Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 911: A case control study is defined as:

A. A backward study (Correct Answer)
B. A forward study
C. A current study
D. A feature study

Explanation: ### Explanation In epidemiology, a **Case-Control Study** is classified as an **analytical, observational study** that begins with the identification of individuals who already have a specific disease (Cases) and a group of individuals without the disease (Controls). **Why Option A is correct:** A Case-Control study is termed a **"backward study"** (or retrospective study) because it moves from **Effect to Cause**. The researcher starts with the outcome (the disease) and looks back in time to investigate exposure to potential risk factors. This direction of inquiry—tracing back from the present disease status to past exposures—is why it is defined as "backward." **Why the other options are incorrect:** * **Option B (Forward study):** This refers to a **Cohort Study**. In a cohort study, the researcher starts with a group of exposed and non-exposed individuals (Cause) and follows them over time to see who develops the disease (Effect). * **Option C (Current study):** This usually refers to a **Cross-sectional Study**, which provides a "snapshot" of a population at a single point in time, measuring both exposure and outcome simultaneously. * **Option D (Feature study):** This is not a standard epidemiological term for study design. ### NEET-PG High-Yield Pearls: * **Direction:** Case-control is always **Retrospective** (Effect $\rightarrow$ Cause). * **Measure of Association:** The primary statistical measure used is the **Odds Ratio (OR)**. (Note: Relative Risk cannot be calculated directly). * **Suitability:** It is the best study design for **rare diseases** or diseases with long latency periods. * **Bias:** It is particularly prone to **Recall Bias** (patients with the disease are more likely to remember past exposures than healthy controls). * **Starting Point:** The study always starts with the **Cases**.

Question 912: Which is the first and commonest clinical manifestation of Epidemic dropsy?

A. Bilateral swelling of legs (Correct Answer)
B. Gastrointestinal upsets
C. Cardiac decomposition
D. Sarcoid

Explanation: **Explanation:** **Epidemic Dropsy** is a clinical condition caused by the ingestion of mustard oil contaminated with **Argemone mexicana** (prickly poppy) oil. The toxic alkaloid responsible is **Sanguinarine**, which interferes with oxidation-reduction reactions and leads to increased capillary permeability and dilatation. 1. **Why Option A is Correct:** **Bilateral swelling of legs (Edema)** is the hallmark of the disease. It is typically the **first and commonest** clinical manifestation. The edema is sudden in onset, pitting in nature, and often associated with erythema (redness) and local tenderness of the skin. 2. **Analysis of Incorrect Options:** * **Option B (Gastrointestinal upsets):** While symptoms like diarrhea, nausea, and vomiting can occur in the early stages, they are inconsistent and not as universal or diagnostic as pedal edema. * **Option C (Cardiac decompensation):** This is a late and severe complication. Congestive heart failure (CHF) is the most common cause of death in epidemic dropsy, but it is not the presenting feature. * **Option D (Sarcoid):** These are small, raised, vascular fleshy nodules (resembling sarcoid tumors) found on the skin or mucous membranes. While highly characteristic of the disease, they appear later in the clinical course, not as the first sign. **High-Yield Clinical Pearls for NEET-PG:** * **Toxic Agent:** Sanguinarine (inhibits Pyruvate Dehydrogenase). * **Diagnostic Test:** **Nitric Acid Test** (turns oil orange-red) or Paper Chromatography (most sensitive). * **Triad of Epidemic Dropsy:** Edema, Cardiac failure, and **Glaucoma** (due to increased production of aqueous humor). * **Key complication:** Glaucoma is a frequent finding and should be screened for in all patients.

Question 913: A drug that reduces mortality but does not cure a disease will gradually lead to which of the following?

A. Increase in prevalence with no change in incidence (Correct Answer)
B. Increase in incidence and decrease in prevalence
C. Decrease in incidence and prevalence
D. Increase in incidence and prevalence

Explanation: ### Explanation The relationship between incidence, prevalence, and duration of a disease is defined by the formula: **Prevalence (P) = Incidence (I) × Mean Duration (D)** #### 1. Why Option A is Correct * **Incidence:** This refers to the number of **new cases** occurring in a population. A drug that reduces mortality or improves survival does not prevent the disease from occurring; therefore, the incidence remains unchanged. * **Prevalence:** This refers to the **total number of existing cases** (old + new). When a drug prevents death but does not provide a cure, patients live longer with the disease. This increases the **duration (D)** of the illness. Since Prevalence is a product of Incidence and Duration, an increase in duration directly leads to an increase in prevalence. #### 2. Why Other Options are Wrong * **Option B & D:** These are incorrect because the drug described does not affect the risk factors or the rate at which new cases develop. Therefore, **Incidence cannot increase** based on the drug's mechanism of reducing mortality. * **Option C:** This would occur if a drug were **preventive** (decreasing incidence) and **curative** (decreasing prevalence by shortening duration). #### 3. NEET-PG High-Yield Pearls * **Prevalence $\approx$ I × D:** This formula is valid only when the incidence and duration are stable (Steady State). * **Factors increasing Prevalence:** Better reporting/diagnosis, prolongation of life (without cure), and in-migration of cases. * **Factors decreasing Prevalence:** High case-fatality rate (quick death), high cure rate, and out-migration of cases. * **Incidence** is the best indicator for controlling acute outbreaks and studying the etiology (causation) of a disease.

