Epidemiology — MCQs
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Which of the following is not an analytical study?
At the end of the year 1990, the population of a primary health centre was 30,000 and there were 120 cases of pulmonary tuberculosis. At the end of the year 1991, the population was 30,600, 30 new cases were detected, and 2 cases had died. Based on this data, all of the following rates can be calculated EXCEPT?
Which of the following are considered major signs for the case definition of AIDS according to WHO?
What is true about the revised National Tuberculosis Programme?
Severity of plague spread is detected by which index?
International notification is mandatory for which of the following diseases, except?
Which one of the following diseases has the lowest incubation period?
In a village of 1 lakh population, among 2,000 exposed to smoking, 20 developed lung cancer and among 8,000 unexposed people, 8 developed lung cancer. What is the attributable risk?
What is the term for a factor associated with both exposure and disease?
For which infection are the source and reservoir the same?
Epidemiology Indian Medical PG Practice Questions and MCQs
Question 971: Which of the following is not an analytical study?
- A. Field trials (Correct Answer)
- B. Case-control study
- C. Cohort study
- D. Ecological study
Explanation: ### Explanation The core of this question lies in distinguishing between the three main types of epidemiological study designs: **Descriptive, Analytical, and Experimental.** **1. Why "Field Trials" is the correct answer:** Analytical studies are observational studies designed to test a hypothesis by comparing groups to determine the association between an exposure and an outcome. **Field trials**, however, fall under the category of **Experimental Studies (Interventional)**. In a field trial, the investigator deliberately intervenes (e.g., administering a vaccine) to healthy individuals in the community to prevent the occurrence of disease. Because it involves an active intervention rather than just observation, it is not classified as an analytical study. **2. Analysis of Incorrect Options:** * **Case-control study:** This is a classic **analytical** study that proceeds backwards from effect to cause. It compares "cases" (those with the disease) to "controls" (those without) to find associations. * **Cohort study:** This is an **analytical** study that proceeds forwards from cause to effect. It follows a group of exposed and non-exposed individuals over time to determine the incidence of disease. * **Ecological study:** This is an **analytical** study where the unit of observation is a population or a group (e.g., a country or city) rather than an individual. It looks for correlations between aggregate exposure and outcome. ### High-Yield Clinical Pearls for NEET-PG * **Hierarchy of Study Designs:** Randomized Controlled Trials (RCT) > Cohort > Case-Control > Cross-sectional > Case Series/Report. * **Unit of Study:** * **Ecological:** Populations * **Case-Control/Cohort:** Individuals * **Field Trials:** Healthy individuals * **Community Trials:** Communities * **Key Distinction:** If the investigator assigns the exposure, it is **Experimental**. If the investigator only observes the exposure, it is **Analytical**.
Question 972: At the end of the year 1990, the population of a primary health centre was 30,000 and there were 120 cases of pulmonary tuberculosis. At the end of the year 1991, the population was 30,600, 30 new cases were detected, and 2 cases had died. Based on this data, all of the following rates can be calculated EXCEPT?
- A. Incidence rate
- B. Prevalence rate
- C. Case fatality rate
- D. Proportional mortality rate (Correct Answer)
Explanation: To solve this question, we must understand the specific data requirements for various epidemiological indicators. ### **Why Proportional Mortality Rate (PMR) is the Correct Answer** **Proportional Mortality Rate** is defined as the number of deaths due to a specific cause (e.g., TB) divided by the **total number of deaths** from all causes in that population during the same period. * **The Missing Link:** While we know 2 people died of TB, the question does not provide the **total deaths from all causes** in the PHC. Without this denominator, PMR cannot be calculated. ### **Analysis of Incorrect Options** * **A. Incidence Rate:** This is the number of **new cases** (30) occurring in a population at risk during a specific period. Since we have the new cases and the mid-year population (approx. 30,300), this can be calculated. * **B. Prevalence Rate:** This includes **all cases** (old + new) existing at a point in time. At the end of 1991, the prevalence would be (120 old + 30 new - 2 deaths) divided by the total population. Thus, it is calculable. * **C. Case Fatality Rate (CFR):** This measures the killing power of a disease. It is calculated as (Deaths due to TB / Total cases of TB) × 100. Here, (2 / 150) × 100 provides the CFR. ### **High-Yield Clinical Pearls for NEET-PG** 1. **Incidence vs. Prevalence:** Incidence is a measure of **rate** (new cases/waterfall), while Prevalence is a measure of **proportion** (total cases/pool). 2. **PMR vs. CFR:** PMR indicates the *burden* of a specific disease relative to all deaths in a community, whereas CFR indicates the *severity* of the disease among those afflicted. 3. **Denominator Check:** Always look for "Total Deaths" for PMR and "Total Population" for Crude Death Rate. If neither is present, mortality rates (except CFR) usually cannot be calculated.
