Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1751: Which measure indicates the diagnostic power of a test to correctly identify those with a disease?

A. Negative predictive value
B. Specificity
C. Sensitivity (Correct Answer)
D. Positive predictive value

Explanation: ***Positive predictive value*** - It refers to the probability that subjects with a positive test result truly have the disease, highlighting the test's **diagnostic accuracy** [1]. - A high positive predictive value indicates that the test is effective at diagnosing the disease in the population tested. *Sensitivity* - Sensitivity measures the ability of a test to correctly identify those with the disease (true positives), but does not account for the test result's predictive capability [1]. - It is important for screening, but **not directly the diagnostic power** for those already tested. *Negative predictive value* - This indicates the probability that subjects with a negative test result truly do not have the disease, focusing on true negatives rather than correct diagnosis of the condition [1]. - While informative, it does not assess the ability to correctly diagnose the disease when the result is positive. *Specificity* - Specificity is the measure of a test's ability to correctly identify those without the disease (true negatives), not diagnosing the disease accurately among those tested [1]. - It is essential for determining false positives but not for assessing the overall diagnostic power of a test. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. (Basic Pathology) introduces the student to key general principles of pathology, both as a medical science and as a clinical activity with a vital role in patient care. Part 2 (Disease Mechanisms) provides fundamental knowledge about the cellular and molecular processes involved in diseases, providing the rationale for their treatment. Part 3 (Systematic Pathology) deals in detail with specific diseases, with emphasis on the clinically important aspects., pp. 253-254.

Question 1752: In a screening test for DM out of 1000 population, 90 were positive. When the gold standard test was applied to the entire population, 100 were found to have the disease. Assuming all 90 screening positives were confirmed as true positives by the gold standard, calculate the sensitivity.

A. All positives identified by the test assumed as true positives (100%)
B. True positives divided by total actual positives (90%) (Correct Answer)
C. Underestimated true positives divided by total actual positives (80%)
D. Total positives identified by the test divided by total actual positives (90%)

Explanation: ***True positives divided by total actual positives (90%)*** - **Sensitivity** is the proportion of true positives correctly identified by a screening test among all individuals who actually have the disease. It is calculated by (Number of True Positives) / (Total Number of Diseased Individuals). - In this case, 90 people screened positive and were confirmed as **true positives**. The total number of people with the disease (actual positives) is 100. So, sensitivity = 90/100 = **90%**. *Total positives identified by the test divided by total actual positives (90%)* - While this option states the correct percentage (90%), the phrasing "total positives identified by the test" is misleading terminology. In screening test evaluation, this could be confused with all test positives (which would include false positives if they existed). - The correct terminology is "true positives" divided by "total actual positives," not "total positives identified by the test." The distinction is important: true positives are confirmed cases, while test positives might include false positives. *All positives identified by the test assumed as true positives (100%)* - This option incorrectly assumes that because all 90 screening positives were confirmed as true positives, the sensitivity must be 100%. However, sensitivity measures how many of ALL diseased individuals were caught, not just those who screened positive. - There were 100 actual diseased individuals, and only 90 were identified by the screening test; therefore, the sensitivity cannot be 100%. The test missed 10 diseased individuals (false negatives). *Underestimated true positives divided by total actual positives (80%)* - This option presents an arbitrary percentage that does not reflect the given data. There is no information to suggest that the true positives were underestimated or that the calculation would result in 80%. - The actual number of true positives (90) and actual positives (100) directly leads to a sensitivity calculation of 90%, not 80%.

Question 1753: Incidence of a disease is 4 per 1000 of population with duration of 2 years. Calculate the prevalence?

A. 8 per 1000 (Correct Answer)
B. 4 per 1000
C. 2 per 1000
D. 6 per 1000

Explanation: ***8 per 1000*** - Prevalence can be estimated by multiplying the **incidence rate** by the **duration of the disease**. - In this case, 4/1000 (incidence) * 2 years (duration) = **8 per 1000**. *4 per 1000* - This value represents the **incidence** of the disease, which is the rate of new cases, not the total number of existing cases (prevalence). - Prevalence includes both new and existing cases over a specified period. *2 per 1000* - This value is obtained by dividing the incidence by the duration (4/2), which is not the correct formula for calculating prevalence in this context. - Doing so would incorrectly imply a lower disease burden than what is indicated by the incidence and duration. *6 per 1000* - This option is simply the sum of incidence and duration (4+2), which does not represent a valid epidemiological calculation for prevalence. - Prevalence is determined by considering both the rate of new cases and how long individuals typically live with the disease.

