Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1881: To determine the endemicity of hepatitis B, what should be measured?

A. HBsAg (Hepatitis B surface antigen) (Correct Answer)
B. HBcAg (Hepatitis B core antigen)
C. HBeAg (Hepatitis B e antigen)
D. Anti-HBeAg (Hepatitis B e antibody)

Explanation: ***HBsAg*** - **HBsAg (Hepatitis B surface antigen)** is the primary marker used to determine the **endemicity** of hepatitis B because its persistent presence indicates **chronic infection**. - A high prevalence of HBsAg in a population signifies a high burden of chronic hepatitis B infection, reflecting the endemic nature of the disease in that region. *HBcAg (Hepatitis B core antigen)* - **HBcAg** is an **intracellular antigen** and is not detectable in the serum, making it unsuitable for population-level screening or endemicity assessment. - While important for viral replication, its absence in routine blood tests means it cannot be used to gauge the prevalence of infection in a community. *HBeAg (Hepatitis B e antigen)* - **HBeAg** indicates **active viral replication** and high infectivity but is not the best marker for overall endemicity. - A positive HBeAg suggests active disease and high transmissibility in an infected individual, but not the general prevalence of chronic infection in a population. *Anti-HBeAg (Hepatitis B e antibody)* - **Anti-HBeAg** indicates a **decrease in viral replication** and a lower risk of transmission, often representing a stage of immune control. - While useful for monitoring disease progression, it is an antibody response and does not directly measure the presence of chronic infection or endemicity.

Question 1882: What is the prevalence in a given population of 1000, where there are 50 new cases of lung cancer and 100 old cases of lung cancer in the same population?

A. 1.50%
B. 15% (Correct Answer)
C. 150
D. 13%

Explanation: ***Correct: 15%*** - **Prevalence** is the proportion of a population living with a disease at a specific time point. It includes both new and existing (old) cases. - **Calculation:** Total cases = 50 (new cases) + 100 (old cases) = 150 cases - **Prevalence rate** = (150 / 1000) × 100% = **15%** - Prevalence answers the question: "What proportion of the population has the disease right now?" *Incorrect: 1.50%* - This value represents a calculation error, likely from dividing 150 by 10,000 instead of 1,000 - It underestimates the actual prevalence by a factor of 10 - Would only be correct if there were 15 total cases, not 150 *Incorrect: 150* - This is the **absolute count** of individuals with lung cancer (both new and old cases) - Prevalence must be expressed as a **proportion or percentage**, not a raw count - Raw counts cannot be compared across populations of different sizes *Incorrect: 13%* - This would only be correct if there were 130 total cases, not 150 - This miscalculation fails to properly sum the new cases (50) and old cases (100) - The arithmetic is incorrect: 50 + 100 ≠ 130

Question 1883: Which of the following is considered the most reliable indicator of mortality in a population?

A. CDR
B. PMR
C. ASDR (Correct Answer)
D. CFR

Explanation: ***ASDR (Age-Specific Death Rate)*** * **ASDR** is considered the most reliable indicator of mortality because it takes into account the different **age structures** of populations being compared. * It provides a more accurate picture of mortality by showing the number of deaths relative to the population in specific age groups, which is crucial for **epidemiological studies** and public health planning. *CDR (Crude Death Rate)* * The **CDR** is the total number of deaths in a given period divided by the total population, which can be misleading when comparing populations with different **age distributions**. * A high CDR in one population might be due to a larger proportion of elderly individuals, rather than higher actual mortality risk across all age groups. *PMR (Proportional Mortality Ratio)* * The **PMR** expresses the proportion of deaths due to a specific cause out of all deaths, rather than the risk of dying from that cause in the entire population. * It does not reflect the **absolute risk** of death and can be influenced by changes in other causes of death. *CFR (Case Fatality Rate)* * The **CFR** measures the proportion of people diagnosed with a specific disease who die from that disease within a certain period. * While useful for understanding the severity of a disease, it is not an indicator of overall mortality in a population but rather the **lethality** of a particular condition among those affected.

Question 1884: The time interval between diagnosis by early detection and diagnosis by other means is what?

