Epidemiology — MCQs

In a cohort of 500 women attending antenatal clinic, 70 % had ultrasonography (USG). This cohort was followed up at delivery. Of the women who had USG, 70 delivered low birth weight (LBW) babies; whereas of the women, who did not undergo USG, 50 delivered LBW babies. The incidence of LBW babies among women who had USG is:

Q1537

Poor hand hygiene of a mess worker in a university college mess led to Hepatitis A cases in the hostel inmates. What type of epidemic will this exposure present with? 1. Propagated 2. Common source-continuous exposure 3. Common source-point exposure Select the correct answer using the code given below:

Q1538

A depression screening tool developed in specialty clinics performs poorly when used in primary care settings. The positive predictive value drops from 85% to 25%. What is the most likely explanation?

Q1539

A cohort study follows 5,000 nurses for 20 years to study night shift work and breast cancer risk. After 10 years, 30% of participants have been lost to follow-up. What is the most significant threat to study validity?

Q1540

A screening test for a rare disease has 99% sensitivity and 95% specificity. In a population where the disease prevalence is 0.1%, what is the most important consideration for implementing this test?

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1531: Which one of the following statements is NOT true for taking a decision on screening for disease?

A. Sensitivity and specificity are high
B. Proportion of false negatives is high (Correct Answer)
C. Disease prevalence should be high
D. Disease is lethal

Explanation: ***Proportion of false negatives is high*** - A **high proportion of false negatives** means that many individuals with the disease are missed by the screening test. - This is **absolutely undesirable** for screening and directly contradicts the fundamental principle that screening tests should have **high sensitivity**. - This statement is clearly NOT true as a criterion for making screening decisions. *Sensitivity and specificity are high* - **High sensitivity** means the test correctly identifies most people who have the disease, minimizing false negatives. - **High specificity** means the test correctly identifies most people who do not have the disease, minimizing false positives. - Both are desirable characteristics according to Wilson-Jungner screening criteria. *Disease prevalence should be high* - This statement is **oversimplified and not strictly accurate** according to Wilson-Jungner criteria. - The actual criterion is: **"The condition should be an important health problem"** - based on disease burden, severity, and consequences, NOT necessarily high prevalence. - Many successful screening programs target **low prevalence diseases** (e.g., phenylketonuria in newborns, congenital hypothyroidism). - What matters is the **positive predictive value (PPV)** and cost-effectiveness, which are influenced by prevalence but don't require "high" prevalence. *Disease is lethal* - The Wilson-Jungner criterion states the disease should have **"serious consequences if left untreated"** - this includes but is not limited to lethality. - Screening is justified for diseases causing **significant morbidity or mortality** where early detection improves outcomes. - The disease should be sufficiently serious to warrant the costs and potential harms of screening.

Question 1532: The sequence of events leading to disability and handicap is:

A. Disease → Disability→Impairment→Handicap
B. Disease → Impairment→Disability→Handicap (Correct Answer)
C. Disease → Disability→Handicap→Impairment
D. Disease → Handicap→Impairment→Disability

Explanation: ***Disease → Impairment→Disability→Handicap*** - This sequence follows the **WHO International Classification of Impairments, Disabilities, and Handicaps (ICIDH, 1980)** model, which correctly illustrates the progression from a health condition to its societal consequences. - A **disease** or health condition leads to **impairment** (loss or abnormality of body structure/function), which then restricts activities (**disability**), and ultimately impacts social roles (**handicap**). *Disease → Disability→Impairment→Handicap* - This order is incorrect because **impairment** (a problem in body function or structure) logically precedes **disability** (a difficulty executing tasks). - **Disability** arises from the *functional limitation* caused by impairment, not the other way around. *Disease → Disability→Handicap→Impairment* - This sequence is incorrect as **impairment** is the initial consequence of disease on a functional level, occurring before **disability** and **handicap**. - **Handicap** represents the societal and environmental disadvantage, which is the final stage in this classic WHO model. *Disease → Handicap→Impairment→Disability* - This order is incorrect because **handicap**, which refers to a social disadvantage, is the last step in the disablement process, following impairment and disability. - **Impairment** is a direct result of the disease, and **disability** follows from that impairment.

