Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1191: In a malaria survey, every 1st and 4th house was chosen for the survey. What type of sampling was used?

A. Cluster sampling
B. Systematic random sampling (Correct Answer)
C. Stratified random sampling
D. Multistage sampling

Explanation: ### Explanation **Why Systematic Random Sampling is Correct:** Systematic random sampling involves selecting units from a population at a **fixed, periodic interval** (the sampling interval, ‘k’). In this scenario, the survey follows a predetermined pattern (every 1st and 4th house). This "kth" unit selection is the hallmark of systematic sampling. It is frequently used in field surveys (like malaria or filaria mapping) because it is simpler to implement than simple random sampling while ensuring the sample is spread evenly across the study area. **Analysis of Incorrect Options:** * **A. Cluster Sampling:** This involves dividing the population into groups (clusters), usually based on geography (e.g., villages), and selecting entire clusters at random. Here, individual houses are selected based on a sequence, not entire blocks or villages. * **C. Stratified Random Sampling:** This is used when the population is heterogeneous. The population is divided into homogenous "strata" (e.g., by age, gender, or socio-economic status), and samples are drawn from each. This scenario does not mention grouping by characteristics. * **D. Multistage Sampling:** This involves multiple levels of sampling (e.g., selecting districts, then villages, then households). While common in large surveys, the specific method described here (fixed interval) refers specifically to the systematic technique. **High-Yield Clinical Pearls for NEET-PG:** * **Sampling Interval (k):** Calculated as $N/n$ (Total Population / Sample Size). * **WHO EPI Cluster Sampling:** Used for immunization coverage. It traditionally uses a **30 x 7 design** (30 clusters, 7 children per cluster). * **Best for Heterogeneous Population:** Stratified Random Sampling. * **Most Representative of General Population:** Stratified Random Sampling. * **Sampling Error:** Is absent in a complete Census; it is highest in Cluster sampling and lowest in Stratified sampling.

Question 1192: A diagnostic test is considered highly sensitive. What does this imply?

A. A low rate of false positives
B. A low rate of false negatives (Correct Answer)
C. A low rate of true negatives
D. A low rate of true positives

Explanation: ### Explanation **1. Why Option B is Correct:** Sensitivity is defined as the ability of a test to correctly identify those **with the disease** (True Positive Rate). Mathematically, it is calculated as: $$\text{Sensitivity} = \frac{\text{True Positives (TP)}}{\text{True Positives (TP)} + \text{False Negatives (FN)}}$$ A highly sensitive test is excellent at "ruling out" a disease. If a test is 100% sensitive, it will catch every person who has the disease, meaning there will be **zero false negatives**. Therefore, a high sensitivity directly implies a very low rate of false negatives. **2. Why Other Options are Incorrect:** * **Option A:** A low rate of false positives is a characteristic of **Specificity**. Specificity measures the ability of a test to correctly identify those without the disease. * **Option C:** A low rate of true negatives would imply poor specificity, meaning the test is bad at identifying healthy individuals. * **Option D:** A low rate of true positives would imply a very low sensitivity, making the test clinically useless for screening. **3. NEET-PG High-Yield Clinical Pearls:** * **SNOUT Mnemonic:** **S**ensitivity rules **OUT** (A negative result in a highly sensitive test reliably rules out the disease). * **SPIN Mnemonic:** **S**pecificity rules **IN** (A positive result in a highly specific test reliably rules in the disease). * **Screening vs. Diagnosis:** Highly **sensitive** tests are preferred for **screening** (e.g., ELISA for HIV) to ensure no cases are missed. Highly **specific** tests are used for **confirmation** (e.g., Western Blot for HIV) to avoid unnecessary treatment of healthy people. * **Inverse Relationship:** As you decrease the "cut-off" point to increase sensitivity, specificity typically decreases, and vice versa.

Question 1193: In a case-control study of buccal carcinoma, what association is demonstrated?

A. Carcinoma is commoner in zarda pan users than non-users.
B. Zarda pan is a cause of buccal carcinoma.
C. Zarda pan is associated with buccal carcinoma. (Correct Answer)
D. If the use of zarda pan is stopped, the number of cases will reduce.

