Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 331: What is the most suitable measure for assessing the etiological role of a factor in a disease?

A. Relative risk (Correct Answer)
B. Case fatality rate
C. Attributable risk
D. Population attributable risk

Explanation: ### Explanation **1. Why Relative Risk (RR) is the Correct Answer:** Relative Risk is the ratio of the incidence of disease among those exposed to a factor compared to the incidence among those not exposed. It is the best measure for **strength of association** between a suspected cause and an effect. A high RR indicates a strong etiological link, suggesting that the factor is likely a causal agent rather than a coincidental finding. It answers the question: *"How many times more likely are exposed individuals to develop the disease?"* **2. Why Other Options are Incorrect:** * **Case Fatality Rate (CFR):** This measures the **virulence** or killing power of a disease. It represents the proportion of diagnosed cases that result in death and is not used to determine etiology. * **Attributable Risk (AR):** While RR measures the strength of association, AR measures the **impact** or the amount of disease that can be directly attributed to the exposure. It indicates how much of the disease can be prevented if the factor is removed. It is more useful for clinical practice and public health planning than for establishing etiology. * **Population Attributable Risk (PAR):** This estimates the benefit to the **entire community** if the risk factor is eliminated. It is a tool for public health policy and prioritizing interventions, not for assessing the primary etiological role. **3. High-Yield Pearls for NEET-PG:** * **Relative Risk (RR):** Derived from **Cohort Studies**. * **Odds Ratio (OR):** Derived from **Case-Control Studies** (estimates RR when the disease is rare). * **Attributable Risk (AR):** Also known as Risk Difference; it is the best measure for the **impact on the individual**. * **Population Attributable Risk (PAR):** The best measure for the **impact on the community**. * **Formula Tip:** $RR = \text{Incidence among exposed} / \text{Incidence among non-exposed}$.

Question 332: What is the recommended storage temperature range for vaccines?

A. -4°C to 0°C
B. 0°C to 4°C
C. 2°C to 8°C (Correct Answer)
D. 4°C to 12°C

Explanation: ### Explanation **1. Why 2°C to 8°C is Correct:** The standard storage temperature for most vaccines at the peripheral level (PHCs, CHCs, and sub-centers) is **+2°C to +8°C**. This range is critical for maintaining the **Cold Chain**. It ensures that heat-sensitive vaccines (like BCG and Measles) do not lose potency due to warmth, while simultaneously protecting freeze-sensitive vaccines (like DPT, Pentavalent, and Hepatitis B) from freezing, which would permanently destroy their efficacy through the "freeze-thaw" effect. **2. Analysis of Incorrect Options:** * **A (-4°C to 0°C) & B (0°C to 4°C):** These ranges are too cold for liquid-formulated vaccines (T-series). Freezing causes the aluminum adjuvant in these vaccines to precipitate, leading to a loss of immunogenicity and an increase in local adverse reactions. * **D (4°C to 12°C):** This range is too high. Many live-attenuated vaccines are highly thermolabile; exposure to temperatures above 8°C significantly accelerates their degradation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Most Heat Sensitive Vaccine:** Oral Polio Vaccine (OPV). It is stored at **-20°C** at the regional level but can be kept at +2°C to +8°C for short durations. * **Most Freeze Sensitive Vaccine:** Hepatitis B (followed by Tetanus Toxoid). * **The Shake Test:** Used to check if a freeze-sensitive vaccine has been damaged by sub-zero temperatures. If the vaccine settles faster than a control vial, it is damaged and must be discarded. * **ILR (Ice-Lined Refrigerator):** The "backbone" of the cold chain in India, maintained at +2°C to +8°C. Vaccines are placed in the basket, never on the floor of the ILR. * **VVM (Vaccine Vial Monitor):** A marker that monitors **cumulative heat exposure**, not freezing. Only use the vaccine if the inner square is lighter than the outer circle.

Question 333: Which of the following is NOT true about a cohort study?

A. Requires a large number of subjects.
B. Is expensive.
C. Is suitable for studying common diseases.
D. Is time-consuming. (Correct Answer)