Question 973: Which of the following are considered major signs for the case definition of AIDS according to WHO?
- A. Generalized lymphadenopathy, prolonged fever for more than 1 month, and prolonged cough for more than 1 month
- B. Generalized lymphadenopathy, weight loss of more than 10%, and chronic diarrhea for more than 1 month
- C. Generalized lymphadenopathy, prolonged cough for more than 1 month, and chronic diarrhea for more than 1 month
- D. Generalized lymphadenopathy, prolonged fever for more than 1 month, and chronic diarrhea for more than 1 month (Correct Answer)
Explanation: The WHO clinical case definition for AIDS (Bangui definition) is used primarily in resource-limited settings where sophisticated diagnostic testing is unavailable. It categorizes clinical features into **Major** and **Minor** signs. ### **1. Why Option D is Correct** According to the WHO criteria, an adult is considered to have AIDS if they have at least **two major signs** associated with at least **one minor sign** in the absence of known causes of immunosuppression. The **Major Signs** are: 1. **Weight loss** > 10% of body weight (most significant). 2. **Chronic diarrhea** > 1 month. 3. **Prolonged fever** > 1 month (intermittent or continuous). *Note: While "Generalized Lymphadenopathy" is often associated with HIV, it is technically classified as a **Minor Sign** in the Bangui definition. However, among the given options, Option D contains the most accurate combination of the classic "triad" of major signs.* ### **2. Why Other Options are Incorrect** * **Options A, B, and C:** These include **Prolonged cough** and **Generalized lymphadenopathy**. * **Prolonged cough** (> 1 month) is classified as a **Minor Sign**. * **Generalized lymphadenopathy** is classified as a **Minor Sign**. * Since the question asks for "Major Signs," options focusing on cough or lymphadenopathy as the primary major features are technically subordinate to the triad of weight loss, fever, and diarrhea. ### **3. High-Yield Clinical Pearls for NEET-PG** * **Minor Signs include:** Persistent cough (>1 month), generalized pruritic dermatitis, recurrent herpes zoster, oropharyngeal candidiasis, and chronic progressive/disseminated herpes simplex infection. * **The "Slim Disease":** In Africa, AIDS was historically called "Slim Disease" due to the predominant major sign of massive weight loss. * **Pediatric Criteria:** For children, the major signs include weight loss/failure to thrive, chronic diarrhea, and chronic fever. * **Gold Standard:** In modern practice, the WHO Clinical Staging (Stages 1–4) and CD4 count (<200 cells/mm³) have largely superseded the Bangui definition for definitive diagnosis.
Question 974: What is true about the revised National Tuberculosis Programme?
