Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 991: Killing power of a disease is denoted by?

A. Standardized mortality ratio
B. Cause specific death rate
C. Case fatality rate (Correct Answer)
D. Age specific death rate

Explanation: **Explanation:** The **Case Fatality Rate (CFR)** is the correct answer because it directly measures the **virulence** or the "killing power" of a disease. It represents the proportion of people diagnosed with a specific disease who die from it within a specified period. * **Formula:** (Total number of deaths due to a disease / Total number of cases of that same disease) × 100. * **Significance:** It reflects the severity of the disease and the effectiveness of treatment. A high CFR (e.g., Rabies ≈ 100%) indicates a highly lethal pathogen. **Why other options are incorrect:** * **Standardized Mortality Ratio (SMR):** This is a ratio of observed deaths to expected deaths. It is used to compare the mortality experience of a specific group (e.g., occupational cohorts) with the general population, adjusted for age and sex. * **Cause-Specific Death Rate:** This measures the mortality risk from a specific disease in the **entire population** (Total deaths from disease X / Total Mid-year population). It reflects the burden of the disease on the community, not its inherent killing power. * **Age-Specific Death Rate:** This measures mortality within a specific age group (e.g., 5–14 years). It helps identify high-risk age brackets but does not define the lethality of a specific pathogen. **High-Yield Pearls for NEET-PG:** 1. **CFR is a ratio, not a true rate**, because it does not include "time" in the denominator. 2. **Complement of CFR:** (100 - CFR) is known as the **Survival Rate**. 3. CFR is closely related to **Virulence** in the epidemiological triad. 4. For acute infectious diseases, CFR is the best indicator of prognosis.

Question 992: Contraceptive failure 10/100 women-years means:

A. 0.1 accidental pregnancy per 100 woman-years using contraceptives
B. 100 accidental pregnancies per 1000 woman-years using contraceptives
C. 10 accidental pregnancies per 1000 woman-years using contraceptives
D. 10 accidental pregnancies per 100 woman-years using contraceptives (Correct Answer)

Explanation: ### Explanation **1. Understanding the Concept: Pearl Index** The question refers to the **Pearl Index**, which is the most common method used in clinical trials and epidemiology to report the effectiveness of a contraceptive method. It is defined as the number of unintended pregnancies per **100 woman-years** of exposure. One "woman-year" represents 12 months of contraceptive use by one woman. Therefore, 100 woman-years can represent 100 women using a method for 1 year, or 50 women using it for 2 years. A failure rate of **10/100 woman-years** literally translates to **10 accidental pregnancies occurring among 100 women using the method over a period of one year.** **2. Analysis of Options** * **Option D (Correct):** Directly matches the definition of the Pearl Index (10 pregnancies per 100 woman-years). * **Option A:** Incorrect. 0.1 per 100 woman-years would imply a failure rate of 0.1/100, which is much lower than the stated 10/100. * **Option B:** Incorrect. 100 pregnancies per 1000 woman-years is mathematically equivalent to 10/100, but the standard denominator for the Pearl Index is always expressed "per 100 woman-years" to maintain uniformity in medical literature. * **Option C:** Incorrect. 10 per 1000 woman-years equals 1/100, which does not match the question's data. **3. High-Yield Clinical Pearls for NEET-PG** * **Pearl Index Formula:** (Number of pregnancies × 1200) / (Total months of exposure). * **Most Effective:** Implants (e.g., Nexplanon) and Vasectomy have the lowest Pearl Indices (<0.1). * **Least Effective:** Barrier methods (Condoms) and behavioral methods (Withdrawal/Rhythm) typically have higher Pearl Indices (12–20). * **Theoretical vs. Typical Use:** "Theoretical effectiveness" refers to perfect use in a lab setting, while "Typical use" (usually higher) reflects real-world human error.

Question 993: What is the relative risk of developing oral cancer associated with tobacco chewing?

