Epidemiology Practice Questions
20 free Epidemiology practice questions for the USMLE Step 1. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
A study reports that 50 new cases of hepatitis A occurred in a population of 100,000 over one year. What does this measure represent?
- A Prevalence
- B Incidence
- C Case-fatality rate
- D Mortality rate
Correct answer: Incidence
Incidence measures the number of new cases occurring in a population over a specific time period. The calculation directly reflects new-onset disease.
A cross-sectional survey finds that 5% of a community currently has asthma. What epidemiologic measure is being reported?
- A Lifetime incidence
- B Incidence density
- C Prevalence
- D Relative risk
Correct answer: Prevalence
Prevalence measures the proportion of individuals with a condition at a specific point in time. A cross-sectional survey typically reports prevalence.
Which of the following will increase prevalence without changing incidence?
- A Shorter duration of disease
- B Higher cure rate
- C Prolonged survival without cure
- D Rapid fatality of the disease
Correct answer: Prolonged survival without cure
Prevalence increases when individuals live longer with a disease, even if no new cases occur. Prolonged survival raises the total number of existing cases.
A disease has an annual incidence of 10 per 1,000 people and an average disease duration of 5 years. What is the approximate prevalence?
- A 0.5%
- B 2%
- C 5%
- D 10%
Correct answer: 5%
Prevalence ≈ incidence × duration. Here, 10/1000 × 5 = 50/1000 = 5%.
A rare disease suddenly becomes more easily detectable due to improved diagnostic tests. What will happen to prevalence and incidence in the short term?
- A Prevalence increases; incidence decreases
- B Prevalence increases; incidence increases
- C Prevalence decreases; incidence stays the same
- D Prevalence stays the same; incidence decreases
Correct answer: Prevalence increases; incidence increases
Improved detection increases identification of both existing and newly diagnosed cases, temporarily raising both prevalence and incidence.
A study tracks a group of disease-free individuals over time to identify new cases. What measure does this design best determine?
- A Point prevalence
- B Period prevalence
- C Incidence
- D Case-control odds ratio
Correct answer: Incidence
Following disease-free individuals over time identifies new cases, which is the definition of incidence. Cohort studies commonly estimate incidence.
Which of the following will decrease prevalence but not incidence?
- A More sensitive diagnostic tests
- B Better cure rates
- C Increased disease duration
- D Population growth
Correct answer: Better cure rates
Better cure rates reduce the number of existing cases but do not affect the rate of new cases. Thus prevalence decreases while incidence is unchanged.
Point prevalence differs from period prevalence in which way?
- A Point prevalence measures new cases only
- B Period prevalence excludes cases that resolve during the period
- C Point prevalence measures cases at a single moment
- D Period prevalence estimates only chronic diseases
Correct answer: Point prevalence measures cases at a single moment
Point prevalence measures all cases at a specific moment in time. Period prevalence includes any case existing at any time during the period.
A population has 1,000 total cases of hypertension in a population of 20,000. What is the prevalence?
- A 0.5%
- B 2%
- C 5%
- D 10%
Correct answer: 5%
Prevalence = (cases ÷ population) × 100 = (1,000 ÷ 20,000) × 100 = 5%. This represents the proportion of affected individuals in the population.
If the incidence of a fatal disease remains constant but survival improves, what happens to prevalence?
- A It increases
- B It decreases
- C It stays the same
- D It becomes unpredictable
Correct answer: It increases
Improved survival increases the number of people living with the disease at any time, increasing prevalence even with unchanged incidence.
In a stable population, which relationship is generally true?
- A Prevalence < incidence
- B Incidence = prevalence × duration
- C Prevalence increases when incidence decreases
- D Incidence = prevalence ÷ duration
Correct answer: Incidence = prevalence ÷ duration
For a stable population prevalence ≈ incidence × duration. Rearranging gives incidence = prevalence ÷ duration, so this is the relationship that holds.
A hospital wants to measure the burden of chronic kidney disease among its current patients. Which measure should be used?
- A Incidence
- B Point prevalence
- C Relative risk
- D Population attributable risk
Correct answer: Point prevalence
Point prevalence indicates the proportion of individuals affected at a specific time, which is ideal for determining disease burden.
A researcher finds that the period prevalence of flu during winter is higher than the point prevalence measured on a single day in winter. Why?
- A Because flu incidence temporarily drops to zero
- B Because period prevalence counts all cases during the season
- C Because point prevalence includes far more individuals
- D Because point prevalence measures only new cases
Correct answer: Because period prevalence counts all cases during the season
Period prevalence counts all cases present at any time during the period, making it higher than point prevalence measured at a single moment.
During an outbreak, the incidence of a disease rises sharply. What immediate effect does this have on prevalence?
- A Prevalence increases
- B Prevalence decreases
- C Prevalence stays the same
- D Prevalence drops to zero
Correct answer: Prevalence increases
Increasing incidence adds new cases, raising prevalence unless the disease is rapidly fatal or cured instantly.
Which scenario best illustrates incidence?
- A The total number of diabetes cases in 2025
- B The number of new diabetes cases in 2025
- C The proportion of adults living with diabetes
- D The number of diabetes-related deaths in 2025
Correct answer: The number of new diabetes cases in 2025
Incidence focuses on new cases arising in a defined period. It measures the risk of developing the disease.
Which scenario best illustrates prevalence?
- A New depression diagnoses in college freshmen
- B Students who currently have depression
- C Risk of developing depression over the next year
- D Deaths from depression-related suicide
Correct answer: Students who currently have depression
Prevalence includes all existing cases at a specific time, reflecting the overall burden of disease.
If incidence remains the same but mortality increases, what happens to prevalence?
- A Prevalence increases
- B Prevalence decreases
- C Prevalence remains unchanged
- D Prevalence becomes equal to mortality
Correct answer: Prevalence decreases
Higher mortality reduces the number of existing cases, lowering prevalence even with stable incidence.
A chronic condition has low incidence but high prevalence. What is the most likely reason?
- A High mortality rate
- B Short duration of illness
- C Long duration of disease
- D High cure rate
Correct answer: Long duration of disease
Low incidence but high prevalence suggests long disease duration, allowing cases to accumulate over time.
Which of the following combinations would result in a very low prevalence?
- A High incidence; long duration
- B Low incidence; long duration
- C Low incidence; short duration
- D High incidence; moderate duration
Correct answer: Low incidence; short duration
Low incidence and short duration both reduce the number of existing cases, producing low prevalence.
A researcher wants to know how many people lived with influenza at any point during a 3-month season. What measure should be calculated?
- A Point prevalence
- B Period prevalence
- C Cumulative incidence
- D Incidence density
Correct answer: Period prevalence
Period prevalence measures the proportion of individuals who had the disease at any time during a specified period.