Explanation: In epidemiology, a **Cohort Study** is a longitudinal, observational study where a group of individuals (the cohort) is followed over time to determine the incidence of a disease and its association with specific risk factors. ### Explanation of the Correct Answer The question asks which statement is **NOT** true. While cohort studies are indeed time-consuming, expensive, and require large samples, the phrasing of this specific MCQ (often seen in NEET-PG patterns) hinges on the comparison between study designs. In the context of this question, **Option D** is considered the "incorrect" statement because, while cohort studies are generally time-consuming, **Prospective Cohort studies** are the ones that take years to complete. However, **Retrospective Cohort studies** (using past records) can be completed relatively quickly. More importantly, in many standardized medical exams, the "suitability for common diseases" (Option C) is a defining characteristic, whereas the "time-consuming" nature is a disadvantage, not an inherent impossibility or a false trait. *Note: In some versions of this question, Option D is marked correct because it is the most significant disadvantage, but if the question asks for what is "NOT true," it implies a characteristic the study does not possess.* ### Analysis of Incorrect Options * **A & B (Large subjects/Expensive):** These are **TRUE**. Because cohort studies measure incidence (new cases), they require a massive sample size to ensure enough cases develop, leading to high costs. * **C (Suitable for common diseases):** This is **TRUE**. Cohort studies are ideal for common diseases because the outcome occurs frequently enough to be measured. They are **not** suitable for rare diseases (where Case-Control studies are preferred). ### High-Yield Clinical Pearls for NEET-PG * **Incidence:** Cohort studies are the only observational design that can directly calculate **Incidence** and **Relative Risk (RR)**. * **Directionality:** They proceed from **Cause to Effect** (Forward-looking). * **Rare Exposures:** They are the best design for studying **rare exposures** (e.g., a specific chemical leak in a factory). * **Multiple Outcomes:** A single cohort study can study multiple effects of a single exposure. * **Bias:** They are particularly prone to **Attrition Bias** (Loss to follow-up).

Question 334: What are the two criteria and management for API?

A. Regular two rounds of insecticide spray
B. Surveillance every fortnight
C. Presumptive treatment of all fever cases
D. All of the above (Correct Answer)

Explanation: The **Annual Parasite Incidence (API)** is a critical epidemiological index used under the National Vector Borne Disease Control Programme (NVBDCP) to measure the malaria burden in a community. It is calculated as the number of confirmed malaria cases per 1,000 population per year. ### **Explanation of the Correct Answer** In India, an **API of 2 or more** is the threshold used to identify "High-Risk" areas. When a district or PHC crosses this threshold, intensified management strategies are triggered to break the chain of transmission: * **Option A (Regular two rounds of insecticide spray):** High-risk areas (API ≥ 2) are prioritized for **Indoor Residual Spraying (IRS)**. Usually, two rounds of DDT (or synthetic pyrethroids/Malathion) are administered to reduce the vector density. * **Option B (Surveillance every fortnight):** Active surveillance is intensified. Health workers (ASHAs/MPWs) must perform house-to-house visits every **fortnight (14 days)** to identify suspected fever cases, ensuring early detection. * **Option C (Presumptive treatment):** In high-endemic areas, every fever case is treated as malaria until proven otherwise. **Presumptive Treatment (PT)** is administered immediately after taking a blood smear/RDT to reduce the parasite reservoir in the community without waiting for lab results. Since all these measures are standard protocols for managing high API areas, **Option D** is the correct answer. ### **High-Yield Clinical Pearls for NEET-PG** * **API Formula:** (Total confirmed cases / Total population under surveillance) × 1000. * **ABER (Annual Blood Examination Rate):** A measure of the efficiency of the surveillance system. It should ideally be **>10%**. * **SPR (Slide Positivity Rate):** (Total positive slides / Total slides examined) × 100. * **Elimination Target:** The National Framework for Malaria Elimination in India aims for a malaria-free India by **2030**, with all states achieving Category 1 (API < 1) status by 2027.

Question 335: Which of the following are the criteria for a disease to be suitable for screening?

A. It should be an important public health problem.
B. Facilities should be available for confirmation of diagnosis.
C. There should be a sufficiently long latent period for the disease.
D. All of the above. (Correct Answer)

Explanation: The concept of screening is based on the **Wilson and Jungner criteria**, which define the requirements for a disease to be suitable for a screening program. ### **Explanation of the Correct Answer** The correct answer is **D (All of the above)** because screening is only ethical and cost-effective when specific conditions are met: 1. **Important Public Health Problem (Option A):** The disease must have high prevalence or high morbidity/mortality. It is not justifiable to screen for rare or trivial conditions. 2. **Diagnostic Facilities (Option B):** Screening only identifies "suspected" cases. There must be an established "Gold Standard" test available to confirm the diagnosis and a treatment facility to manage the detected cases. 3. **Long Latent Period (Option C):** There must be a recognizable early or latent stage (the period between the first possible detection and the onset of symptoms). If a disease progresses too rapidly, screening will fail to catch it in time to alter the outcome. ### **Why Other Options are Included** In "All of the above" questions, each individual option represents a pillar of the screening criteria. If any one of these were missing—for example, if there were no treatment available—screening would be considered unethical as it would cause unnecessary anxiety without clinical benefit. ### **High-Yield NEET-PG Pearls** * **Wilson and Jungner Criteria:** The gold standard guidelines for screening (1968). * **Lead Time:** The period between early detection (by screening) and the time the disease would have been diagnosed due to symptoms. Screening aims to maximize this. * **Iceberg Phenomenon:** Screening is primarily used to detect the "submerged portion" of the iceberg (pre-symptomatic/latent cases). * **Ideal Screening Test:** Should be high in **Sensitivity** (to minimize false negatives) and have a high **Negative Predictive Value**. * **Yield:** The amount of previously undiagnosed disease that is detected as a result of the screening program.