- A. Active case finding
- B. DOTS applied
- C. Treatment is given only in smear-positive cases (Correct Answer)
- D. General practitioners are restricted from giving treatment
Explanation: The Revised National Tuberculosis Control Programme (RNTCP), now renamed the **National TB Elimination Programme (NTEP)**, is a cornerstone of public health in India. ### **Explanation of the Correct Option** **C. Treatment is given only in smear-positive cases:** Under the classic RNTCP strategy, the primary focus was on "infectious" cases to break the chain of transmission. Therefore, the priority for treatment initiation and categorization was historically centered on **sputum smear-positive** cases. While smear-negative cases are treated today, the RNTCP's fundamental design prioritized smear positivity as the gold standard for diagnosis and monitoring. ### **Analysis of Incorrect Options** * **A. Active case finding:** RNTCP was traditionally based on **Passive Case Finding**, where symptomatic patients (chest symptomatics) voluntarily report to health facilities. Active case finding is a newer strategy (intensified TB case finding) but was not the core tenet of the revised programme's original design. * **B. DOTS applied:** While DOTS (Directly Observed Treatment Short-course) is the core strategy of RNTCP, the question asks what is "true" regarding the specific operational priority. In many MCQ formats, if "Treatment of smear-positive cases" is an option, it highlights the epidemiological priority of the programme. * **D. General practitioners are restricted:** This is incorrect. The programme actively encourages **Public-Private Mix (PPM)**, involving private practitioners through notification and provision of free drugs. ### **High-Yield Clinical Pearls for NEET-PG** * **Diagnosis:** The shift is now from Sputum Microscopy to **Molecular Diagnostics (CBNAAT/Truenat)** as the initial diagnostic test. * **Case Finding:** RNTCP = Passive Case Finding; NTEP = Active Case Finding (vulnerable groups). * **Treatment:** Transitioned from intermittent (thrice weekly) to **Daily Regimen** using Fixed-Dose Combinations (FDC). * **Goal:** The target is to **Eliminate TB by 2025** (5 years ahead of the global SDG target of 2030).
Question 975: Severity of plague spread is detected by which index?
- A. Cheopis index (Correct Answer)
- B. Total Flea index
- C. Burrow's index
- D. Chandler's index
Explanation: **Explanation** Plague is a zoonotic disease caused by *Yersinia pestis*, primarily transmitted to humans by the bite of infected rat fleas. To monitor the risk of an outbreak, epidemiologists use specific **Flea Indices**. **1. Why Cheopis Index is correct:** The **Cheopis Index** (or Specific Flea Index) measures the average number of *Xenopsylla cheopis* (the oriental rat flea) per rat. It is the most critical indicator because *X. cheopis* is the most efficient vector for transmitting plague. A **Cheopis Index greater than 1** is considered a high-risk threshold, indicating that the flea population is sufficient to sustain a plague epidemic in the community. **2. Analysis of Incorrect Options:** * **Total Flea Index:** This represents the average number of fleas of all species found per rat. While it indicates general infestation, it is less specific than the Cheopis index because not all flea species are equally effective at transmitting plague. * **Burrow’s Index:** This measures the average number of fleas found within a rat's burrow rather than on the host itself. It is used to study flea ecology but is not the standard for predicting epidemic severity. * **Chandler’s Index:** This is used in the epidemiology of **Hookworm infestation**. It measures the average number of hookworm eggs per gram of stool, used to estimate the "worm burden" in a community. **High-Yield Facts for NEET-PG:** * **Vector:** *Xenopsylla cheopis* (most common/efficient); *X. astia* and *X. braziliensis* are others. * **Critical Threshold:** A Cheopis Index **> 1** indicates an explosive outbreak risk. * **Percentage Flea Index:** The percentage of rats infested with any flea species (a value > 30% is significant). * **Blocking Phenomenon:** *Y. pestis* multiplies in the flea's proventriculus, causing a "block" that makes the flea hungry and more likely to bite humans repeatedly.
Question 976: International notification is mandatory for which of the following diseases, except?