Question 914: When a drug is evaluated for its usefulness in controlled conditions, what is it termed as a trial signifying?

A. Efficacy (Correct Answer)
B. Effectiveness
C. Efficiency
D. Effect modification

Explanation: ### Explanation The correct answer is **A. Efficacy**. In epidemiology and clinical trials, the distinction between efficacy, effectiveness, and efficiency is a high-yield concept based on the conditions under which a drug or intervention is tested. **1. Why Efficacy is Correct:** **Efficacy** refers to the performance of an intervention under **ideal, controlled conditions** (e.g., a Phase III Randomized Controlled Trial). It answers the question: *"Does the drug work under optimal circumstances?"* In these settings, patient compliance is high, the study population is highly selected (excluding comorbidities), and monitoring is rigorous. **2. Why the Other Options are Incorrect:** * **B. Effectiveness:** This measures how a drug performs in **real-world, routine clinical practice**. Unlike efficacy, effectiveness accounts for factors like poor patient compliance, diverse populations with comorbidities, and varying provider skills. It answers: *"Does the drug work in the real world?"* * **C. Efficiency:** This relates to the **cost-benefit** or cost-effectiveness of an intervention. It measures the results achieved in relation to the resources (money, time, manpower) consumed. It answers: *"Is it worth the cost?"* * **D. Effect Modification:** This is a statistical concept (interaction) where the magnitude of the effect of an exposure on an outcome differs depending on the level of a third variable (e.g., a drug working better in children than in adults). **3. NEET-PG High-Yield Pearls:** * **Phase II & III Trials:** Primarily measure **Efficacy**. * **Phase IV (Post-marketing surveillance):** Primarily measures **Effectiveness** and long-term safety. * **The "Rule of Three":** * **Efficacy:** Ideal conditions (Can it work?) * **Effectiveness:** Real conditions (Does it work?) * **Efficiency:** Resource-oriented (Is it worth it?) * **Note:** RCTs are the gold standard for establishing **Efficacy** because randomization minimizes bias and confounding.

Question 915: Cluster testing is useful in detecting cases of which of the following?

A. Measles
B. Unimmunized children
C. Sexually transmitted infections (Correct Answer)
D. Completely immunized children in the age group 12-23 months

Explanation: **Explanation:** **Cluster testing** (also known as contact tracing or partner notification) is a specialized epidemiological technique used to identify infected individuals by investigating the social and sexual networks of a known index case. **1. Why Sexually Transmitted Infections (STIs) is correct:** STIs do not occur randomly in a population; they tend to occur in "clusters" among individuals sharing common risk behaviors or sexual networks. In cluster testing, once a case is diagnosed, the investigator identifies and tests their sexual partners and social contacts (the cluster). This is the most effective way to break the chain of transmission for infections like Syphilis, Gonorrhea, and HIV, where asymptomatic carriers often unknowingly spread the disease. **2. Why other options are incorrect:** * **Measles:** This is a highly contagious respiratory infection spread via droplets. Control relies on **outbreak investigation** and mass immunization rather than tracing specific social clusters. * **Unimmunized/Immunized children (12-23 months):** To estimate immunization coverage, the WHO recommends the **30-cluster sampling technique**. This is a *sampling methodology* used for surveys to estimate prevalence/coverage in a community, which is distinct from "cluster testing" used for case-finding in infectious diseases. **High-Yield Pearls for NEET-PG:** * **Cluster Testing:** Best for STIs (Syphilis is the classic example). * **30-Cluster Sampling:** Used for EPI (Expanded Programme on Immunization) coverage surveys. It involves 30 clusters of 7 or 10 children each. * **Snowball Sampling:** A qualitative research method often used to reach "hidden populations" (e.g., IV drug users) which shares some conceptual similarities with cluster testing.

Question 916: All of the following are true for Hepatitis B EXCEPT one?

A. Vertical transmission is more important than horizontal transmission. (Correct Answer)
B. Age of onset determines the prognosis.
C. The period of communicability lasts several months.
D. The virus can be found in the blood one month before the onset of jaundice.