Question 1754: What is the term used to describe the occurrence of a disease in a susceptible person after they have been in contact with a primary case within the incubation period?

A. Secondary attack rate (Correct Answer)
B. Case fatality rate
C. Primary attack rate
D. Tertiary attack rate

Explanation: ***Secondary attack rate*** - This term specifically measures the **frequency of new cases** among contacts of primary cases within the incubation period. - It is a key epidemiological measure to assess the **transmissibility** of an infectious agent within a defined population group. - Calculated as: (Number of cases among contacts / Number of susceptible contacts) × 100 *Case fatality rate* - This metric represents the **proportion of deaths** among individuals diagnosed with a specific disease, indicating its severity. - It does not describe the occurrence of disease transmission from a primary case to susceptible contacts. *Primary attack rate* - This refers to the **number of cases occurring among the total population at risk** during the initial period of an outbreak. - It differs from secondary attack rate, which specifically measures transmission from a **known primary case to their contacts**. - Primary attack rate does not distinguish between primary and secondary cases. *Tertiary attack rate* - This term is not a commonly used or recognized epidemiological measure. - While disease transmission can occur beyond secondary contacts, there isn't a standard "tertiary attack rate" used in epidemiological practice.

Question 1755: What is the primary ecological unit of study in epidemiology for understanding disease patterns and public health?

A. Population (Correct Answer)
B. Community
C. Individual patient
D. Case study

Explanation: ***Population*** - In public health and epidemiology, a **population** is the fundamental unit for studying disease patterns, incidence, prevalence, and risk factors across groups. - Understanding disease at the population level allows for the development of **prevention strategies**, public health interventions, and policy making that impact many individuals. *Individual patient* - While critical for clinical diagnosis and treatment, the **individual patient** represents a single case and does not provide insights into broader disease patterns or public health trends. - Studying individuals primarily informs **patient management** and understanding disease pathophysiology rather than population-level epidemiology. *Community* - A **community** is a group of people living in the same place or having a particular characteristic in common, which is a broader concept than a population. - While public health interventions often target communities, the underlying data and epidemiological analyses are typically based on defined **populations within** or across communities. *Case study* - A **case study** is an in-depth analysis of a single individual, group, or event, offering rich, detailed information. - While valuable for generating hypotheses or understanding rare conditions, a case study does not provide the **statistical power** or generalizability needed to understand disease patterns across large groups.

Question 1756: What is the definition of Population Attributable Risk?

A. The difference between incidence in population and incidence in exposed.
B. The difference between incidence in population and incidence in non-exposed. (Correct Answer)
C. The difference between incidence in population and incidence in non-exposed compared with incidence in exposed.
D. The difference between incidence in exposed and incidence in non-exposed.

Explanation: ***Correct: The difference between incidence in population and incidence in non-exposed.*** - **Population Attributable Risk (PAR)** quantifies the excess incidence of disease in the total population that is attributable to a specific exposure. - Formula: **PAR = Incidence in total population - Incidence in unexposed** - It represents the amount of disease burden that would be eliminated from the entire population if the exposure were completely removed. - PAR accounts for both the strength of association and the prevalence of exposure in the population. *Incorrect: The difference between incidence in population and incidence in exposed.* - This formula (I(population) - I(exposed)) does not correctly capture PAR. - This calculation does not isolate the portion of disease attributable to the exposure across the entire population. - It fails to provide meaningful information about attributable risk. *Incorrect: The difference between incidence in population and incidence in non-exposed compared with incidence in exposed.* - This option introduces unnecessary complexity and is not the standard definition of PAR. - PAR is a simple difference, not a comparative ratio involving exposed individuals. - This description confuses PAR with other epidemiological measures. *Incorrect: The difference between incidence in exposed and incidence in non-exposed.* - This describes **Attributable Risk (AR)** or **Risk Difference (RD)**, not Population Attributable Risk. - Formula: **AR = I(exposed) - I(unexposed)** - AR measures excess risk in the exposed group only, without considering the prevalence of exposure in the total population. - PAR differs from AR by accounting for how common the exposure is in the population.

Question 1757: What is a key benefit of Randomized Controlled Trials (RCTs) in clinical research?

A. They can be conducted more quickly than other study types.
B. They minimize selection bias. (Correct Answer)
C. They are ideal for studying rare diseases.
D. They are generally less expensive than other study types.