Explanation: ### Explanation **1. Why Option C is Correct:** The primary objective of a **Case-Control Study** is to identify an **association** between an exposure (Zarda pan) and an outcome (Buccal carcinoma). In this study design, we start with the disease and look backward (retrospective) to determine the frequency of exposure. While it can demonstrate a statistical relationship and calculate the **Odds Ratio**, it cannot definitively prove causation or absolute risk. **2. Why Other Options are Incorrect:** * **Option A:** This describes **Prevalence** or **Incidence**. Case-control studies do not provide the frequency of a disease in a population; they only compare the proportion of exposure among cases versus controls. * **Option B:** This implies **Causality**. A case-control study is the first step in testing a hypothesis but is insufficient to establish a "cause-effect" relationship. Stronger evidence from Cohort studies or Randomized Controlled Trials (RCTs), satisfying Bradford Hill’s criteria, is required to claim causation. * **Option D:** This refers to **Attributable Risk** or **Preventable Fraction**, which can only be calculated from Cohort studies where the incidence of the disease is known. **3. High-Yield Clinical Pearls for NEET-PG:** * **Direction of Study:** Retrospective (Effect to Cause). * **Measure of Association:** **Odds Ratio (OR)** is the only measure of association calculated in case-control studies. * **Key Feature:** It is ideal for **rare diseases** or diseases with long latency periods. * **Bias:** Highly susceptible to **Recall Bias** and **Selection Bias** (Berkson’s Bias). * **Matching:** Done in case-control studies to eliminate the effects of **confounding variables**.

Question 1194: Which statement is true regarding Kyasanur Forest Disease (KFD)?

A. Transmitted by ticks
B. It is an arboviral infection
C. Also known as the 'monkey disease'
D. All of the above. (Correct Answer)

Explanation: **Explanation:** Kyasanur Forest Disease (KFD) is a high-yield topic in NEET-PG, representing a classic example of a viral zoonotic disease endemic to India. 1. **Arboviral Infection:** KFD is caused by the Kyasanur Forest Disease Virus (KFDV), a member of the family *Flaviviridae*. As it is transmitted by arthropod vectors, it is classified as an arbovirus. 2. **Vector (Ticks):** The primary vector is the hard tick, specifically ***Haemaphysalis spinigera***. Humans usually contract the infection through the bite of an infected nymphal tick or via contact with an infected animal. 3. **'Monkey Disease':** It is colloquially known as "Monkey Disease" because it causes significant epizootics (outbreaks) among black-faced langurs and bonnet macaques. In endemic areas, the sudden death of monkeys is often the first "sentinel" sign of an impending human outbreak. Since all three statements are accurate descriptions of the disease's etiology, transmission, and nomenclature, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Geography:** First identified in 1957 in the Shimoga district of **Karnataka**. * **Reservoirs:** Rodents, shrews, and monkeys. Humans are **dead-end hosts**. * **Clinical Features:** Characterized by sudden onset chills, high fever, frontal headache, and severe myalgia. A biphasic illness may occur, leading to hemorrhagic manifestations or neurological complications. * **Prevention:** A **formalin-inactivated vaccine** is used in endemic areas (given in two doses at a 1-month interval, followed by boosters).

Question 1195: In a population of 500,000 people, there were 100 deaths in a year. Out of these 100 deaths, 10 individuals died due to pulmonary TB. What is the proportional mortality rate from TB?

A. 10% (Correct Answer)
B. 50%
C. 75%
D. None of the above

Explanation: ### Explanation **1. Why Option A is Correct:** Proportional Mortality Rate (PMR) is a measure of the relative importance of a specific cause of death within a population. It expresses the number of deaths due to a particular cause as a percentage of the **total deaths** from all causes in that same period. The formula is: $$\text{Proportional Mortality Rate} = \frac{\text{Number of deaths from a specific disease}}{\text{Total deaths from all causes}} \times 100$$ In this scenario: * Deaths from TB = 10 * Total deaths = 100 * Calculation: $(10 / 100) \times 100 = \mathbf{10\%}$. **2. Why Other Options are Incorrect:** * **Option B (50%) and C (75%):** These are mathematically incorrect based on the provided data. They do not represent any standard epidemiological metric derived from the numbers given. * **Note on Population Size:** The total population (500,000) is a "distractor" in this question. If you used the total population as the denominator, you would be calculating the **Cause-Specific Mortality Rate**, not the Proportional Mortality Rate. **3. High-Yield Clinical Pearls for NEET-PG:** * **PMR vs. Case Fatality Rate (CFR):** While PMR uses total deaths as the denominator, CFR uses the total number of *cases* of that disease. CFR indicates the killing power of a disease. * **PMR vs. Cause-Specific Mortality Rate:** PMR uses "Total Deaths" as the denominator, whereas Cause-Specific Mortality Rate uses the "Total Mid-year Population." * **Utility:** PMR is useful when population data is unavailable. It indicates the "burden" of a disease as a cause of death but does not represent the risk of dying from the disease (which is measured by mortality rates). * **Common Trap:** Always check the denominator. If the question asks for "Proportional Mortality," ignore the total population and focus only on the total death count.

