Epidemiology Practice Questions

Q: A suspected cause preceding an observed effect is an example of which criterion for causality?

Temporal association. ### Explanation **1. Why Temporal Association is Correct:** Temporal association is the most fundamental and indispensable criterion among **Bradford Hill’s criteria** for causality. It states that for a factor to be considered a cause, it must precede the occurrence of the disease (the effect). In this question, the "suspected cause preceding an observed effect" directly describes this chronological requirement. Without establishing that the exposure occurred before the outcome, a causal relationship cannot be validated. **2. Why Other Options are Incorrect:** * **Consistency of Association:** This refers to the repeated observation of the association in different populations, under different circumstances, and by different investigators (reproducibility). * **Strength of Association:** This is measured by the magnitude of the Relative Risk (RR) or Odds Ratio (OR). A stronger association (e.g., RR > 10) is more likely to be causal than a weak one. * **Coherence of Association:** This implies that the cause-and-effect interpretation of the data should not seriously conflict with the generally known facts of the natural history and biology of the disease. **3. High-Yield Clinical Pearls for NEET-PG:** * **Bradford Hill Criteria:** There are 9 criteria (Strength, Consistency, Specificity, Temporality, Biological Gradient, Plausibility, Coherence, Experiment, and Analogy). * **The "Must-Have":** Temporality is the only criterion that is **absolutely essential**. * **Dose-Response Relationship:** Also known as the **Biological Gradient**; as the dose of exposure increases, the risk of disease increases (e.g., more cigarettes smoked per day leads to higher lung cancer risk). * **Study Design & Temporality:** Prospective Cohort studies are the best observational designs to establish temporality, whereas Cross-sectional studies are the weakest because they measure exposure and outcome simultaneously.

Q: Which vaccine has the highest efficacy?

DPT. **Explanation:** The efficacy of a vaccine refers to its ability to prevent disease under ideal, controlled conditions. In the context of NEET-PG, vaccine efficacy is a high-yield topic often categorized by the percentage of protection provided. **Why DPT is the Correct Answer:** The **DPT (Diphtheria, Pertussis, and Tetanus)** vaccine, specifically the Tetanus toxoid component, is considered one of the most efficacious vaccines available. Tetanus toxoid has an efficacy of nearly **100%** after a primary series. Diphtheria toxoid also boasts high efficacy (over 95%). When compared as a combined unit or by its strongest component (Tetanus), DPT ranks higher in protective efficacy than live viral or oral bacterial vaccines. **Analysis of Incorrect Options:** * **Measles:** This is a live-attenuated vaccine. While highly effective, its efficacy is approximately **95%** when given at 9 months and increases to 99% only after the second dose. * **Oral Typhoid (Ty21a):** This vaccine has a relatively lower efficacy, ranging from **50% to 80%**. It requires multiple doses and periodic boosters. * **Tetanus:** While Tetanus alone has ~100% efficacy, in the context of this specific standard MCQ, **DPT** is often the preferred collective answer in community medicine textbooks (like Park’s) when listed alongside these specific options. *Note: If DPT were not an option, Tetanus would be the standalone winner.* **High-Yield NEET-PG Pearls:** * **Highest Efficacy:** Tetanus toxoid (~100%), Measles (95-99%), Yellow Fever (>99%). * **Lowest Efficacy:** BCG (highly variable, 0-80%) and Cholera vaccine (~50%). * **Cold Chain Sensitivity:** Polio (OPV) is the most heat-sensitive; Tetanus/DPT are the most heat-stable but are highly sensitive to freezing (freeze-sensitive). * **Herd Immunity:** Measles requires the highest herd immunity threshold (~94-95%) due to its high $R_0$.