Explanation: ***They minimize selection bias.*** - **Randomization** in RCTs ensures that participants have an equal chance of being assigned to any of the treatment groups, thereby balancing potential **confounding factors** across groups. - This balance helps to ensure that any observed differences in outcomes between groups are more likely due to the intervention being studied rather than pre-existing differences among participants, thus minimizing **selection bias**. *They can be conducted more quickly than other study types.* - RCTs often require **extensive planning**, recruitment, and follow-up periods, making them one of the **most time-consuming** study designs. - The need for sufficient **power** to detect meaningful differences often translates into longer study durations. *They are ideal for studying rare diseases.* - Due to the requirement for **large sample sizes** to demonstrate statistical significance, RCTs are **not practical** for diseases with low prevalence. - Recruiting enough participants with a rare disease for an RCT can be extremely challenging and often **unfeasible**. *They are generally less expensive than other study types.* - RCTs are typically among the **most expensive** study designs because they involve extensive participant recruitment, intervention administration, data collection, and long-term follow-up. - The costs associated with staff, resources, and monitoring for ethical compliance contribute to their **high financial burden**.

Question 1758: Multiphasic screening means-

A. Application of two or more screening tests in combination at different geographical areas
B. Application of separate screening tests for different diseases
C. Application of two or more screening tests in combination at one time (Correct Answer)
D. Application of two or more screening tests in combination at different times

Explanation: ***Application of two or more screening tests in combination at one time*** - **Multiphasic screening** involves performing several screening tests simultaneously during a single screening session. - This approach aims to detect multiple diseases or risk factors efficiently within a single visit or examination. *Application of two or more screening tests in combination at different times* - This describes repeated screening or sequential screening, where tests are administered over a period, not the immediate, combined approach of multiphasic screening. - **Multiphasic screening** specifically refers to the concurrent application of multiple tests, not their staggered use. *Application of two or more screening tests in combination at different geographical areas* - This concept relates more to large-scale public health programs or epidemiological studies across regions, rather than the definition of multiphasic screening itself. - Geographical variation is not a defining characteristic of multiphasic screening. *Application of separate screening tests for different diseases* - While multiphasic screening indeed uses separate tests for different diseases, the key aspect is their **simultaneous application** at one time to a single individual, which this option omits. - This option describes the general nature of screening for various conditions but misses the crucial element of combination and timing.

Question 1759: What is the most important criterion in a causal relationship hypothesis?

A. Temporal association (Correct Answer)
B. Coherence of association
C. Specificity of association
D. Strength of association

Explanation: ***Temporal association*** - This is the **sine qua non** of causality, meaning the exposure or cause must always precede the outcome or effect in time. - Without the exposure occurring before the disease, a causal link cannot be established, even if other criteria are met. *Coherence of association* - This refers to the consistency of findings with current scientific knowledge and **biological plausibility**. - While important for supporting causality, a coherent explanation is not sufficient in itself to prove causation and may even be misleading if current knowledge is incomplete. *Specificity of association* - This criterion suggests that a single exposure should lead to a single outcome, or a single outcome should be caused by a single exposure. - However, many diseases have **multiple causes**, and many exposures can lead to multiple effects, making this a weak criterion in modern epidemiology. *Strength of association* - A **strong association**, often measured by a high relative risk or odds ratio, makes a causal relationship more likely but does not guarantee it. - Strong associations can still be due to **confounding factors** or bias, and weak associations can be causal.

Question 1760: What is the primary benefit of screening for diseases?

A. Early detection of diseases (Correct Answer)
B. Providing support for patients after diagnosis
C. Identifying all potential cases of a disease
D. Timely treatment of identified conditions

Explanation: ***Early detection of diseases*** - This is the **primary benefit** and defining purpose of **screening programs** in public health. - Screening identifies diseases in their **presymptomatic or early stage** when individuals are apparently healthy, allowing for intervention before clinical symptoms appear. - According to epidemiological principles, the goal of screening is to detect disease **earlier than it would be found through routine clinical practice**. - Early detection enables better prognosis through **lead time** and **length time bias** advantages. *Timely treatment of identified conditions* - While treatment is the **ultimate goal** of healthcare, it is not specific to screening—treatment occurs whether disease is found through screening or clinical presentation. - Treatment is the **consequence** of early detection, not the primary benefit of the screening process itself. - The unique value of screening lies in **detection**, not treatment per se. *Providing support for patients after diagnosis* - **Patient support** is an important aspect of healthcare but is not the purpose of screening programs. - This is **post-diagnostic care**, which follows after the screening process has identified cases. *Identifying all potential cases of a disease* - **Screening tests** cannot identify all cases due to inherent limitations in **sensitivity** and **specificity**. - Screening aims to identify a significant proportion of cases in a population, accepting that some will be missed (**false negatives**) and some healthy individuals may test positive (**false positives**).

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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