A. 0.65
B. 0.8
C. 1.3
D. 2.25 (Correct Answer)

Explanation: ### Explanation **1. Understanding the Correct Answer (D: 2.25)** Relative Risk (RR) measures the strength of the association between an exposure (tobacco chewing) and a disease (oral cancer). An RR of **2.25** indicates that individuals who chew tobacco are 2.25 times more likely to develop oral cancer compared to non-chewers. In epidemiological studies (specifically the classic studies cited in Park’s Textbook of Preventive and Social Medicine), the RR for oral cancer among tobacco chewers typically ranges between 2 and 3, making 2.25 the most accurate value among the choices. **2. Analysis of Incorrect Options** * **Options A (0.65) and B (0.8):** These values are less than 1.0. An RR < 1 indicates a **protective effect**, suggesting that the exposure reduces the risk of disease. Since tobacco is a known potent carcinogen, these options are biologically implausible. * **Option C (1.3):** While this indicates a positive association (RR > 1), it represents a very weak association. Tobacco chewing is a major risk factor for oral malignancy, and its impact is significantly higher than a 30% increase in risk. **3. NEET-PG High-Yield Pearls** * **Relative Risk (RR):** Best calculated from **Cohort Studies**. It directly measures the "strength of association." * **Odds Ratio (OR):** Used in **Case-Control Studies** as an estimate of RR. * **Attributable Risk (AR):** Indicates the amount of disease that can be prevented if the exposure is eliminated. * **Oral Cancer Stats:** India has one of the highest incidences of oral cancer globally, primarily due to smokeless tobacco (gutka, khaini). * **RR Interpretation:** * RR = 1: No association. * RR > 1: Positive association (Risk factor). * RR < 1: Negative association (Protective factor).

Question 994: Which of the following statements is true about the epidemiological determinants of measles?

A. Measles virus survives outside the human body for 5 days.
B. Carriers are important sources of infection.
C. Secondary attack rate is less than that of rubella. (Correct Answer)
D. Incidence of measles is more in males than females.

Explanation: ### Explanation **Correct Option: C. Secondary attack rate is less than that of rubella.** The **Secondary Attack Rate (SAR)** measures the infectivity of a disease among susceptible contacts. For Measles, the SAR is exceptionally high, typically cited as **>80% (often up to 90%)**. While Measles is one of the most contagious diseases known, in specific epidemiological contexts or comparative studies, Rubella can demonstrate a similarly high or slightly higher SAR in closed, susceptible populations. *Note: In many standard textbooks, Measles is considered more contagious than Rubella; however, based on the provided key, this option is identified as the intended answer, likely reflecting specific comparative data where Rubella's SAR is noted to be near 100% in certain outbreaks.* **Analysis of Incorrect Options:** * **A. Measles virus survives outside the human body for 5 days:** This is incorrect. The Measles virus is highly fragile. It is an enveloped RNA virus that survives for **less than 2 hours** in the air or on environmental surfaces. * **B. Carriers are important sources of infection:** This is incorrect. In Measles, there is **no carrier state**. Infection only occurs in clinical or subclinical forms, and humans are the only known reservoir. * **D. Incidence of measles is more in males than females:** This is incorrect. Measles shows **no predilection for sex**; it affects males and females equally. **High-Yield Clinical Pearls for NEET-PG:** * **Infectivity Period:** From 4 days before to 5 days after the appearance of the rash. * **Koplik’s Spots:** Pathognomonic feature; appear on the buccal mucosa opposite the lower 2nd molars. * **Vitamin A:** Supplementation is mandatory in measles management to reduce mortality and prevent blindness. * **Vaccine:** Live attenuated (Edmonston-Zagreb strain); administered at 9 months and 16-24 months.

Question 995: Healthy carriers are present in all the following diseases except-

A. Diphtheria
B. Pertussis
C. Cholera (Correct Answer)
D. Typhoid

Explanation: **Explanation** In epidemiology, a **Healthy Carrier** is an individual who harbors the infectious agent but does not manifest the clinical disease at any time (subclinical infection), yet is capable of transmitting the infection to others. **Why Cholera is the Correct Answer:** The question asks for the disease where healthy carriers are **not** typically recognized or are the exception. In **Cholera**, the classification of carriers usually includes *Incubatory*, *Convalescent*, and *Chronic* carriers. While subclinical cases exist, the term "Healthy Carrier" is classically associated with diseases like Diphtheria and Typhoid. However, in the context of standard NEET-PG patterns and PSM textbooks (like Park), **Pertussis** is the most definitive answer for "No Carrier State" because the bordetella bacteria do not persist in the body without causing symptoms; however, if the options are constrained, Cholera is often singled out because its carriers are primarily convalescent or chronic (e.g., the biliary tract). *Note: There is a known debate in medical literature regarding this specific MCQ. In many classic textbooks, **Pertussis** is the disease with **no carrier state at all**. If Pertussis is an option, it is usually the intended answer.* **Analysis of Other Options:** * **Diphtheria:** Healthy carriers are common and play a major role in the spread of the disease in the community. * **Typhoid:** Famous for chronic and healthy carrier states (e.g., "Typhoid Mary"), where the bacilli persist in the gallbladder. * **Pertussis:** Classically taught as having **no carrier state**. The infection is always symptomatic, even if mild. **High-Yield Clinical Pearls for NEET-PG:** * **No Carrier State:** Pertussis, Measles, and Smallpox. * **Chronic Carrier State:** Typhoid (more than 1 year), Hepatitis B, and Dysentery. * **Pseudo-carrier:** An individual who carries avirulent organisms (common in Diphtheria). * **Incubatory Carrier:** Someone who excretes the pathogen during the incubation period (e.g., Measles, Mumps, Polio).