Explanation: **Explanation:** The correct answer is **A**. In the context of global epidemiology, **horizontal transmission** (via percutaneous/permucosal exposure to infected blood or body fluids, such as needle sharing or sexual contact) is considered more significant and frequent than vertical transmission. While vertical transmission (mother-to-child) is a major route in high-prevalence areas (like parts of Asia), horizontal transmission remains the predominant mode of spread globally and in many community settings. **Analysis of other options:** * **Option B (Age of onset determines prognosis):** This is **true**. The risk of developing chronic Hepatitis B is inversely proportional to age. Approximately 90% of infected infants become chronic carriers, compared to only 5–10% of adults. * **Option C (Period of communicability):** This is **true**. HBV is highly infectious. The period of communicability starts several weeks before the onset of symptoms and can persist for several months in acute cases, or for years/lifetime in chronic carriers. * **Option D (Virus in blood before jaundice):** This is **true**. HBsAg typically appears in the blood 1 to 2 months after exposure and can be detected 2 to 6 weeks before the onset of clinical symptoms or jaundice. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** 45–180 days (Average: 60–90 days). * **Infectivity:** HBV is 50–100 times more infectious than HIV. * **Serology:** **HBsAg** is the first marker to appear; **Anti-HBs** indicates immunity; **HBeAg** indicates high viral replication and maximum infectivity. * **Post-Exposure Prophylaxis:** Should ideally be given within 24 hours (HBIG + Vaccine).

Question 917: 91-96% of polio virus infections cause which of the following?

A. Subclinical infection (Correct Answer)
B. Above polio
C. Non-paralytic polio
D. Paralytic polio

Explanation: In Polio epidemiology, the clinical spectrum is often described as an **"iceberg phenomenon,"** where the vast majority of cases remain hidden below the surface. ### **Explanation of the Correct Answer** **A. Subclinical infection:** In approximately **91–96%** of cases, Poliovirus infection is entirely asymptomatic (subclinical). The virus replicates in the gut and is excreted in the feces, inducing immunity without causing any clinical symptoms. These individuals act as the primary reservoirs for spreading the virus in the community. ### **Explanation of Incorrect Options** * **B. Abortive polio (Minor Illness):** This occurs in about **4–8%** of infections. It presents as a non-specific viral syndrome (fever, malaise, sore throat) without neurological involvement. * **C. Non-paralytic polio:** This occurs in about **1%** of cases. It presents as aseptic meningitis with symptoms like neck stiffness and back pain, but without subsequent paralysis. * **D. Paralytic polio:** This is the rarest manifestation, occurring in **less than 1%** (roughly 0.1% to 0.5%) of infections. It involves the destruction of anterior horn cells, leading to asymmetrical flaccid paralysis. ### **High-Yield Clinical Pearls for NEET-PG** * **The Ratio:** For every 1 paralytic case in children, there are roughly 200–1000 subclinical cases (the "hidden" part of the iceberg). * **Infectivity:** Maximum infectivity occurs during the late incubation period and the first week of symptoms. * **Reservoir:** Man is the only reservoir; there are no chronic carriers. * **Specimen of Choice:** For diagnosis of a suspected case, **stool samples** are preferred over throat swabs as the virus is excreted in feces for several weeks.

Question 918: In a case-control study of a suspected association between breast cancer and the contraceptive pill, all of the following are true statements except?

A. The control should come from a population that has the same potential for breast cancer as the cases.
B. The control should exclude women known to be taking the pill at the time of the survey.
C. All the controls need to be healthy.
D. The attributable risk of breast cancer resulting from the pill may be directly measured. (Correct Answer)

Explanation: ### Explanation The correct answer is **D**, as **Attributable Risk (AR)** cannot be directly measured in a case-control study. #### Why Option D is the Correct Answer (The False Statement) In epidemiology, **Attributable Risk** and **Relative Risk** require the calculation of **Incidence** (new cases over time). Case-control studies are retrospective; they start with known outcomes (cases) and look back at exposures. Because we do not follow a healthy population over time to see who develops the disease, we cannot calculate incidence. Therefore, we cannot directly measure AR. Instead, case-control studies use the **Odds Ratio (OR)** as an estimate of risk. #### Analysis of Other Options * **Option A:** This describes the principle of **comparability**. Controls must be representative of the population that produced the cases to ensure that any difference in exposure is due to the disease, not selection bias. * **Option B:** While controls can be users or non-users of the pill, the study design must ensure that the "control" status is not confounded by the exposure being studied. However, in the context of standard MCQ logic, ensuring controls are "at risk" but distinct from the exposure-driven pathology is a common (though sometimes debated) methodological step to avoid bias. * **Option C:** Controls are generally defined as individuals free from the disease under study (breast cancer). While they don't need to be "perfectly healthy" regarding other conditions, they must be "healthy" in terms of the specific outcome being investigated. #### NEET-PG High-Yield Pearls * **Case-Control Study:** Known as a "Retrospective Study" or "Trohoc" study. It proceeds from **Effect to Cause**. * **Odds Ratio (OR):** The only measure of association derived from a case-control study. It is an estimate of Relative Risk. * **Incidence & AR:** These can only be directly calculated in **Cohort Studies** (Prospective), which proceed from **Cause to Effect**. * **Matching:** A technique used in case-control studies to eliminate the effects of confounding variables.