Q: A Chandler's index of 312 is considered as:

Important public health problem. **Explanation:** **Chandler’s Index** is a classic epidemiological tool used to measure the prevalence and intensity of **Hookworm infection** (*Ancylostoma duodenale* and *Necator americanus*) in a community. It is calculated by taking the average number of eggs per gram (EPG) of stool from a representative sample of the population. **Why Option D is Correct:** The severity of the public health problem is categorized based on the numerical value of the index: * ** 500:** Severe public health problem (Potential danger). Since the value **312** falls within the 250–500 range, it is classified as an **important public health problem**. **Analysis of Incorrect Options:** * **Option A:** An index 500 is considered a potential danger or severe problem. **High-Yield Clinical Pearls for NEET-PG:** * **Hookworm and Anemia:** Hookworm is a leading cause of Iron Deficiency Anemia in India. *A. duodenale* causes more blood loss (0.2 ml/day) than *N. americanus* (0.03 ml/day). * **Other Indices:** While Chandler’s Index measures egg load, the **Prevalence Rate** (percentage of people infected) is also used to assess the extent of the endemic. * **Control:** Periodic deworming (Albendazole 400mg) and improved sanitation (use of sanitary latrines) are the primary preventive strategies.

Q: Which of the following statements are true about Cholera?

Food can transmit cholera. **Explanation:** **Cholera** is an acute diarrheal infection caused by the ingestion of food or water contaminated with the bacterium *Vibrio cholerae*. 1. **Why Option A is Correct:** Cholera is primarily transmitted via the **fecal-oral route**. While contaminated water is the most common vehicle, **food** (especially raw or undercooked shellfish, contaminated fruits, and vegetables) acts as a significant reservoir for transmission. In endemic areas, "street foods" and moist grains are frequent sources of outbreaks. 2. **Why Other Options are Incorrect:** * **Option B:** Modern Oral Cholera Vaccines (OCVs) like Shanchol and Dukoral provide significant protection, but their efficacy is generally around **60–80%**, not 90%. Protection also wanes significantly after 2–3 years. * **Option C:** In Cholera, **"Healthy carriers" do not exist** in the epidemiological sense of long-term transmission. While "Asymptomatic cases" occur (and outnumber clinical cases), the term "Healthy carrier" usually refers to chronic carriage (like in Typhoid). In Cholera, the carrier state is transient (incubatory or convalescent), usually lasting less than 2–4 weeks. * **Option D:** *Vibrio cholerae* is highly sensitive to chlorine. **Chlorination** of water supplies (maintaining a free residual chlorine level of 0.5 mg/L) is one of the most effective public health interventions to control epidemics. **High-Yield NEET-PG Pearls:** * **Rice Water Stools:** The hallmark clinical sign. * **Haldane’s Rule:** The ratio of asymptomatic to symptomatic cases is high (approx. 10:1 to 100:1 for El Tor biotype). * **Darting Motility:** Observed under dark-ground microscopy. * **Treatment of Choice:** Prompt rehydration (ORS/IV fluids) and **Doxycycline** (drug of choice to reduce stool volume and duration).

Q: Which type of country does the age pyramid denote?

Developed country. ***Developed country*** - Shows a **constrictive or rectangular pyramid** with a narrow base, indicating **low birth rates** and **low fertility rates**. - Features a significant proportion of **elderly population** due to **increased life expectancy** and **low mortality rates**. *Developing country* - Characterized by an **expansive pyramid** with a **broad base**, reflecting **high birth rates** and **high fertility rates**. - Shows a **rapid tapering** towards the top due to **higher mortality rates** in older age groups. *Underdeveloped country* - Displays a **very broad base** with extremely **high birth rates** and **high infant mortality rates**. - Has a **very narrow top** due to **low life expectancy** and **high death rates** across all age groups. *Poor country* - This term is **economically descriptive** rather than demographically specific and doesn't correspond to a distinct **age pyramid pattern**. - **Economic status** alone doesn't determine demographic structure; poor countries can show various pyramid shapes depending on their **demographic transition stage**.

Q: Which influenza strain, not of human origin, can cause a pandemic?

