Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 791: Which of the following research methods include only people who are initially free of the disease of interest?

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

Explanation: **Explanation:** The core principle of a **Cohort Study** is that it begins with a group of individuals (the cohort) who are **initially free of the disease** or outcome of interest. These individuals are classified based on their exposure status (exposed vs. non-exposed) and followed forward in time to observe the development of the disease. Because the study starts before the disease occurs, it is the gold standard for establishing **temporality** (exposure precedes outcome) and calculating **Incidence**. **Why the other options are incorrect:** * **Case series:** These describe a group of patients who **already have the disease**. There is no comparison group, and it is purely descriptive. * **Cross-sectional study:** This is a "snapshot" where exposure and disease are measured **simultaneously**. It includes both diseased and non-diseased individuals at a single point in time, making it impossible to determine if the exposure came before the disease. * **Case-control study:** This study starts with people who **already have the disease** (Cases) and compares them to those who do not (Controls). It looks backward in time to identify risk factors. **High-Yield Clinical Pearls for NEET-PG:** * **Cohort Study:** Best for rare exposures; can calculate Relative Risk (RR) and Attributable Risk (AR). * **Case-Control:** Best for rare diseases; can only calculate Odds Ratio (OR). * **Incidence:** Can only be directly calculated in a Cohort study (since participants start disease-free). * **Mnemonic:** **C**ohort = **C**ause to Effect; **C**ase-Control = **E**ffect to Cause.

Question 792: Which one of the following is not a special incidence rate?

A. Attack rate
B. Secondary attack rate
C. Hospital admission rate
D. Standardized mortality rate (Correct Answer)

Explanation: ### Explanation To answer this question, it is essential to distinguish between **Incidence** (new cases) and **Mortality** (deaths). **Why the Correct Answer is Right:** **Standardized Mortality Rate (SMR)** is a measure of **mortality**, not morbidity (incidence). It is used in longitudinal studies to compare the observed number of deaths in a study population with the expected number of deaths derived from a standard population. Since it measures death events rather than the occurrence of new disease cases, it cannot be classified as an incidence rate. **Analysis of Incorrect Options:** * **A. Attack Rate:** This is a type of incidence rate used specifically during an epidemic for a narrow population over a short period (e.g., food poisoning). It is calculated as: *(Number of new cases / Population at risk) × 100*. * **B. Secondary Attack Rate (SAR):** This is a special incidence rate that measures the spread of a communicable disease from a primary case to contacts within a closed group (like a household). It reflects the infectivity of an agent. * **C. Hospital Admission Rate:** This is considered a special incidence rate in administrative epidemiology, representing the number of new "events" (admissions) occurring in a population over a specific timeframe. **High-Yield NEET-PG Pearls:** * **Incidence** = New cases / Population at risk. It is the best indicator for the **etiology** of a disease and the **efficacy** of preventive programs. * **Prevalence** = Total cases (Old + New) / Total population. It is best for estimating the **burden** of disease and planning health services. * **SAR Formula:** (Number of cases among contacts / Total number of susceptible contacts) × 100. Note: The denominator excludes those already immune or the primary case. * **SMR Formula:** (Observed deaths / Expected deaths) × 100. An SMR > 100 indicates higher-than-expected mortality.

Question 793: In a population, changing harmful lifestyles through education to prevent coronary artery disease is referred to as:

A. High risk strategy
B. Primary prevention (Correct Answer)
C. Secondary prevention
D. Tertiary prevention

Explanation: ### Explanation **Correct Answer: B. Primary Prevention** **Why it is correct:** Primary prevention aims to prevent the **onset of disease** by controlling causes and risk factors. It is applied during the **pre-pathogenesis phase** (before the disease process has started). In this scenario, educating a population to change harmful lifestyles (like smoking cessation or healthy dieting) is a form of **Health Promotion**, which is one of the two main modes of intervention in primary prevention (the other being Specific Protection). By modifying these risk factors, we prevent the development of Coronary Artery Disease (CAD) in healthy individuals. **Why incorrect options are wrong:** * **A. High-risk strategy:** This is a *subset* of primary prevention that targets only those individuals at the highest risk (e.g., those with a strong family history or morbid obesity). The question refers to the general population, which aligns more broadly with the "Mass Strategy" of primary prevention. * **C. Secondary prevention:** This involves **early diagnosis and prompt treatment** (e.g., screening for hypertension or using stress tests to detect early CAD). It aims to halt disease progression and prevent complications after the disease process has already begun. * **D. Tertiary prevention:** This occurs in the late pathogenesis phase. It focuses on **disability limitation and rehabilitation** (e.g., cardiac rehabilitation after a myocardial infarction) to restore function and prevent further deterioration. **High-Yield Clinical Pearls for NEET-PG:** * **Primordial Prevention:** Often confused with Primary. It is the prevention of the **emergence of risk factors** themselves (e.g., discouraging children from starting smoking). If the risk factor is already present (as "harmful lifestyles" implies), the intervention is **Primary**. * **Modes of Intervention:** * **Primary:** Health Promotion & Specific Protection (e.g., Immunization). * **Secondary:** Early Diagnosis & Treatment. * **Tertiary:** Disability Limitation & Rehabilitation. * **Population Strategy:** Primary prevention is often the most cost-effective method for controlling non-communicable diseases (NCDs) like CAD.

