalence is moderate and sensitivity is imperfect.

Question 2

An occupational medicine physician evaluates a 52-year-old sandblaster with 20 pack-years of smoking and progressive dyspnea. HRCT shows bilateral upper-lobe nodules with eggshell calcification. The MOST critical immediate regulatory obligation under OSHA's Respirable Crystalline Silica Standard (29 CFR 1926.1153) for the employer is:

Accepted Answer

Initiate medical removal protection if the physician recommends the worker should not be exposed to silica

Answer

Under OSHA's crystalline silica standard, medical removal protection (MRP) is the critical employer obligation when the PLHCP recommends removal from silica exposure. The employer must transfer the worker to a comparable job with no silica exposure while maintaining earnings, seniority, and benefits. Eggshell calcification is pathognomonic for silicosis (or sarcoidosis), and given the exposure history, silicosis is the diagnosis — warranting permanent removal. Respirator use and air monitoring are prevention/compliance steps but do not address this clinical scenario. NIOSH reporting is not mandated by OSHA's standard.

Question 3

In a prospective cohort study of 10,000 adults followed for 10 years, 200 develop type 2 diabetes. The incidence rate in exposed (high-sugar diet) is 4 per 100 person-years; in unexposed it is 1 per 100 person-years. The attributable fraction among the exposed (AFe) is calculated as:

Accepted Answer

75%, meaning three-quarters of diabetes cases among the exposed are attributable to high-sugar diet

Answer

The attributable fraction among the exposed (AFe), also called the etiologic fraction, is calculated as (RR - 1)/RR. Here RR = 4/1 = 4, so AFe = (4-1)/4 = 3/4 = 75%. This means that among exposed individuals who develop diabetes, 75% of those cases are attributable to the high-sugar diet (assuming causality). This is distinct from the population attributable fraction (PAF), which accounts for the proportion of the population exposed.

Question 4

A 38-year-old HIV-positive woman on suppressive antiretroviral therapy (CD4 count 620 cells/µL, undetectable viral load) presents for preconception counseling. Regarding live attenuated vaccines, which of the following is the MOST appropriate recommendation?

Accepted Answer

MMR and varicella vaccines are recommended if seronegative, as her CD4 count exceeds 200 cells/µL

Answer

Current ACIP and CDC guidelines recommend MMR and varicella vaccines for HIV-positive adults with CD4 ≥200 cells/µL who lack immunity, as the risk of the actual disease outweighs the attenuated vaccine risk at this immunocompetence threshold. LAIV is contraindicated in immunocompromised patients — inactivated influenza is used instead. RZV (Shingrix) is actually the preferred zoster vaccine for HIV-positive adults, not the live zoster vaccine (Zostavax), which is contraindicated when CD4 <200. Her CD4 of 620 and suppressed viremia make her a candidate for MMR and varicella if non-immune.

Question 5

The epidemiologic concept of 'healthy worker effect' is MOST likely to cause which specific bias in occupational cohort studies, and what is the appropriate methodological correction?

Accepted Answer

Selection bias causing underestimation of occupational risk; corrected by using a general-population comparison group that includes the disabled and chronically ill

Answer

The healthy worker effect is a form of selection bias: employed workers are healthier than the general population because those with serious illness are either not hired or leave the workforce. When occupational cohort mortality is compared to general population rates, the worker cohort appears artificially healthy, underestimating true occupational risk. The appropriate correction is to use an internal comparison group (e.g., unexposed workers in the same industry) or to use a comparison population that also excludes the severely disabled — not the general population which includes high-mortality groups. Using internal comparisons eliminates the healthy worker bias.

SPEX Exam Practice Test

SPEX Exam Practice Test

SPEX Exam Preventive Medicine 6