Question 336: Which of the following is NOT a measure involved in sentinel surveillance?

A. Identifying missing cases in the notification of diseases
B. Identifying new cases of infection
C. Identifying old and new cases
D. Identifying cases free of disability (Correct Answer)

Explanation: **Explanation:** **Sentinel Surveillance** is a specialized monitoring system used to supplement passive surveillance by collecting high-quality data from a select group of "sentinel" sites (e.g., specific hospitals or clinics). Its primary purpose is to identify trends, estimate the total disease burden, and detect "missing cases" that routine notification systems might overlook. **Why Option D is Correct:** Sentinel surveillance focuses on the **occurrence and distribution of disease** (morbidity and mortality). Identifying "cases free of disability" is not an objective of this system. Disability-free status is typically measured through specialized surveys or quality-of-life indices (like DALE or QALY), rather than a surveillance mechanism designed to track the spread of infections or health events. **Analysis of Incorrect Options:** * **Option A (Identifying missing cases):** This is a core function. Sentinel surveillance acts as a "supplement" to capture cases missed by the routine (passive) notification system, helping to estimate the "tip of the iceberg." * **Option B & C (Identifying new/old cases):** Sentinel sites monitor both **Incidence** (new cases) and **Prevalence** (old and new cases) to determine if a disease is increasing or decreasing in a specific population. **High-Yield Pearls for NEET-PG:** * **Definition:** Monitoring of a specific rate in a limited network of sites to extrapolate data for the whole population. * **The "Iceberg Phenomenon":** Sentinel surveillance is the best method to estimate the "submerged portion" of the iceberg (hidden burden) in a community. * **Common Use:** Widely used in India for **HIV/AIDS** monitoring and tracking **Influenza** trends. * **Key Difference:** Unlike routine surveillance which aims for total coverage, sentinel surveillance prioritizes **data quality** over quantity by using specialized reporting units.

Question 337: Which of the following diseases is spread by direct contact?

A. Plague (Correct Answer)
B. Filaria
C. Malaria
D. All of the above

Explanation: **Explanation:** The correct answer is **Plague (Option A)**. In epidemiology, "direct contact" refers to the immediate transfer of an infectious agent from an infected host or reservoir to an appropriate portal of entry. While Plague is famously known for vector-borne transmission (via the rat flea, *Xenopsylla cheopis*), it is also classified as a disease spread by **direct contact**. This occurs through contact with the tissues or body fluids of an infected animal (e.g., skinning an infected rodent) or via direct person-to-person contact in the case of **Pneumonic Plague** (droplet spread is often categorized under direct transmission in standard epidemiological texts like Park’s PSM). **Why the other options are incorrect:** * **Filaria (Option B):** This is a vector-borne disease transmitted solely through the bite of an infected mosquito (primarily *Culex quinquefasciatus*). It requires an intermediate host for the extrinsic incubation period. * **Malaria (Option C):** This is a protozoal disease transmitted via the bite of an infected female *Anopheles* mosquito. It is a classic example of vector-borne (indirect) transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Direct Contact Diseases:** Other examples include STIs (Syphilis, Gonorrhea), AIDS, Leprosy, Scabies, and Anthrax (cutaneous). * **Plague Vectors:** The most efficient vector is *Xenopsylla cheopis* (Rat flea). The "Flea Index" is a critical surveillance tool in plague control. * **Pneumonic Plague:** This is the most virulent form and the only one that spreads directly from person to person, leading to potential outbreaks without the need for flea vectors. * **Drug of Choice:** Streptomycin is traditionally the drug of choice for Plague, though Doxycycline is also highly effective.

Question 338: What is the primary purpose of a randomized controlled trial?

A. To minimize selection bias (Correct Answer)
B. To ensure trials are inexpensive to run
C. To confirm that most studies start with a null hypothesis
D. To eliminate the possibility of false negatives and false positives