Question 1533: Disability-adjusted life years (DALYs) include:

A. Both YLL and YLD (Correct Answer)
B. Neither YLL nor YLD
C. Years lost to disability (YLD)
D. Years of lost life (YLL)

Explanation: ***Both YLL and YLD*** - **DALYs (Disability-Adjusted Life Years)** are a measure of overall disease burden, expressed as the number of years lost due to ill-health, disability, or early death. - They are calculated as the sum of **Years of Life Lost (YLL)** due to premature mortality and **Years Lost due to Disability (YLD)** for people living with a health condition or its consequences. *Neither YLL nor YLD* - This statement is incorrect because DALYs are explicitly designed to integrate both premature mortality (YLL) and the impact of non-fatal health outcomes (YLD). - Excluding both components would render DALYs meaningless as a comprehensive health metric. *Years lost to disability (YLD)* - While **YLD** is a crucial component of DALYs, focusing solely on YLD would ignore the impact of premature mortality. - A healthy life lost due to early death is as significant as years living with a disability in assessing disease burden. *Years of lost life (YLL)* - While **YLL** is an essential component, considering only YLL would overlook the burden of morbidity and disability. - Many chronic diseases cause significant disability and reduced quality of life without necessarily leading to premature death, which YLD accounts for.

Question 1534: Predictive accuracy of a screening test depends on the following EXCEPT:

A. Specificity of screening test
B. Disease prevalence
C. Disease incidence (Correct Answer)
D. Sensitivity of screening test

Explanation: ***Disease incidence*** - **Disease incidence** refers to the rate at which new cases of a disease occur in a population over a specified period. While related to disease prevalence, it is not a direct factor in calculating the predictive accuracy of a screening test. - Predictive accuracy, specifically **positive predictive value (PPV)** and **negative predictive value (NPV)**, relies on the test's inherent properties (sensitivity and specificity) and the **prevalence** of the disease, not its incidence. *Specificity of screening test* - **Specificity** is crucial for predictive accuracy as it determines the probability that a test correctly identifies those *without* the disease. - A test with high specificity will have fewer **false positives**, which directly impacts the positive predictive value. *Disease prevalence* - **Disease prevalence** profoundly influences the predictive accuracy of a screening test. The **positive predictive value** increases with higher disease prevalence. - In populations with low disease prevalence, even highly sensitive and specific tests can yield a large number of **false positives**. *Sensitivity of screening test* - **Sensitivity** is a key determinant of predictive accuracy, as it measures the proportion of *true positives* correctly identified by the test. - A test with high sensitivity helps ensure that most individuals *with* the disease are detected, which affects both **positive and negative predictive values**.

Question 1535: Which one of the following is NOT a function of Epidemiology?

A. Searching for the causes and risk factors
B. Identifying syndromes
C. To study historically the rise and fall of disease in the population
D. Making a clinical diagnosis (Correct Answer)

Explanation: ***Making a clinical diagnosis*** - **Epidemiology** focuses on **population-level health patterns** and determinants of disease, not individual patient diagnosis. - Making a **clinical diagnosis** is the role of a healthcare provider based on a patient's symptoms, physical examination, and diagnostic tests. *Searching for the causes and risk factors* - A primary function of epidemiology is to **identify potential causes** and **risk factors** for diseases within populations. - This involves investigating associations between **exposures** and **health outcomes**. *Identifying syndromes* - Epidemiologists help identify new or unrecognized **patterns of disease** presentation that may constitute a **syndrome**. - This involves observing **clusters of symptoms** or conditions in a population. *To study historically the rise and fall of disease in the population* - **Historical epidemiology** tracks changes in disease incidence, prevalence, and mortality over time. - This function helps understand disease trends and the **impact of public health interventions**.