A. Incubation period
B. Serial interval
C. Latent period
D. Lead time (Correct Answer)

Explanation: ***Lead time*** - **Lead time** refers to the interval between diagnosis by screening or early detection and the time at which the diagnosis would have been made by usual clinical presentation or other means. - A longer lead time in screening programs can make it seem like screened individuals live longer, even if the treatment efficacy is the same (known as **lead time bias**). *Incubation period* - The **incubation period** is the time from exposure to an infectious agent to the onset of symptoms for an infectious disease. - It is not related to the comparison of diagnosis times using different methods. *Serial interval* - The **serial interval** in epidemiology is the time between symptom onset in an infected person and symptom onset in a secondary case infected by the first person. - This concept is specific to the transmission dynamics of infectious diseases and not to diagnostic timing. *Latent period* - The **latent period** can refer to various concepts depending on the context; in infectious diseases, it's the time from infection to infectivity, or in chronic diseases, it can be the time from exposure to a causal agent to the development of detectable disease. - While it relates to disease progression, it specifically measures the time until detectability or infectivity, not the difference in diagnostic timings between early detection and other methods.

Question 1885: A disease has three times more incidence in females as compared to males, with the same prevalence in both males and females. The TRUE statement is:

A. Increase fatality in women (Correct Answer)
B. More survival in women
C. Less fatality in men
D. Better prognosis in men

Explanation: ***Increase fatality in women*** - **Prevalence = Incidence × Duration of disease** - Given: Incidence in females = 3 × Incidence in males, but Prevalence is same in both - For males: Prevalence = I_m × D_m - For females: Prevalence = 3I_m × D_f - Since prevalences are equal: I_m × D_m = 3I_m × D_f - Therefore: **D_f = D_m/3** (females have 1/3 the disease duration of males) - **Shorter disease duration means worse survival and increased fatality in women** *More survival in women* - This would be incorrect because if women had better survival, their disease duration would be longer - With 3× higher incidence AND longer duration, the prevalence in women would be much higher than men, not equal - The equal prevalence despite higher incidence indicates women are dying faster (shorter duration) *Better prognosis in men* - While men do have longer disease duration (3× that of women), this option is vague - "Prognosis" could refer to recovery or survival, but the question specifically asks about the relationship between incidence and prevalence - The more precise statement is about increased fatality in women, which directly explains the epidemiological relationship *Less fatality in men* - This is essentially the same as saying "more survival in men" or "better prognosis in men" - While men do have less fatality (longer duration), the question stem focuses on the paradox of higher incidence in women with equal prevalence - The **key insight** is recognizing increased fatality in women, which is the direct answer to why higher incidence doesn't lead to higher prevalence

Question 1886: The ability of a screening test to detect true positives is known as:

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

Explanation: ***Sensitivity*** - **Sensitivity** refers to the ability of a screening test to correctly identify individuals who truly **have a disease** (true positives). - A highly sensitive test will have a low rate of **false negatives**. - **Clinical application (SnNout)**: When a highly **sensitive** test is **negative**, it helps rule **out** the disease. *Specificity* - **Specificity** is the ability of a test to correctly identify individuals who do **not have the disease** (true negatives). - A highly specific test has a low rate of **false positives**. - **Clinical application (SpPin)**: When a highly **specific** test is **positive**, it helps rule **in** the disease. *Positive predictive value* - **Positive predictive value (PPV)** is the probability that an individual with a **positive test result** actually has the disease. - PPV is influenced by the **prevalence of the disease** in the population being tested. *Negative predictive value* - **Negative predictive value (NPV)** is the probability that an individual with a **negative test result** actually does not have the disease. - NPV is also affected by the **prevalence of the disease**; a lower prevalence generally leads to a higher NPV.

Question 1887: A study is to be conducted to compare the fat content in the expressed breast milk of pre-term infants with that of term infants. Which study design is best suited?

A. Longitudinal study
B. Ambispective
C. Case control
D. Prospective cohort (Correct Answer)

Explanation: ***Prospective cohort*** - Among the given options, a **prospective cohort study** is the most appropriate design for this comparative study. - The study involves identifying two groups (mothers of pre-term vs. term infants) and **prospectively collecting breast milk samples** to measure and compare fat content between these groups. - This design allows for **standardized data collection** moving forward in time, ensuring consistent measurement protocols for both groups. - While this is essentially a comparative cross-sectional measurement, the prospective nature ensures proper sample collection and reduces recall bias. *Case control* - This design is used to compare **exposures** between those with and without an outcome (typically a disease). - Fat content in breast milk is a **continuous biological variable**, not a disease outcome, making case-control design inappropriate. - Case-control studies work backward from outcome to exposure, which doesn't fit this scenario where we're comparing groups defined by infant term status. *Longitudinal study* - While **prospective cohort** is a type of longitudinal study, this term is too broad and non-specific. - Longitudinal studies involve repeated measurements over time, but this question asks for a specific study design for comparing two groups. - Simply stating "longitudinal study" doesn't specify the comparative framework needed. *Ambispective* - An **ambispective (or ambi-directional) study** combines retrospective and prospective components, using existing historical data plus new follow-up. - This design is unnecessary here as there's no indication of existing historical data to utilize. - The study can be conducted entirely prospectively by identifying mothers and collecting fresh breast milk samples for analysis.

