Epidemiology Practice Questions

Q: A 60-year-old man presents with a chronic cough and is found to have a lung lesion on chest X-ray. What is the significance of high sensitivity in a diagnostic test for lung cancer?

It correctly identifies those with the disease. ***It correctly identifies those with the disease*** - A test with **high sensitivity** is good at ruling out disease when the result is negative because it has a low chance of missing true cases. - In lung cancer screening, high sensitivity helps ensure that individuals who actually have lung cancer are **identified**, reducing false negatives. *It incorrectly identifies those without the disease* - This statement describes a test with **low specificity**, where many healthy individuals are misidentified as having the disease (many false positives). - A high sensitivity test focuses on identifying true positives, not on incorrectly identifying healthy individuals. *It indicates the proportion of false negatives among all actual positives.* - The proportion of **false negatives among all actual positives** is the complement of sensitivity (1 - sensitivity). - High sensitivity implies a **low false-negative rate**, meaning few true cases are missed. *It indicates the proportion of true positives among all positive results.* - This describes **positive predictive value (PPV)**, which is the probability that a positive test result is truly positive. - While related to test utility, PPV is influenced by disease prevalence, whereas sensitivity is an intrinsic property of the test.

Q: A researcher calculates the relative risk (RR) of developing a disease among exposed individuals compared to non-exposed individuals and finds it to be 2. What does this indicate?

The exposure doubles the risk of disease. ***The exposure doubles the risk of disease*** - A **relative risk (RR)** of 2 means that the incidence of the disease in the exposed group is **twice** that in the unexposed group. - This indicates a **two-fold increase** in the likelihood of developing the disease due to the exposure. *The exposure does not affect disease risk* - This would be indicated by an **RR of 1**, meaning the risk in the exposed group is the same as in the unexposed group. - An RR of 2 clearly shows a **difference** in risk between the two groups. *The exposure halves the risk of disease* - This would be indicated by an RR of **0.5 (or 1/2)**, meaning the incidence in the exposed group is half that of the unexposed group. - A value of 2 signifies an **increase**, not a decrease, in risk. *The exposure is protective against the disease* - A protective effect would result in an RR **less than 1**, indicating a lower risk in the exposed group. - An RR of 2 demonstrates an **increased risk**, which is the opposite of a protective effect.

Q: In a community-based intervention, the incidence of a disease is found to decrease after vaccination. What study design is most appropriate to confirm vaccine efficacy?

Randomized controlled trial. ***Randomized controlled trial*** - A **randomized controlled trial (RCT)** is the most appropriate study design to confirm vaccine efficacy because it allows for a robust comparison between a vaccinated group and a control group (placebo or no intervention) by **randomly assigning participants** to each. - This design minimizes bias and allows researchers to establish a direct **cause-and-effect relationship** between the vaccine and the prevention of the disease by controlling for confounding factors. *Cohort study* - A **cohort study** observes groups over time, but participants are not randomized to receive the intervention (vaccine), which can introduce **selection bias** if those who choose to be vaccinated differ from those who do not. - While useful for studying prognosis or the effectiveness of interventions in real-world settings, it is less ideal for definitively establishing efficacy compared to an RCT due to the lack of **randomized assignment**. *Case-control study* - A **case-control study** compares individuals with a disease (cases) to those without the disease (controls) and looks back retrospectively for exposure to a risk factor (e.g., lack of vaccination). - This design is prone to **recall bias** and is suitable for rare outcomes, but it cannot directly measure vaccine efficacy or incidence in the same way an RCT can. *Cross-sectional study* - A **cross-sectional study** assesses exposure and outcome simultaneously at a single point in time, providing a snapshot of prevalence. - This design cannot establish temporality (which came first, exposure or outcome) and is therefore unsuitable for confirming vaccine efficacy or a **cause-and-effect relationship**.

Q: Admission rate bias is?

Berksonian bias. ***Berksonian bias*** - **Berksonian bias** is a form of selection bias, also known as **admission rate bias**, that occurs when different rates of admission to a hospital or clinical setting distort the association between diseases or between a disease and a risk factor. - This bias arises because the hospitalized population may not be representative of the general population, leading to spurious associations or masking real ones. *Reporting bias* - **Reporting bias** is a type of information bias where the outcome or exposure information is reported inaccurately, often due to social desirability or recall issues. - It does not specifically refer to distortions stemming from hospital admission rates. *Response bias* - **Response bias** occurs when participants in a study alter their answers or behavior from what is true due to factors like leading questions, social desirability, or acquiescence. - This is an issue related to data collection, not an unrepresentative study population due to hospital admission protocols. *None of the options* - Berksonian bias directly corresponds to the definition of admission rate bias, making this option incorrect.