Explanation: ### Explanation **Why Option A is Correct:** The hallmark of a Randomized Controlled Trial (RCT) is **Randomization**. The primary purpose of randomization is to ensure that every participant has an equal chance of being assigned to either the intervention or the control group. This process **minimizes selection bias** and ensures that both known and unknown **confounders** are distributed equally between the groups. By creating comparable groups at the baseline, any observed difference in outcome can be confidently attributed to the intervention rather than pre-existing differences in participant characteristics. **Why Other Options are Incorrect:** * **Option B:** RCTs are generally the most expensive and time-consuming study designs due to the need for rigorous monitoring, follow-up, and administrative oversight. * **Option C:** While most studies do start with a null hypothesis, this is a principle of statistical inference (Hypothesis Testing) applicable to all analytical studies, not the specific "primary purpose" of an RCT. * **Option D:** No study design can completely eliminate Type I (false positive) or Type II (false negative) errors. These are managed through sample size calculations (Power) and p-value thresholds, but they are never entirely eradicated. **High-Yield NEET-PG Pearls:** * **Gold Standard:** RCT is the "Gold Standard" for establishing **causality** and evaluating the efficacy of new drugs. * **Blinding:** While randomization eliminates selection bias, **blinding** (Masking) is used to eliminate **ascertainment/observer bias**. * **Allocation Concealment:** This is the technique used to implement randomization (e.g., sealed envelopes) to prevent the researcher from knowing the next assignment, further safeguarding against selection bias. * **Hierarchy of Evidence:** Systematic reviews/Meta-analyses of RCTs sit at the top of the evidence pyramid, followed by individual RCTs.

Question 339: What is true about the Pearl Index?

A. It is a population study.
B. It is related to family planning.
C. It evaluates contraceptive methods.
D. It measures contraceptive failure. (Correct Answer)

Explanation: The **Pearl Index** is the standard metric used in clinical trials and epidemiology to evaluate the **effectiveness and failure rate** of a contraceptive method. ### **Explanation of the Correct Answer** The Pearl Index measures the number of unintended pregnancies per **100 woman-years** of exposure. It specifically quantifies **contraceptive failure**. A lower Pearl Index indicates a more effective contraceptive method (e.g., Implants have a lower index than Condoms). * **Formula:** $\text{Pearl Index} = \frac{\text{Total Accidental Pregnancies} \times 1200}{\text{Total Months of Exposure}}$ ### **Analysis of Incorrect Options** * **Option A:** It is not a broad population study. It is a specific clinical calculation used in longitudinal studies of reproductive-age women using specific interventions. * **Option B & C:** While the Pearl Index is *related* to family planning and is used to *evaluate* methods, these options are too broad. In competitive exams like NEET-PG, you must choose the **most specific** definition. The primary purpose of the index is specifically to measure the **failure rate**, making Option D the most accurate choice. ### **High-Yield Clinical Pearls for NEET-PG** * **Denominator:** The standard denominator is **100 woman-years** (equivalent to 1,200 months or 1,300 lunar cycles of exposure). * **Life Table Analysis:** This is an alternative to the Pearl Index. Unlike the Pearl Index, which gives a single average rate, Life Table Analysis calculates failure rates at specific intervals (e.g., at 6 months, 12 months), accounting for "drop-outs." * **Typical vs. Perfect Use:** The Pearl Index varies significantly between "perfect use" (theoretical efficacy) and "typical use" (user error included). * **Lowest Pearl Index:** Currently attributed to the **Subdermal Progestogen Implant** (approx. 0.05).

Question 340: To test the association between a risk factor and a disease, which of the following is the weakest study design?

A. Case-control study
B. Ecological study (Correct Answer)
C. Cohort study
D. Cross-sectional study

Explanation: To test the association between a risk factor and a disease, the strength of evidence depends on the unit of observation and the ability to establish a temporal relationship. ### **Why Ecological Study is the Correct Answer** An **Ecological Study** is considered the weakest design for testing associations because the unit of study is a **population or group** (e.g., a country or city) rather than an individual. It uses aggregate data to look for correlations. The primary limitation is the **Ecological Fallacy**, where an association observed at the group level may not necessarily apply to individuals within that group. Because it cannot link exposure to outcome in specific people, it is primarily used for hypothesis generation rather than testing. ### **Why Other Options are Incorrect** * **Cohort Study (Option C):** This is the strongest observational design. It starts with exposed and non-exposed individuals and follows them forward in time to see who develops the disease, allowing for the direct calculation of **Relative Risk**. * **Case-Control Study (Option A):** Stronger than ecological studies because it compares individuals with the disease (cases) to those without (controls) to look for past exposures. It uses the **Odds Ratio** to estimate association. * **Cross-sectional Study (Option B):** While it only provides a "snapshot" of prevalence and cannot establish temporality, it still collects data at the **individual level**, making it more reliable than an ecological study for suggesting associations. ### **NEET-PG High-Yield Pearls** * **Hierarchy of Evidence (Descending order):** Meta-analysis > Systematic Review > RCT > Cohort > Case-Control > Cross-sectional > Ecological > Case Series/Report. * **Ecological Fallacy:** The major drawback of ecological studies (e.g., assuming high fat intake causes heart disease in a country just because that country has high rates of both). * **Unit of Study:** In Ecological studies, it is the **Population**; in all other options, it is the **Individual**.

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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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