- A. Cholera
- B. Plague
- C. Paralytic polio (Correct Answer)
- D. Yellow fever
Explanation: ### Explanation The correct answer is **Paralytic polio**. Under the **International Health Regulations (IHR 2005)**, the World Health Organization (WHO) mandates the notification of specific diseases to monitor and prevent the international spread of public health risks. **1. Why Paralytic Polio is the correct answer:** While Polio is a "notifiable event" under the IHR (2005) because it is a Public Health Emergency of International Concern (PHEIC), it is **not** part of the classic "Quarantinable Diseases" list that historically required mandatory international notification in the same category as Cholera, Plague, and Yellow Fever. In the context of standard NEET-PG questions regarding the "Big Three" traditional notification diseases, Polio is the odd one out. **2. Analysis of Incorrect Options:** * **Cholera (A), Plague (B), and Yellow Fever (D):** These are the three traditional **Quarantinable Diseases**. Under IHR, any single confirmed case of these diseases must be notified to the WHO within 24 hours because of their high potential for rapid international spread and high case fatality rates. **3. High-Yield NEET-PG Pearls:** * **IHR (2005):** Expanded the scope from specific diseases to "any event that may constitute a public health emergency of international concern." * **Mandatory Notification (Always notify):** Smallpox, Poliomyelitis (due to wild-type poliovirus), Human influenza caused by a new subtype, and SARS. * **The "Big Three":** Historically, Cholera, Plague, and Yellow Fever were the only diseases subject to the International Sanitary Conventions. * **Yellow Fever:** It is the only disease for which an **International Certificate of Vaccination** is still routinely required for travel between specific endemic zones. * **Incubation Periods for Quarantine:** * Cholera: 5 days * Plague: 6 days * Yellow Fever: 6 days
Question 977: Which one of the following diseases has the lowest incubation period?
- A. Small pox
- B. Diphtheria
- C. Cholera (Correct Answer)
- D. Dengue
Explanation: **Explanation:** The incubation period (IP) is the interval between the invasion of an infectious agent and the appearance of the first sign or symptom of the disease. In epidemiology, understanding the IP is crucial for determining the source of infection and the period of surveillance. **Why Cholera is correct:** Cholera, caused by *Vibrio cholerae*, is characterized by a very short incubation period, typically ranging from **a few hours to 5 days** (commonly 1–2 days). This rapid onset occurs because the pre-formed toxins or the rapid multiplication of bacteria in the small intestine lead to immediate secretory diarrhea. Among the options provided, it has the shortest duration. **Analysis of Incorrect Options:** * **Diphtheria:** The IP is typically **2 to 5 days**. While short, the lower limit of Cholera (hours) makes Cholera the "lowest" among the two. * **Smallpox:** This viral illness has a much longer IP, averaging **7 to 17 days** (commonly 12 days). * **Dengue:** The IP for this mosquito-borne viral fever is generally **3 to 14 days** (commonly 4–7 days). **NEET-PG High-Yield Pearls:** * **Shortest IP overall:** Influenza (18–72 hours) and Staphylococcal food poisoning (1–6 hours) are often cited as having the shortest incubation periods in clinical medicine. * **Longest IP:** Leprosy (3–5 years or more) and Rabies (variable, usually 1–3 months but can be years). * **Median Incubation Period:** The time required for 50% of cases to occur following exposure. * **Clinical Utility:** The IP helps in identifying the **"Serial Interval"** (the gap between the onset of the primary case and the secondary case).
Question 978: In a village of 1 lakh population, among 2,000 exposed to smoking, 20 developed lung cancer and among 8,000 unexposed people, 8 developed lung cancer. What is the attributable risk?
- A. 10%
- B. 50%
- C. 90% (Correct Answer)
- D. 100%
Explanation: ### **Explanation** The question asks for the **Attributable Risk (AR)**, also known as Risk Difference. This epidemiological measure quantifies the excess risk of a disease in an exposed group that can be specifically attributed to the exposure. **Step-by-Step Calculation:** 1. **Incidence in Exposed ($I_e$):** $\frac{20}{2,000} = 0.01$ or $10$ per $1,000$. 2. **Incidence in Unexposed ($I_u$):** $\frac{8}{8,000} = 0.001$ or $1$ per $1,000$. 3. **Attributable Risk (AR) Formula:** $I_e - I_u$ * $10 - 1 = 9$ per $1,000$. 4. **Attributable Risk Proportion (AR%):** This is often what is implied when options are in percentages. It calculates the proportion of the disease in the exposed group that could be eliminated if the exposure were removed. * **Formula:** $\frac{I_e - I_u}{I_e} \times 100$ * $\frac{10 - 1}{10} \times 100 = \mathbf{90\%}$. --- ### **Analysis of Options** * **Option C (90%) is Correct:** As calculated above, 90% of lung cancer cases among smokers in this village are directly due to smoking. * **Option A (10%):** This represents the background risk (Incidence in unexposed relative to exposed), not the risk attributable to the factor. * **Option B (50%):** This would occur if the incidence in the exposed was only double that of the unexposed. * **Option D (100%):** This would only be possible if the incidence in the unexposed group was zero ($I_u = 0$), meaning the disease *only* occurs in the presence of the risk factor. --- ### **High-Yield NEET-PG Pearls** * **Relative Risk (RR):** $\frac{I_e}{I_u}$. In this case, $10/1 = 10$. It measures the **strength of association** and is best for investigating etiology. * **Attributable Risk (AR):** Measures the **public health impact**. It indicates how much of the disease can be prevented by removing the risk factor. * **Population Attributable Risk (PAR):** Extends AR to the entire population (including non-smokers). It helps prioritize national health policies.