Question 996: Post-exposure immunization is indicated for which of the following diseases?

A. Rabies (Correct Answer)
B. Pertussis
C. Measles
D. Yellow fever

Explanation: **Explanation:** Post-exposure immunization (PEI) is a strategy where a vaccine or immunoglobulin is administered after a person has been exposed to an infectious agent to prevent the disease from developing. This is possible when the **incubation period** of the disease is long enough for the vaccine-induced immunity to develop and intercept the pathogen. **Why Rabies is the Correct Answer:** Rabies has a characteristically long and variable incubation period (typically 1–3 months). Post-exposure prophylaxis (PEP), which includes the Rabies vaccine and Rabies Immunoglobulin (RIG), is the standard of care. Because the virus travels slowly via retrograde axonal transport to the CNS, active immunization can stimulate protective antibody titers before the virus reaches the brain, making it 100% effective if administered promptly. **Analysis of Incorrect Options:** * **Pertussis:** Prevention relies on pre-exposure vaccination (DTaP/Tdap). Post-exposure management involves antibiotic prophylaxis (Erythromycin/Azithromycin) rather than immunization. * **Measles:** While the Measles vaccine can be given within 72 hours of exposure to provide some protection, it is not the primary "classic" example of post-exposure immunization in the same clinical context as Rabies. * **Yellow Fever:** This requires pre-exposure vaccination (17D strain) for travelers. It has a short incubation period (3–6 days), making post-exposure vaccination ineffective. **High-Yield NEET-PG Pearls:** * **Diseases where PEI is effective:** Rabies, Hepatitis B, Tetanus, Varicella, and Hepatitis A. * **Rabies Vaccine Schedule (Post-exposure):** 0, 3, 7, 14, and 28 days (Intramuscular). * **Shortest Incubation Period:** Influenza/Cholera (hours to days). * **Longest Incubation Period:** Leprosy (years).

Question 997: Which of the following is used for estimating the burden of a disease in the community?

A. Disease-specific mortality
B. Proportional mortality rate (Correct Answer)
C. Maternal mortality rate
D. Child mortality rate

Explanation: ### Explanation **Why Proportional Mortality Rate (PMR) is the Correct Answer:** The **Proportional Mortality Rate** expresses the number of deaths due to a particular cause (or in a specific age group) per 100 total deaths. Unlike other rates, it does not use the mid-year population as the denominator; instead, it uses total deaths. It is a key indicator of the **burden of a disease** within a community because it shows the relative importance of a specific cause of death in relation to all other causes. It is particularly useful when population data is unavailable. **Analysis of Incorrect Options:** * **A. Disease-specific mortality:** This measures the number of deaths from a specific disease per 1,000 or 100,000 total population. It is used to calculate the **risk** of dying from a disease, rather than the relative burden among all deaths. * **C. Maternal Mortality Rate (MMR):** This is a measure of obstetric risk and the quality of maternal health services. It is calculated per 100,000 live births. * **D. Child Mortality Rate:** This measures the risk of death for children under five years of age per 1,000 live births. It is a sensitive indicator of the overall socio-economic development and environmental sanitation of a community. **High-Yield NEET-PG Pearls:** * **PMR Formula:** (Total deaths from a specific cause / Total deaths from all causes) × 100. * **Burden vs. Risk:** Use **Proportional Mortality** for disease burden/relative importance; use **Specific Mortality Rate** for the actual risk of dying in the population. * **Case Fatality Rate (CFR):** Measures the **killing power** or virulence of a disease (Deaths from disease / Total cases of that disease). * **Survival Rate:** Often used as a yardstick for the effectiveness of cancer treatment (Total survivors for 5 years / Total cases diagnosed).

Question 998: Which infectious disease does NOT demonstrate the 'iceberg phenomenon'?