Question 794: Which of the following statements is true regarding the Mantoux test?

A. The test is read before 48 hours.
B. An induration of 6-9 mm indicates the maximum chances of developing TB.
C. A positive test does not necessarily indicate that the person is suffering from active TB disease. (Correct Answer)
D. New cases of TB are more likely to occur in tuberculin-negative individuals than in those who are tuberculin reactors.

Explanation: ### Explanation **1. Why the Correct Answer is Right:** The Mantoux test (Tuberculin Skin Test) is a delayed hypersensitivity reaction (Type IV) to Purified Protein Derivative (PPD). A positive result indicates that the individual’s immune system has been **sensitized** to the tubercle bacilli. However, it cannot distinguish between **latent TB infection (LTBI)** and **active TB disease**. Therefore, a positive test confirms infection but does not necessarily mean the patient is currently suffering from clinical disease. **2. Why the Other Options are Wrong:** * **Option A:** The test must be read between **48 to 72 hours**. Reading it before 48 hours may lead to a false-negative result as the delayed hypersensitivity reaction takes time to peak. * **Option B:** In the Indian context (and per WHO), an induration of **≥10 mm** is generally considered positive. The risk of developing active TB is significantly higher in "strong reactors" (induration **>20 mm**), not in the 6-9 mm range. * **Option D:** New cases of TB are actually **more likely to occur in tuberculin reactors** (positive testers) because they already harbor the bacilli (latent infection), which can reactivate. Tuberculin-negative individuals are "at risk" of infection, but those already positive are "at risk" of disease. **3. High-Yield Clinical Pearls for NEET-PG:** * **Standard Dose:** 0.1 ml of 5 TU (Tuberculin Units) injected intradermally (ID) using a 26G needle (Omega/Mantoux syringe). * **Measurement:** Only the **induration** (palpable hardening) is measured, not the erythema (redness). * **False Negatives:** Can occur in miliary TB, malnutrition, HIV/AIDS (due to anergy), and recent viral infections (e.g., Measles). * **BCG Effect:** Prior BCG vaccination can cause a false-positive Mantoux test, though the induration is usually smaller and wanes over time.

Question 795: What does specificity measure?

A. True negatives (Correct Answer)
B. False positives
C. True positives
D. None of the above

Explanation: **Explanation:** **Specificity** is a measure of a diagnostic test's ability to correctly identify those **without the disease**. It is defined as the proportion of truly healthy individuals (non-diseased) who are correctly identified as negative by the test. 1. **Why Option A is Correct:** Specificity focuses on the "healthy" column of a 2x2 contingency table. The formula is: **Specificity = [True Negatives (TN) / (True Negatives + False Positives)] × 100.** A highly specific test has very few "False Positives," meaning if the test result is positive, you can be highly confident the patient actually has the disease (Rule: **SpPIn** – Specificity rules **In**). 2. **Why Other Options are Incorrect:** * **Option B (False Positives):** Specificity aims to *minimize* false positives, but it measures the True Negatives. The "False Positive Rate" is actually calculated as (1 - Specificity). * **Option C (True Positives):** This is the definition of **Sensitivity**. Sensitivity measures the ability of a test to correctly identify those *with* the disease (Rule: **SnNOut** – Sensitivity rules **Out**). **NEET-PG High-Yield Pearls:** * **Screening vs. Diagnosis:** Use a highly **Sensitive** test for screening (to catch all cases) and a highly **Specific** test for confirmation (to avoid unnecessary treatment). * **Ideal Test:** An ideal diagnostic test has 100% Sensitivity and 100% Specificity. * **Inverse Relationship:** As you change the "cut-off" point to increase sensitivity, specificity usually decreases, and vice versa. This relationship is visualized using the **ROC (Receiver Operating Characteristic) Curve**.

Question 796: Which one of the following factors is NOT considered when evaluating a screening program?