H5N1. **Explanation:** The potential for an influenza virus to cause a pandemic depends on its ability to undergo **Antigenic Shift**—a major genetic change resulting in a new surface antigen (Hemagglutinin or Neuraminidase) to which the human population has little to no pre-existing immunity. **Why H5N1 is the correct answer:** H5N1 is an **Avian Influenza** strain (Bird Flu). It is primarily of non-human origin and currently circulates in poultry. While it does not yet spread efficiently between humans, it is considered a significant pandemic threat because if it acquires the ability for sustained human-to-human transmission (via reassortment), it could trigger a global pandemic due to the lack of "herd immunity" against the H5 subtype. **Analysis of Incorrect Options:** * **H1N1:** This is a well-established human influenza strain. It caused the 1918 Spanish Flu and the 2009 Swine Flu pandemic. Since it is already circulating widely in the human population, it is now considered a seasonal strain rather than a "new" non-human threat. * **H2N2:** This strain caused the 1957 Asian Flu pandemic. Like H1N1, it has a history of human adaptation. * **H9N1:** While H9 subtypes exist in birds, H5 and H7 strains are clinically more significant and more frequently associated with severe zoonotic outbreaks and pandemic potential in WHO surveillance. **High-Yield NEET-PG Pearls:** * **Antigenic Shift:** Sudden, major change (New subtype); leads to **Pandemics**. * **Antigenic Drift:** Gradual, minor point mutations; leads to **Epidemics** and necessitates annual vaccine updates. * **Pandemic Criteria:** A new virus emerges, infects humans, causes serious illness, and spreads easily and sustainably among humans. * **Host of Influenza A:** Wild aquatic birds are the natural reservoirs for all subtypes of Influenza A.

Question 1

What are randomized controlled trials?

Accepted Answer

Clinical trials

Answer

Preventive trials

Answer

Before and after comparison studies

Answer

Evaluation of health services

Question 2

A suspected cause preceding an observed effect is an example of which criterion for causality?

Accepted Answer

Temporal association

Answer

Consistency of association

Answer

Strength of association

Answer

Coherence of association

Question 3

Which vaccine has the highest efficacy?

Accepted Answer

DPT

Answer

Measles

Answer

Oral typhoid

Answer

Tetanus

Question 4

A Chandler's index of 312 is considered as:

Accepted Answer

Important public health problem

Answer

The infection is not of much significance

Answer

Minor public health problem

Answer

Potential danger

Question 5

What is the couple protection rate when the Net Reproduction Rate (NRR) becomes 1?

Accepted Answer

60%

Answer

50%

Answer

70%

Answer

80%

Question 6

Which of the following statements are true about Cholera?

Accepted Answer

Food can transmit cholera

Answer

Vaccination gives 90% efficiency

Answer

Healthy carriers can transmit cholera

Answer

Chlorination is not effective

Question 7

Which type of country does the age pyramid denote?

Accepted Answer

Developed country

Answer

Developing country

Answer

Underdeveloped country

Answer

Poor country

Question 8

Bias is unlikely to invalidate cohort studies used to assess the risk of exposure because?

Accepted Answer

Exposure is usually determined prior to disease occurrence

Answer

Data collection is prospective

Answer

A large number of subjects is usually included

Answer

Actual relative risk can be determined

Question 9

Which influenza strain, not of human origin, can cause a pandemic?

Accepted Answer

H5N1

Answer

H1N1

Answer

H2N2

Answer

H9N1

Question 10

If two screening tests are used in parallel, what is the effect on their predictive values?

Accepted Answer

Decrease Positive Predictive Value (PPV) and Increase Negative Predictive Value (NPV)

Answer

Increase NPV and Decrease PPV

Answer

Increase PPV and Increase NPV

Answer

Decrease PPV and Decrease NPV

Epidemiology — MCQs

Epidemiology — MCQs

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