Question 1196: For the prevention of yellow fever, a single vaccination with the 17-D nonpathogenic strain of virus gives full protection to the individual for at least how many years?

A. 10 years (Correct Answer)
B. 12 years
C. 15 years
D. 18 years

Explanation: ### Explanation **Correct Option: A (10 years)** The Yellow Fever vaccine (17-D strain) is a live attenuated vaccine. According to the **International Health Regulations (IHR)**, the validity of the certificate of vaccination against yellow fever was historically set at **10 years**. While the WHO updated its position in 2014/2016 stating that a single dose confers life-long immunity, for the purpose of competitive exams like NEET-PG (which often follow standard textbook timelines and IHR certification rules), the conventional answer remains **10 years**. The protection starts 10 days after vaccination. **Analysis of Incorrect Options:** * **B, C, and D:** These timeframes (12, 15, and 18 years) do not correspond to any historical or current WHO guidelines or clinical trial milestones regarding the duration of the International Certificate of Vaccination or Prophylaxis (ICVP). **High-Yield Clinical Pearls for NEET-PG:** * **Strain:** 17-D strain (chick embryo derived). * **Route & Dose:** 0.5 ml, Subcutaneous (SC). * **Immunity Timeline:** Immunity begins on the **10th day** post-vaccination. If a person is revaccinated before the expiry of 10 years, immunity is considered continuous from the day of revaccination. * **Validity:** For international travel, the certificate becomes valid 10 days after primary vaccination and lasts for the duration of the life of the person (as per updated WHO IHR), but the "10-year rule" is the classic exam answer. * **Contraindications:** Infants <6 months (risk of encephalitis), egg allergy, and immunocompromised individuals (e.g., symptomatic HIV, thymic disorders). * **Cold Chain:** Must be stored between **-15°C and +5°C**. It is highly heat-sensitive.

Question 1197: Which of the following is a killed vaccine?

A. Hepatitis A (Correct Answer)
B. Measles
C. Oral Polio Vaccine (OPV)
D. Bacillus Calmette-Guérin (BCG)

Explanation: **Explanation:** The classification of vaccines into Live Attenuated, Killed (Inactivated), and Subunit types is a high-yield topic for NEET-PG. **1. Why Hepatitis A is correct:** The Hepatitis A vaccine is a **killed (inactivated) vaccine**. It is prepared by growing the virus in cell culture and subsequently inactivating it using chemicals like formaldehyde. Because the virus is dead, it cannot replicate or cause disease, making it safe for immunocompromised individuals, though it typically requires multiple doses (booster shots) to maintain long-term immunity. **2. Why the other options are incorrect:** * **Measles (Option B):** This is a **Live Attenuated Vaccine**. It contains a weakened form of the virus that replicates in the host to induce a strong immune response. * **Oral Polio Vaccine (OPV/Sabin) (Option C):** This is a **Live Attenuated Vaccine**. In contrast, the Injectable Polio Vaccine (IPV/Salk) is a killed vaccine. * **BCG (Option D):** This is a **Live Attenuated Bacterial Vaccine** derived from *Mycobacterium bovis*. It is the only live bacterial vaccine commonly used in the National Immunization Schedule. **3. Clinical Pearls for NEET-PG:** * **Mnemonic for Killed Vaccines:** "**K**illed **P**olio (**S**alk), **R**abies, **I**nfluenza, **H**epatitis **A**, **P**ertussis" (Remember: **K**atty **P**erry **R**ides **I**n **H**er **P**orsche). * **Mnemonic for Live Vaccines:** "**BOY** **R**omes **M**y **C**hicken **I**s **V**ery **T**asty" (**B**CG, **O**PV, **Y**ellow Fever, **R**otavirus, **M**easles/Mumps/MR, **C**hicken Pox, **I**ntranasal Influenza, **V**aricella, **T**yphoid [Ty21a]). * **Key Distinction:** Live vaccines are generally contraindicated in pregnancy and severe immunodeficiency (except HIV patients before they reach the AIDS stage).

Question 1198: Selection bias in a research study can be eliminated by which of the following procedures?