Question 1536: In a cohort of 500 women attending antenatal clinic, 70 % had ultrasonography (USG). This cohort was followed up at delivery. Of the women who had USG, 70 delivered low birth weight (LBW) babies; whereas of the women, who did not undergo USG, 50 delivered LBW babies. The incidence of LBW babies among women who had USG is:

A. 25 %
B. 10 %
C. 20 % (Correct Answer)
D. 15 %

Explanation: ***20 %*** - Total women are 500, and 70% had USG, which equals 350 women. - Of these 350 women, 70 delivered **low birth weight (LBW)** babies. Therefore, the incidence is (70/350) * 100% = **20%**. *25 %* - This option would imply a higher number of LBW babies among those who had USG than the actual data. - It does not align with the calculation based on the given figures (70 LBW babies out of 350 women who had USG). *10 %* - This option represents a lower incidence and does not correspond to the calculation of (70/350) * 100%. - This value might be obtained if the total number of women with USG was incorrectly assumed to be 700. *15 %* - This option is incorrect as it does not match the calculated incidence of LBW babies among women who underwent USG. - This would mean only 52.5 LBW babies were born, which contradicts the information given that 70 delivered LBW babies.

Question 1537: Poor hand hygiene of a mess worker in a university college mess led to Hepatitis A cases in the hostel inmates. What type of epidemic will this exposure present with? 1. Propagated 2. Common source-continuous exposure 3. Common source-point exposure Select the correct answer using the code given below:

A. 1 only
B. 1 and 3
C. 2 only (Correct Answer)
D. 1 and 2

Explanation: ***2 only*** - A mess worker with **ongoing poor hand hygiene** represents a **continuous common source exposure**. The worker continues to handle food over days or weeks with persistent poor hygiene, leading to **repeated contamination** and cases occurring over an extended period rather than clustered around a single incubation period. This produces a plateau-like epidemic curve characteristic of continuous exposure. *1 only* - **Propagated epidemic** occurs through **person-to-person transmission** (e.g., measles, chickenpox), where each case can generate new cases, creating successive waves with progressively larger peaks. Hepatitis A from a food handler is a **common source outbreak**, not propagated, as cases trace back to contaminated food rather than spreading between inmates. *1 and 3* - Option 1 (propagated) is incorrect as explained above. Option 3 (**common source-point exposure**) would apply if there was a **single, brief contamination event** (e.g., one contaminated meal), resulting in cases appearing within one incubation period with a sharp peak. The scenario describes **persistent poor hygiene** suggesting ongoing contamination, not a single point event. *1 and 2* - Option 1 (propagated) is incorrect as this is a common source outbreak from contaminated food, not person-to-person transmission.

Question 1538: A depression screening tool developed in specialty clinics performs poorly when used in primary care settings. The positive predictive value drops from 85% to 25%. What is the most likely explanation?

A. The tool is fundamentally flawed
B. Patients in primary care are less honest about symptoms
C. Disease prevalence is lower in primary care than specialty clinics (Correct Answer)
D. Primary care physicians are using the tool incorrectly

Explanation: ***Disease prevalence is lower in primary care than specialty clinics*** - **Positive predictive value (PPV)** is highly dependent on **disease prevalence**. A lower prevalence in primary care will naturally lead to a lower PPV even if the test's sensitivity and specificity remain constant. - In specialty clinics, the population has a higher pre-test probability of having the condition due to referral patterns, which inflates the PPV. *The tool is fundamentally flawed* - While a flawed tool could certainly perform poorly, the fact that it performed well (85% PPV) in specialty clinics suggests the tool itself is not fundamentally flawed in its ability to identify depression in an appropriate population. - The drop in performance is more likely due to a change in the characteristics of the population being screened, rather than an inherent flaw in the tool's design. *Patients in primary care are less honest about symptoms* - There is no evidence to suggest that patients in primary care settings are inherently less honest about their symptoms compared to those in specialty clinics. - Such an assumption is a generalization and does not account for the statistical relationship between prevalence and predictive values. *Primary care physicians are using the tool incorrectly* - While possible, there's no information provided to support this claim; the question implies the tool is being used for its intended purpose. - Improper use would affect test results, but a consistent drop in PPV across a broad setting points to a more systemic issue related to population characteristics.