Q: Nosocomial infections are defined as infections that develop after how many hours of hospital admission?

A) 48 hours. ***A) 48 hours*** - Nosocomial infections, also known as **hospital-acquired infections (HAI)**, are defined as infections that develop **48 hours or more** after hospital admission. - This is the **standard definition** used by the **CDC, WHO**, and major medical textbooks including **Park's Textbook of Preventive and Social Medicine**. - The 48-hour threshold helps differentiate infections acquired during hospitalization from those that were **incubating at the time of admission** (typical incubation periods for most common infections are less than 48 hours). - Infections can also be classified as nosocomial if they occur **within 3 days after discharge** or **within 30 days after surgery**. *B) 72 hours* - While **72 hours** is occasionally mentioned in some contexts or specific institutional protocols, it is **not the standard definition** for nosocomial infections. - Using 72 hours would be too restrictive and could miss true hospital-acquired infections that manifest between 48-72 hours. - The universally accepted standard remains **48 hours**. *C) 24 hours* - An infection developing within **24 hours** is very likely to have been **present or incubating prior to admission**. - This timeframe is too short to establish that the infection was acquired during hospitalization. - Most common bacterial and viral infections have incubation periods longer than 24 hours. *D) 50 hours* - This is **not a standard threshold** for defining nosocomial infections. - The conventional definitions use **48 hours** as the cutoff point, which is based on typical incubation periods and epidemiological evidence.

Q: All are true regarding Japanese encephalitis except which of the following?

Horses are amplifier hosts. ***Horses are amplifier hosts*** - **Horses** can act as **sentinel animals** and develop severe neurological disease, but they are generally considered **dead-end hosts** for Japanese encephalitis virus, meaning they do not amplify the virus to a level sufficient to transmit it further. - The primary **amplifier hosts** for Japanese encephalitis are **pigs and wading birds**, which maintain the transmission cycle. *Caused by flavivirus* - Japanese encephalitis is indeed caused by a **flavivirus**, specifically the **Japanese encephalitis virus (JEV)**, which belongs to the Flaviviridae family. - This characteristic is consistent with the general epidemiology and pathogenesis of other well-known flaviviruses like Dengue and West Nile virus. *Humans are dead-end hosts* - **Humans** are considered **dead-end hosts** for Japanese encephalitis because the **viremia** (virus levels in the blood) in infected humans is typically too low to infect mosquitoes that feed on them. - This means that humans do not contribute to the ongoing transmission cycle of the virus between mosquitoes and amplifier hosts. *Transmitted by culex* - The primary vectors for Japanese encephalitis virus are mosquitoes of the **Culex genus**, particularly **Culex tritaeniorhynchus**. - These mosquitoes typically breed in rice paddies and other agricultural wetlands, which are common in regions where the disease is endemic.

Q: Which observational study design is most appropriate for analyzing the long-term effects of an exposure or treatment over a specified timeline?

Cohort Study. ***Cohort Study*** - A **cohort study** tracks a group of individuals (cohort) who share a common characteristic or exposure over a period, making it ideal for observing **long-term effects** and outcomes. - Researchers follow participants forward in time to see who develops the outcome of interest, providing strong evidence for **causality** in observational settings. *Cross-sectional study* - A **cross-sectional study** assesses exposure and outcome simultaneously at a single point in time, failing to establish temporality or long-term effects. - This design is suitable for determining **prevalence** but not for analyzing changes over time or causal relationships. *Randomized Control Trials* - **Randomized controlled trials (RCTs)** are interventional studies where participants are randomly assigned to treatment or control groups, providing the strongest evidence for **causality** in the short to medium term. - While excellent for assessing treatment efficacy, RCTs are often impractical or unethical for very long-term follow-up due to cost, logistical challenges, and participant retention issues. *Interventional Studies* - **Interventional studies** involve researchers actively manipulating an intervention (e.g., a treatment) and observing its effects. - This broad category includes RCTs, but simply being an "interventional study" does not inherently make it the most appropriate for analyzing **long-term effects** over an extended timeline, which is better suited for the observational follow-up of a cohort study.

Q: Which type of study determines the odds ratio?