A. Disease burden
B. Physician's knowledge of the disease
C. Cost of the screening test (Correct Answer)
D. Efficacy of available treatments

Explanation: ### Explanation The evaluation of a screening program is based on the **Wilson and Jungner criteria**, which outline the requirements for a screening test to be ethically and medically justifiable. **Why "Cost of the screening test" is the correct answer:** While economic feasibility is a consideration in public health policy, the **unit cost of the test itself** is not a primary criterion for evaluating the scientific validity or effectiveness of a screening program. Instead, the focus is on the **cost-effectiveness** of the entire program (including diagnosis and treatment) relative to the benefits gained. A cheap test for a disease with no cure is useless, whereas an expensive test for a treatable condition may be highly valuable. **Analysis of incorrect options:** * **Disease burden (A):** The condition must be an important health problem (high prevalence or high mortality/morbidity) to justify mass screening. * **Physician's knowledge (B):** While often phrased as "the natural history of the disease should be well understood," this includes the medical community's ability to recognize the latent and early symptomatic stages. Without this knowledge, screening cannot be timed effectively. * **Efficacy of available treatments (D):** This is a core ethical requirement. There must be an accepted, effective treatment for patients discovered through screening. Screening without the ability to improve the outcome is considered unethical. **High-Yield Clinical Pearls for NEET-PG:** * **Wilson and Jungner Criteria:** The gold standard for screening evaluation. * **Lead Time Bias:** The apparent increase in survival time due to early diagnosis, even if the actual time of death is not delayed. * **Length Bias:** Screening tends to detect slowly progressing cases (better prognosis) more than rapidly progressing ones. * **Yield:** The amount of previously undiagnosed disease that is detected as a result of the screening program.

Question 797: Which of the following is NOT a feature of epidemic dropsy?

A. Glaucoma
B. Diarrhoea
C. Convulsions (Correct Answer)
D. Heart failure

Explanation: **Explanation:** Epidemic dropsy is a clinical condition caused by the ingestion of mustard oil contaminated with **Argemone mexicana** (prickly poppy) oil. The toxic alkaloids responsible are **Sanguinarine** and **Dihydrosanguinarine**, which interfere with cellular oxidative phosphorylation. **Why Convulsions is the correct answer:** Epidemic dropsy primarily affects the vascular system, leading to increased capillary permeability and dilatation. While it involves the cardiovascular, gastrointestinal, and ocular systems, it **does not typically involve the Central Nervous System (CNS)**. Therefore, neurological symptoms like convulsions are not a feature of this condition. **Analysis of other options:** * **Glaucoma (Option A):** This is a hallmark feature. Sanguinarine causes dilatation of the uveal tract capillaries, leading to increased aqueous humor production and **open-angle glaucoma**. * **Diarrhoea (Option B):** Gastrointestinal symptoms, including vomiting and diarrhoea, are often the earliest manifestations of the toxicity. * **Heart Failure (Option D):** The toxin causes extensive capillary leakage and peripheral vasodilatation, leading to high-output cardiac failure, which is a common cause of death in these patients. **High-Yield Clinical Pearls for NEET-PG:** * **Causative Agent:** Argemone mexicana (Sanguinarine toxin). * **Test for Detection:** **Nitric Acid Test** (turns orange-red) and **Paper Chromatography** (most sensitive). * **Key Clinical Triad:** Bilateral pitting edema (dropsy), Erythema/Pigmentation of skin, and Cardiac failure. * **Distinguishing Feature:** Unlike nutritional edema, epidemic dropsy is associated with **erythema** and **normal serum albumin** levels. * **Treatment:** No specific antidote; management is symptomatic (bed rest, high protein diet, and antioxidants).

Question 798: Which of the following diseases is NOT included in WHO surveillance programs?