Question 1539: A cohort study follows 5,000 nurses for 20 years to study night shift work and breast cancer risk. After 10 years, 30% of participants have been lost to follow-up. What is the most significant threat to study validity?

A. Twenty years is too long for meaningful results
B. Nurses are not representative of the general population
C. The study should be converted to a case-control design
D. Loss to follow-up may introduce bias if related to exposure or outcome (Correct Answer)

Explanation: ***Loss to follow-up may introduce bias if related to exposure or outcome*** - A 30% **loss to follow-up** in a cohort study is substantial and can lead to **selection bias** if the reasons for loss are related to either night shift work (exposure) or breast cancer risk (outcome). - For example, if nurses experiencing early symptoms of breast cancer or those struggling with night shift demands selectively drop out, the study's **risk estimates** will be skewed. *Twenty years is too long for meaningful results* - A 20-year follow-up period is often necessary to observe chronic diseases like cancer, as they have long **latency periods**. - While longer study durations can increase the likelihood of loss to follow-up, the duration itself is not inherently a threat to validity when studying diseases with long developmental phases. *Nurses are not representative of the general population* - Selecting nurses as a cohort is a common and often appropriate strategy in **occupational epidemiology**, as it allows for focused study of specific exposures (like shift work) in a relatively homogenous group. - While findings might not be directly generalizable to the entire population, they can still provide valuable insights into the **exposure-outcome relationship** within this group. *The study should be converted to a case-control design* - The study began as a **cohort design**, which is strong for assessing incidence and temporal relationships between exposure and outcome. - Converting to a **case-control design** retrospectively would mean losing the prospective data collection and the ability to calculate **absolute risk**, which are strengths of the original design.

Question 1540: A screening test for a rare disease has 99% sensitivity and 95% specificity. In a population where the disease prevalence is 0.1%, what is the most important consideration for implementing this test?

A. High specificity ensures accurate results
B. High sensitivity makes it excellent for screening
C. Low prevalence will result in many false positives (Correct Answer)
D. The test performance is ideal for any population

Explanation: ***Low prevalence will result in many false positives*** - In populations with **low disease prevalence**, even a test with high specificity can yield a large number of **false positives** because the vast majority of tested individuals do not have the condition. - For example, with a 95% specificity and 0.1% prevalence, 5% of healthy individuals will test positive, which is significantly more than the true positives from a 0.1% prevalence. - The **positive predictive value (PPV)** will be very low (~1.9%), meaning most positive results will be false positives. *High specificity ensures accurate results* - While high specificity (95%) means a low rate of false positives among healthy individuals, in a **rare disease** scenario, the absolute number of healthy people is very large, leading to many healthy individuals being incorrectly flagged. - This high number of false positives can lead to unnecessary follow-up procedures, anxiety, and resource strain, undermining the screening program's effectiveness. *High sensitivity makes it excellent for screening* - **High sensitivity** (99%) ensures that most true cases are identified, which is crucial for a good screening test. - However, in a low prevalence setting, high sensitivity alone does not address the issue of **false positives** among the vast majority of healthy individuals, which can overwhelm the system and cause patient harm. *The test performance is ideal for any population* - The performance of a screening test is highly dependent on the **prevalence of the disease** in the target population, especially the predictive values (PPV and NPV). - A test that performs well in a high-prevalence population may be unsuitable for screening a low-prevalence population due to a high rate of **false positives** and low PPV, making it impractical for population-wide screening.

Practice by Chapter

Principles of Epidemiology

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Measures of Disease Frequency

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Epidemiological Study Designs

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Descriptive Epidemiology

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Analytical Epidemiology

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Experimental Epidemiology

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Screening for Disease

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Surveillance Systems

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Investigation of an Epidemic

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Association and Causation

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Modern Epidemiological Methods

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Critical Appraisal of Epidemiological Studies

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