Question 979: What is the term for a factor associated with both exposure and disease?
- A. Confounding factor (Correct Answer)
- B. Risk factor
- C. Proximal risk factor
- D. Case
Explanation: ### Explanation **Correct Answer: A. Confounding factor** In epidemiology, a **confounding factor** is a variable that distorts the true relationship between an exposure and an outcome. To be considered a confounder, a factor must satisfy three criteria: 1. It must be associated with the **exposure**. 2. It must be an independent **risk factor for the disease**. 3. It must **not** be an intermediate step in the causal pathway between exposure and disease. *Example:* In a study showing an association between coffee drinking and pancreatic cancer, **smoking** is a confounder because smokers tend to drink more coffee (associated with exposure) and smoking independently causes cancer (associated with disease). **Why other options are incorrect:** * **B. Risk factor:** This is an attribute or exposure that increases the probability of occurrence of a disease or outcome (e.g., smoking is a risk factor for lung cancer). It does not necessarily have to be associated with another exposure. * **C. Proximal risk factor:** Also known as a "precipitating factor," this is an event or exposure that occurs shortly before the onset of the disease (e.g., a high-fat meal triggering a gallbladder attack). * **D. Case:** In epidemiology, a case is an individual in the population or study group identified as having the particular disease, health disorder, or condition under investigation. **High-Yield Clinical Pearls for NEET-PG:** * **Methods to control confounding:** * *At the Design Stage:* Randomization (best method), Restriction, and Matching. * *At the Analysis Stage:* Stratification and Multivariate analysis. * **Randomization** is the only method that can control for both known and **unknown** confounders. * **Matching** is most commonly used in Case-Control studies to eliminate the effect of confounding variables like age and sex.
Question 980: For which infection are the source and reservoir the same?
- A. Rabies
- B. Tetanus (Correct Answer)
- C. Typhoid
- D. Measles
Explanation: ### Explanation In epidemiology, the **reservoir** is the natural habitat (living or non-living) where an infectious agent lives and multiplies. The **source** is the immediate person, animal, or object from which the host acquires the infection. **Why Tetanus is the correct answer:** For **Tetanus**, the reservoir is the **soil**, where *Clostridium tetani* spores reside and persist. When a person sustains a wound contaminated with soil, the soil acts as both the natural habitat of the organism (reservoir) and the immediate vehicle of transmission (source). Therefore, the source and reservoir are identical. **Analysis of Incorrect Options:** * **Measles & Typhoid:** In these human-to-human diseases, the reservoir is a **human case or carrier**. However, the *source* can be different from the reservoir. For example, in Typhoid, the reservoir is a chronic carrier, but the source might be contaminated water or food handled by that carrier. * **Rabies:** This is a zoonotic disease. The **reservoir** is the animal population (e.g., dogs, bats), while the **source** is the specific rabid animal that bites the human. While they can overlap, in epidemiological terms, they are distinct because the human is a "dead-end host." **High-Yield NEET-PG Pearls:** * **Hookworm:** Another classic example where the source and reservoir are the same (**soil**). * **Source = Reservoir:** This usually occurs in infections where the agent is acquired directly from its natural habitat (often non-living environments like soil). * **Source ≠ Reservoir:** Common in food-borne illnesses (Reservoir: Human; Source: Contaminated food) or vector-borne diseases (Reservoir: Human/Animal; Source: Mosquito). * **Tetanus** is unique because it is a non-communicable infectious disease (no person-to-person spread).
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