A. Polio
B. Viral encephalitis (Correct Answer)
C. Malaria
D. Relapsing fever

Explanation: The World Health Organization (WHO) maintains specific surveillance systems for diseases that pose a significant threat to global health security, are targeted for eradication/elimination, or are subject to the **International Health Regulations (IHR)**. **Explanation of the Correct Answer:** **Viral Encephalitis (Option B)** is the correct answer because it is not part of the standardized global WHO surveillance program. While specific types (like Japanese Encephalitis) may be monitored in endemic regions, "Viral Encephalitis" as a broad category is not a mandatory reportable disease under the IHR or a specific global WHO surveillance mandate. **Analysis of Incorrect Options:** * **Polio (Option A):** Included under the Global Polio Eradication Initiative (GPEI). It is a "Public Health Emergency of International Concern" (PHEIC) and requires mandatory reporting. * **Malaria (Option C):** Subject to intense global surveillance under the "Global Malaria Programme" to monitor trends, drug resistance, and elimination status. * **Relapsing Fever (Option D):** Historically, this is one of the "Louse-borne diseases" (along with Epidemic Typhus) specifically mentioned in older and updated WHO surveillance frameworks due to its potential for rapid outbreaks in vulnerable populations. **High-Yield NEET-PG Pearls:** 1. **IHR Mandatory Reporting:** Under the International Health Regulations (2005), four diseases **must** be notified regardless of the context: **Smallpox, Polio (wild type), Human Influenza (new subtype), and SARS.** 2. **Surveillance Types:** Remember the difference between **Passive** (routine reporting), **Active** (health staff visiting facilities), and **Sentinel** (selected sites for specific data). 3. **Integrated Disease Surveillance Programme (IDSP):** In India, diseases are categorized into L (Laboratory), P (Presumptive), and S (Syndromic) formats. Viral Encephalitis (as AES - Acute Encephalitis Syndrome) is monitored under IDSP, but not the global WHO mandatory list.

Question 799: Which of the following is an example of an amplifier host?

A. Pig in Japanese encephalitis (Correct Answer)
B. Dog in hydatid disease
C. Cattle in mad cow disease
D. Rat in plague

Explanation: **Explanation:** In epidemiology, an **amplifier host** is an animal in which an infectious agent (usually a virus) multiplies rapidly to high levels, providing a significant source of infection for vectors (like mosquitoes) to transmit the pathogen to humans. **1. Why Pig in Japanese Encephalitis (JE) is correct:** In the transmission cycle of JE, pigs serve as the primary amplifier hosts. When a *Culex* mosquito bites an infected pig, the virus replicates extensively in the pig's blood (**viremia**) without causing serious illness to the animal. This high viral load ensures that any mosquito biting the pig becomes infected, subsequently spreading the virus to humans (who are "dead-end" hosts). **2. Analysis of Incorrect Options:** * **Dog in Hydatid Disease:** The dog is the **definitive host** because it harbors the adult stage of the parasite *Echinococcus granulosus*. * **Cattle in Mad Cow Disease:** Cattle are the **primary hosts/reservoirs** for the prions causing Bovine Spongiform Encephalopathy (BSE). It is not a vector-borne disease requiring amplification for transmission. * **Rat in Plague:** Rats are **reservoirs** or **maintenance hosts**. While they harbor *Yersinia pestis*, the term "amplifier host" is specifically used in the context of increasing the viral/pathogen titer for vector transmission, whereas rats often die from the plague (epizootic). **Clinical Pearls for NEET-PG:** * **Dead-end Host:** Humans in JE and Rabies (the pathogen cannot be transmitted further). * **Incidental Host:** Humans in JE (we are not part of the natural maintenance cycle). * **JE Vector:** *Culex tritaeniorhynchus* (breeds in rice fields). * **Arbo-viral amplification:** Pigs are to JE what birds are to West Nile Virus.

Question 800: What is the Pearl index?

A. Failures per 1000 women-years of exposure
B. Failures per 100 women-years of exposure (Correct Answer)
C. Failures per 10 women-years of exposure
D. Failures per women-years of exposure

Explanation: The **Pearl Index** is the most common method used in clinical trials and epidemiological studies to express the **efficacy of a contraceptive method**. ### Why Option B is Correct The Pearl Index is defined as the number of unintended pregnancies (failures) per **100 woman-years** of exposure. It standardizes the failure rate over a specific duration of time, allowing for a direct comparison between different contraceptive methods. The formula is: $$\text{Pearl Index} = \frac{\text{Total number of accidental pregnancies} \times 1200}{\text{Total months of exposure (usage)}}$$ *(Note: 1200 represents 100 women multiplied by 12 months in a year).* ### Why Other Options are Incorrect * **Option A, C, and D:** These are mathematically incorrect. The standard denominator for the Pearl Index is always **100 woman-years**. Using 1000, 10, or 1 woman-year would lead to an inaccurate representation of the standardized failure rate used in global medical literature. ### High-Yield Facts for NEET-PG * **Lower is Better:** A lower Pearl Index indicates a more effective contraceptive method (e.g., Implants have a Pearl Index of ~0.05, while the rhythm method can be >20). * **Limitation:** The Pearl Index assumes a constant failure rate over time. However, failure rates usually decrease as users become more experienced with a method. * **Alternative:** The **Life Table Analysis** is considered superior to the Pearl Index because it calculates the failure rate for each month of use, accounting for "drop-outs" or users who switch methods. * **Theoretical vs. Typical Use:** Always distinguish between "Perfect Use" (lowest Pearl Index) and "Typical Use" (higher Pearl Index due to human error).

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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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