A. Randomization (Correct Answer)
B. Single blinding
C. Double blinding
D. Matching

Explanation: ### Explanation **Selection bias** occurs when there is a systematic difference between the characteristics of the people selected for a study and those who are not, or between the study groups themselves. **Why Randomization is the Correct Answer:** Randomization is the "heart" of the Randomized Controlled Trial (RCT). It ensures that every participant has an equal chance of being assigned to either the study or control group. By doing so, it eliminates selection bias by distributing both **known and unknown confounders** equally between the groups. This makes the groups comparable at the baseline, ensuring that any difference in outcome is due to the intervention and not pre-existing differences. **Why the Other Options are Incorrect:** * **Blinding (Single and Double):** Blinding is primarily used to eliminate **Measurement (Information) bias** and **Observer bias**. While it prevents participants or investigators from knowing the treatment assignment, it does not influence how participants were selected or assigned initially. * **Matching:** Matching is a technique used in Case-Control studies to eliminate **Confounding**. While it makes groups similar regarding specific known variables (like age or sex), it can actually lead to selection bias if not done carefully and cannot account for unknown confounders. **High-Yield Clinical Pearls for NEET-PG:** * **Randomization** = Eliminates Selection Bias + Controls Confounding (Known & Unknown). * **Blinding** = Eliminates Observation/Information Bias. * **Matching** = Controls known Confounders (used in Case-Control). * **Restriction** = Limiting study entry to a narrow group to control confounding. * **Confounding** is often called a "Hidden Bias," but it is technically a source of error distinct from systematic selection bias.

Question 1199: Which type of study design allows for the investigation of exposure in relation to multiple outcomes?

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

Explanation: ### Explanation **Why Cohort Study is the Correct Answer:** A **Cohort study** is an observational, longitudinal study that starts with a group of individuals (the cohort) who are currently free of the disease but differ in their exposure to a specific factor. Because the study follows these individuals forward in time (prospective) or reconstructs their history (retrospective) from exposure to outcome, it allows researchers to observe the development of **multiple different diseases or outcomes** resulting from a single exposure. For example, a cohort of smokers can be monitored to study the incidence of lung cancer, COPD, and coronary artery disease simultaneously. **Why the Other Options are Incorrect:** * **B. Case-control study:** This design starts with the "outcome" (cases) and looks backward to identify exposures. It is ideal for studying **multiple exposures** for a single outcome, but not multiple outcomes. * **C. Cross-sectional study:** This provides a "snapshot" of a population at a single point in time. It measures prevalence rather than incidence and is generally used to generate hypotheses rather than establish temporal relationships for multiple outcomes. * **D. Ecological study:** This uses populations or groups as the unit of analysis rather than individuals. It is used to look for correlations at a macro level and is prone to "ecological fallacy." **NEET-PG High-Yield Pearls:** * **Cohort Study:** Best for rare exposures; can calculate **Incidence, Relative Risk (RR), and Attributable Risk (AR).** * **Case-Control Study:** Best for rare diseases; can calculate **Odds Ratio (OR).** * **Mnemonic:** * **C**ase-Control = Multiple **E**xposures (**C-E**) * **C**ohort = Multiple **O**utcomes (**C-O**) * The most common bias in Cohort studies is **Loss to follow-up (Attrition bias)**, whereas in Case-control studies, it is **Recall bias.**

Question 1200: A false positive in a screening test is mostly due to what characteristic?

A. High specificity
B. High sensitivity
C. High prevalence
D. Low prevalence (Correct Answer)

Explanation: ### Explanation The correct answer is **Low prevalence (Option D)**. The relationship between screening test results and disease frequency is governed by **Bayes' Theorem**. While sensitivity and specificity are inherent properties of a test, the **Positive Predictive Value (PPV)**—the probability that a person with a positive test actually has the disease—is heavily dependent on the **prevalence** of the disease in the population. When the prevalence of a disease is low, the vast majority of the population is healthy (true negatives). Even a highly specific test will produce a small percentage of false positives. Because the actual number of diseased individuals is so small, these few false positives will outnumber the true positives, leading to a low PPV. Therefore, in a low-prevalence setting, a positive result is more likely to be a **false positive**. #### Analysis of Incorrect Options: * **A. High specificity:** High specificity means the test is good at identifying those *without* the disease. Increasing specificity actually **decreases** the number of false positives. * **B. High sensitivity:** High sensitivity ensures that most diseased individuals are caught (fewer false negatives). It does not directly cause false positives; rather, it is the "trade-off" decrease in specificity that would do so. * **C. High prevalence:** In a high-prevalence population, a positive test is much more likely to be a **true positive**, thereby increasing the PPV and decreasing the proportion of false positives. #### NEET-PG High-Yield Pearls: * **Prevalence vs. Predictive Value:** Prevalence is directly proportional to PPV and inversely proportional to NPV (Negative Predictive Value). * **Screening Strategy:** To minimize false positives, screening should be targeted at **high-risk groups** (increasing the effective prevalence). * **Fixed Properties:** Sensitivity and Specificity do **not** change with prevalence; they are independent of the population being tested. * **Formula:** $PPV = \frac{\text{True Positives}}{\text{True Positives} + \text{False Positives}}$

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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