Question 1888: Which of the following is not a measure of reliability in screening tests?

A. Validity (accuracy of measurement) (Correct Answer)
B. Consistency of results
C. Reproducibility of results
D. Precision of results

Explanation: ***Validity (accuracy of measurement)*** - **Validity** refers to how accurately a test measures what it intends to measure, often assessed by comparing it to a **gold standard** - It is a measure of a test's **accuracy**, not its reliability or consistency when repeated - **This is NOT a measure of reliability** - it's a separate concept assessing whether the test identifies true positives and true negatives correctly *Consistency of results* - **Consistency of results** is a key aspect of reliability, indicating that the test yields similar outcomes under similar conditions - A reliable test should produce consistent results if repeated multiple times on the same individual (test-retest reliability) *Reproducibility of results* - **Reproducibility of results** is another term used to describe reliability, meaning that the test yields the same outcome when performed by different observers or in different settings - This ensures that the test results are not dependent on the administrator or environment (inter-rater/inter-observer reliability) *Precision of results* - **Precision of results** refers to how close repeated measurements are to each other, irrespective of whether they are close to the true value - It is a measure of the consistency and reliability of the test instrument or method

Question 1889: Consanguineous marriages increase the risk of which of the following diseases?

A. Autosomal dominant diseases
B. Autosomal recessive diseases (Correct Answer)
C. X linked dominant diseases
D. Environmental diseases

Explanation: ***Autosomal recessive diseases*** - Consanguineous marriages increase the likelihood of offspring inheriting two copies of a **recessive deleterious allele** from a common ancestor. - This significantly raises the risk of expressing **autosomal recessive conditions**, as both parents are more likely to be carriers of the same rare recessive gene. - Examples include **thalassemia, sickle cell disease, and cystic fibrosis**. *Autosomal dominant diseases* - These diseases manifest with only **one copy of the mutated allele**, regardless of consanguinity. - The risk is primarily linked to whether one parent carries the dominant gene, not the relatedness of the parents. *X linked dominant diseases* - These conditions are caused by mutations on the **X chromosome** and are expressed dominantly. - Consanguinity does not specifically increase the risk, as the disease manifests when the mutated X-linked gene is inherited from an affected parent. - The inheritance pattern depends on the affected parent's sex, not on parental relatedness. *Environmental diseases* - These diseases are primarily caused by **external factors** such as toxins, diet, lifestyle choices, or infections. - While genetic predisposition may play a role, consanguinity does not directly increase the risk for environmentally triggered diseases.

Question 1890: Sandfly acts as a vector to all of the following diseases, except which of the following?

A. Kala-azar
B. Pappataci fever
C. Onchocerciasis (Correct Answer)
D. Leishmaniasis

Explanation: ***Onchocerciasis*** - **Onchocerciasis**, or river blindness, is transmitted by the **blackfly (Simulium species)**, not the sandfly. - The disease is caused by the filarial nematode *Onchocerca volvulus*, which is introduced into humans through the bite of an infected blackfly. *Pappataci fever* - **Pappataci fever**, also known as sandfly fever, is a viral disease transmitted by the bite of infected **sandflies (Phlebotomus species)**. - This disease is characterized by a sudden onset of fever, headache, and muscle pain. *Kala-azar* - **Kala-azar**, or visceral leishmaniasis, is a severe form of **leishmaniasis** transmitted by the bite of an infected female **sandfly (Phlebotomus species)**. - It is caused by intracellular parasites of the genus *Leishmania* and affects internal organs. *Leishmaniasis* - **Leishmaniasis** in its various forms (cutaneous, mucocutaneous, visceral) is transmitted by the bite of an infected female **sandfly**. - The sandfly acts as the definitive host and vector for the *Leishmania* parasites.

Practice by Chapter

Principles of Epidemiology

Practice Questions

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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