DHA Community & Public Health Medicine exam questions

6 worked Community & Public Health Medicine practice questions

Original exam-style questions from the bank, spread across the blueprint domains. They are not real or recalled exam questions. Try each one, then open the answer for the rationale and reference. None of them appear in the free mock exam, so reading them will not spoil it.

1. Question 1 · Biostatistics

   A screening test with fixed sensitivity and specificity is moved from a high-risk clinic to the general population, where prevalence is much lower. What change is expected?

   1. APPV falls and NPV rises
   2. BPPV rises and NPV falls
   3. CSensitivity falls and specificity rises
   4. DBoth predictive values stay the same
   Show answer and explanation

   Correct answer: A. PPV falls and NPV rises

   Predictive values depend on prevalence: as prevalence falls, false positives make up a larger share of positive results, so PPV falls while NPV rises. Sensitivity and specificity are properties of the test and do not change with prevalence.

   Reference: Gordis Epidemiology, 6th ed.

2. Question 2 · Communicable Disease Control

   Within 2 hours of eating reheated fried rice at a restaurant, several diners develop vomiting without fever. Which agent is most likely?

   1. ASalmonella enteritidis
   2. BBacillus cereus
   3. CClostridium perfringens
   4. DCampylobacter jejuni
   Show answer and explanation

   Correct answer: B. Bacillus cereus

   The emetic toxin of Bacillus cereus, classically linked to rice held warm and reheated, causes vomiting within about 1-6 hours. Clostridium perfringens causes diarrhoea after a longer incubation of roughly 8-16 hours.

   Reference: Heymann, Control of Communicable Diseases Manual, 21st ed.

3. Question 3 · Communicable Disease Control

   A child with suspected measles is admitted to hospital. Which transmission-based precaution is required?

   1. ADroplet precautions in a shared bay of four beds
   2. BContact precautions with gloves and gown only
   3. CStandard precautions only, without any isolation
   4. DAirborne precautions in a single negative-pressure room
   Show answer and explanation

   Correct answer: D. Airborne precautions in a single negative-pressure room

   Measles spreads by the airborne route and needs an airborne infection isolation room with respirators for susceptible staff. Droplet precautions are not enough because the virus stays suspended in air for long periods.

   Reference: CDC Guideline for Isolation Precautions 2007

4. Question 4 · Environmental and Occupational Health

   For effective disinfection of piped drinking water at pH below 8, what minimum free chlorine residual is recommended after at least 30 minutes of contact time?

   1. A0.5 mg/L
   2. B0.05 mg/L
   3. C2.0 mg/L
   4. D5.0 mg/L
   Show answer and explanation

   Correct answer: A. 0.5 mg/L

   WHO recommends a free chlorine residual of at least 0.5 mg/L after 30 minutes of contact at pH below 8. The 5 mg/L figure is the health-based guideline maximum, not the target residual.

   Reference: WHO Guidelines for Drinking-water Quality, 4th ed.

5. Question 5 · Epidemiology and Research Methods

   A new screening test detects pancreatic cancer 2 years earlier than clinical diagnosis. Five-year survival from diagnosis rises, but the average age at death is unchanged. What best explains this?

   1. ALead-time bias
   2. BLength-time bias
   3. COverdiagnosis bias
   4. DVolunteer bias
   Show answer and explanation

   Correct answer: A. Lead-time bias

   Earlier detection adds time to survival measured from diagnosis without delaying death, which is lead-time bias. Length-time bias is the preferential detection of slower-growing tumours by screening, which is a different mechanism.

   Reference: Gordis Epidemiology, 6th ed.

6. Question 6 · Immunization and Vaccine-Preventable Diseases

   A healthy, non-pregnant 25-year-old health worker with no record of measles vaccination was exposed to a measles case 48 hours ago. What is the best action?

   1. AGive MMR vaccine in 3 weeks
   2. BGive oral ribavirin prophylaxis
   3. CGive MMR vaccine now
   4. DNo action; the window has passed
   Show answer and explanation

   Correct answer: C. Give MMR vaccine now

   MMR vaccine given within 72 hours of exposure can prevent or modify measles in susceptible contacts. Delaying vaccination loses this window, and there is no role for ribavirin prophylaxis.

   Reference: CDC ACIP: Prevention of Measles, Rubella, CRS and Mumps 2013