Case control. ***Case control*** - **Case-control studies** compare individuals with a disease (cases) to individuals without the disease (controls) and look back in time to identify previous exposures. - The **odds ratio** is the primary measure of association used in case-control studies, quantifying the odds of exposure among cases versus controls. *Cohort* - **Cohort studies** follow groups of individuals over time, some exposed to a risk factor and some not, to determine the incidence of a disease. - They typically determine **relative risk**, which is the ratio of incidence rates in exposed versus unexposed groups. *Cross sectional* - **Cross-sectional studies** assess the prevalence of disease and exposure at a single point in time. - They primarily measure **prevalence** and can be used to calculate a **prevalence odds ratio**, but they do not establish temporality between exposure and outcome. *RCT* - **Randomized controlled trials (RCTs)** are interventional studies where participants are randomly assigned to an intervention or control group to determine the effectiveness of a treatment or exposure. - The main measure of effect in RCTs is often the **relative risk reduction**, **absolute risk reduction**, or **number needed to treat**, rather than the odds ratio for observational exposure.

Q: What is the expected percentage reduction in mortality risk from breast cancer with annual mammography screening after the age of 50?

15-20%. ***15-20%*** - Current evidence from **meta-analyses and systematic reviews** shows that annual mammography screening in women aged 50 and older reduces breast cancer mortality by approximately **15-20%**. - This estimate is supported by the **U.S. Preventive Services Task Force (USPSTF)** and **Canadian Task Force on Preventive Health Care** based on modern randomized controlled trials. - The benefit reflects **earlier detection** and treatment of breast cancer in this age group. *20-25%* - This range **slightly overestimates** the mortality reduction demonstrated in contemporary evidence-based reviews. - While some older studies reported benefits in this range, more rigorous modern analyses support a lower estimate. *25-30%* - This represents **older estimates** from earlier meta-analyses that have been revised downward with more recent data. - Contemporary evidence-based guidelines do **not support** this level of mortality reduction for routine mammography screening. - This figure may have included methodological biases present in older trials. *30-35%* - This range **significantly overestimates** the mortality benefit of mammography screening. - No current evidence-based guidelines support this level of mortality reduction for average-risk women aged 50 and above. - Such high estimates are not supported by modern randomized controlled trials.

Question 1

A 60-year-old man presents with a chronic cough and is found to have a lung lesion on chest X-ray. What is the significance of high sensitivity in a diagnostic test for lung cancer?

Accepted Answer

It correctly identifies those with the disease

Answer

It incorrectly identifies those without the disease

Answer

It indicates the proportion of true positives among all positive results

Answer

It indicates the proportion of false negatives among all actual positives

Question 2

A researcher calculates the relative risk (RR) of developing a disease among exposed individuals compared to non-exposed individuals and finds it to be 2. What does this indicate?

Accepted Answer

The exposure doubles the risk of disease

Answer

The exposure does not affect disease risk

Answer

The exposure halves the risk of disease

Answer

The exposure is protective against the disease

Question 3

In a community-based intervention, the incidence of a disease is found to decrease after vaccination. What study design is most appropriate to confirm vaccine efficacy?

Accepted Answer

Randomized controlled trial

Answer

Cohort study

Answer

Case-control study

Answer

Cross-sectional study

Question 4

Admission rate bias is?

Accepted Answer

Berksonian bias

Answer

Reporting bias

Answer

Response bias

Answer

None of the options

Question 5

The difference in incidence between exposed and non-exposed groups in epidemiological studies is best described as:

Accepted Answer

Difference in incidence between exposed and non-exposed groups

Answer

Measure of association between exposure and outcome

Answer

Total impact of exposure on disease occurrence in a population

Answer

Ratio of incidence in exposed versus non-exposed groups

Question 6

Nosocomial infections are defined as infections that develop after how many hours of hospital admission?

Accepted Answer

A) 48 hours

Answer

B) 72 hours

Answer

C) 24 hours

Answer

D) 50 hours

Question 7

All are true regarding Japanese encephalitis except which of the following?

Accepted Answer

Horses are amplifier hosts

Answer

Humans are dead-end hosts

Answer

Caused by flavivirus

Answer

Transmitted by culex

Question 8

Which observational study design is most appropriate for analyzing the long-term effects of an exposure or treatment over a specified timeline?

Accepted Answer

Cohort Study

Answer

Cross sectional study

Answer

Randomized Control Trials

Answer

Interventional Studies

Question 9

Which type of study determines the odds ratio?

Accepted Answer

Case control

Answer

Cohort

Answer

Cross sectional

Answer

RCT

Question 10

What is the expected percentage reduction in mortality risk from breast cancer with annual mammography screening after the age of 50?

Accepted Answer

15-20%

Answer

30-35%

Answer

25-30%

Answer

20-25%

Epidemiology — MCQs

Epidemiology — MCQs

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