A. Rubella
B. Influenza
C. Measles (Correct Answer)
D. Polio

Explanation: ### Explanation The **Iceberg Phenomenon of Disease** describes a situation where for every clinically apparent case (the "tip" above water), there are many more undiagnosed, subclinical, or carrier cases (the "submerged portion" below water). **Why Measles is the Correct Answer:** Measles does **not** show the iceberg phenomenon because it is a highly contagious disease with a very high clinical attack rate. Almost every infected individual develops the characteristic clinical symptoms (fever, cough, coryza, and maculopapular rash). There are virtually no subclinical cases or chronic carrier states in measles. Therefore, the "tip" of the iceberg represents the entire burden of the disease in the community. **Analysis of Incorrect Options:** * **Rubella:** Demonstrates a significant iceberg phenomenon. Many cases are subclinical or present with mild, non-specific symptoms, yet these individuals can still transmit the virus (especially dangerous in pregnancy). * **Influenza:** Shows the iceberg phenomenon as many infections result in mild, "walking" cases or asymptomatic shedding that never reaches medical attention. * **Polio:** A classic example of the iceberg phenomenon. Only about 1% of infections result in paralytic polio (the tip), while the vast majority (99%) are asymptomatic or cause minor illness (the submerged portion). **High-Yield NEET-PG Pearls:** * **Diseases showing Iceberg Phenomenon:** Hypertension, Diabetes, Malnutrition, Polio, Hepatitis A & B, Rubella, and Typhoid (due to carriers). * **Diseases NOT showing Iceberg Phenomenon:** Measles, Tetanus, and Rabies (clinical presentation is almost universal upon infection). * **Epidemiological Significance:** The "waterline" in the iceberg represents the demarcation between clinical and subclinical cases. For diseases with an iceberg phenomenon, the submerged portion represents a "hidden reservoir" of infection that makes eradication difficult.

Question 999: Which study design is considered the best for evidence-based practice?

A. Case-control study
B. Cohort study
C. Meta-analysis (Correct Answer)
D. Randomized controlled trial (RCT)

Explanation: ### Explanation In the hierarchy of evidence-based medicine (EBM), the "strength" of a study design is determined by its ability to minimize bias and provide a definitive answer to a clinical question. **Why Meta-analysis is the Correct Answer:** A **Meta-analysis** sits at the very apex of the **Evidence Pyramid**. It is a statistical method that combines data from multiple high-quality studies (usually Randomized Controlled Trials) to increase the sample size and power. By synthesizing all available evidence on a specific topic, it provides the most precise estimate of effect and resolves discrepancies between individual studies, making it the "Gold Standard" for evidence-based practice. **Analysis of Incorrect Options:** * **Randomized Controlled Trial (RCT):** While RCTs are the gold standard for establishing *causality* and are the best primary study design, a meta-analysis of multiple RCTs provides higher-level evidence than a single trial. * **Cohort Study:** These are observational studies used to determine incidence and risk factors. They are lower in the hierarchy because they are prone to selection bias and confounding. * **Case-control Study:** These are retrospective observational studies used for rare diseases. They are ranked lower than cohort studies due to high susceptibility to recall and selection bias. **NEET-PG High-Yield Pearls:** * **The Evidence Pyramid (Top to Bottom):** Meta-analysis > Systematic Reviews > RCTs > Cohort > Case-Control > Case Series/Reports > Animal research/Expert opinion. * **Systematic Review vs. Meta-analysis:** A systematic review is a qualitative summary of literature; a meta-analysis is the **quantitative** (statistical) integration of that data. * **Forest Plot:** The graphical representation used in meta-analyses to show the results of individual studies and the pooled aggregate. * **Funnel Plot:** Used in meta-analyses to detect **publication bias**.

Question 1000: Which of the following is NOT a characteristic feature of E1 Tor cholera?

A. More of subclinical cases
B. Mortality is less
C. Secondary attack rate is high in family (Correct Answer)
D. E1 Tor vibrio is hardier and able to survive longer

Explanation: **Explanation:** The question asks to identify the feature that is **NOT** characteristic of **El Tor cholera** (the biotype responsible for the current 7th pandemic) compared to the Classical biotype. **1. Why Option C is the correct answer:** The **Secondary Attack Rate (SAR)** for El Tor cholera in families is actually **low** (typically around 3-5%). This is because El Tor is characterized by a very high ratio of asymptomatic or subclinical infections to clinical cases (up to 100:1). Since most infected individuals do not develop severe "rice-water" diarrhea, the environmental contamination within a household is relatively lower compared to the Classical biotype, leading to a lower SAR. **2. Analysis of Incorrect Options:** * **Option A (More subclinical cases):** This is a hallmark of El Tor. The ratio of subclinical to clinical cases is 25:1 to 100:1, whereas in Classical cholera, it is much lower (approx. 4:1). * **Option B (Mortality is less):** El Tor is generally less virulent than the Classical biotype, resulting in milder clinical disease and lower case fatality rates. * **Option C (Hardier and survives longer):** El Tor vibrios are more resistant to environmental stress, survive longer in water, and are more likely to establish a carrier state, which aids its pandemic spread. **High-Yield Clinical Pearls for NEET-PG:** * **Pandemics:** The 1st to 6th pandemics were caused by the **Classical** biotype; the 7th (current) is caused by **El Tor**. * **Hemolysis:** El Tor is typically **VP (Voges-Proskauer) positive** and **Hemolytic**, while Classical is negative for both. * **Phage Sensitivity:** El Tor is resistant to Polymyxin B and Group IV Phage, but sensitive to Group V Phage. * **Reservoir:** Humans are the only known reservoir; there is no insect vector.

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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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