Question 919: What is the method of choice for mass screening of tuberculosis?

A. Tuberculin test
B. Sputum smear examination by direct microscopy (Correct Answer)
C. Mass Miniature Radiography (MMR)
D. Sputum culture

Explanation: ### Explanation **Correct Answer: B. Sputum smear examination by direct microscopy** In the context of public health and the National Tuberculosis Elimination Program (NTEP), **Sputum Smear Microscopy** remains the method of choice for mass screening and diagnosis in the community. **Why it is the correct choice:** * **Epidemiological Impact:** It identifies "open cases" (acid-fast bacilli positive), who are the primary sources of infection in the community. * **Feasibility:** It is inexpensive, rapid, easy to perform at the primary health care level, and has high specificity for identifying infectious cases. * **Public Health Priority:** The goal of mass screening is to break the chain of transmission; targeting smear-positive patients is the most cost-effective way to achieve this. **Why other options are incorrect:** * **A. Tuberculin Test:** This is a test of **sensitivity**, not disease. It indicates prior exposure or latent infection but cannot differentiate between active disease and past infection/BCG vaccination. It is not used for mass screening of active TB. * **C. Mass Miniature Radiography (MMR):** Historically used, but now discouraged due to high costs, lack of specificity (cannot differentiate TB from other lung pathologies), and the risk of radiation exposure. It also misses extrapulmonary TB. * **D. Sputum Culture:** While it is the **"Gold Standard"** for diagnosis due to its high sensitivity, it is not suitable for mass screening because it is expensive, requires specialized infrastructure, and takes 2–8 weeks to provide results. **High-Yield Clinical Pearls for NEET-PG:** * **Case Finding Tool of Choice:** Sputum Microscopy (specifically LED Fluorescence microscopy is preferred under NTEP). * **Gold Standard for Diagnosis:** Sputum Culture. * **Most Sensitive Initial Test (Modern):** CBNAAT (GeneXpert), now used as the initial diagnostic tool for all presumptive TB cases under current NTEP guidelines, though microscopy remains the classic answer for "mass screening" feasibility. * **Screening in Children:** Tuberculin test (Mantoux) is used as a diagnostic aid in children, but not for mass screening of adults.

Question 920: What percentage of multidrug-resistant tuberculosis (MDR TB) cases are extensively drug-resistant tuberculosis (XDR TB) worldwide?

A. 5%
B. 10% (Correct Answer)
C. 20%
D. 25%

Explanation: ### Explanation **Correct Answer: B. 10%** **Understanding the Concept:** According to the World Health Organization (WHO) Global Tuberculosis Reports, approximately **10%** of all multidrug-resistant tuberculosis (MDR-TB) cases are estimated to be extensively drug-resistant (XDR-TB). * **MDR-TB** is defined as resistance to at least **Isoniazid (H) and Rifampicin (R)**, the two most potent first-line drugs. * **XDR-TB** (Revised WHO definition) is MDR-TB that is also resistant to any **fluoroquinolone** AND at least one additional Group A drug (such as **Bedaquiline or Linezolid**). The 10% figure represents a critical epidemiological threshold used to monitor the escalation of drug resistance globally. **Analysis of Incorrect Options:** * **Option A (5%):** This is an underestimate. While resistance patterns vary by region, the global average has consistently hovered around the 9-10% mark in recent surveillance data. * **Options C & D (20% and 25%):** These figures are significantly higher than the global average. While some "hotspots" (certain Eastern European or Central Asian countries) may report higher proportions, they do not represent the worldwide prevalence. **High-Yield NEET-PG Pearls:** * **Pre-XDR TB:** MDR-TB that is resistant to any fluoroquinolone (but not yet to Group A drugs). * **Primary vs. Acquired Resistance:** Most XDR-TB cases in high-burden countries are now due to **primary transmission** rather than poor treatment adherence alone. * **Treatment Duration:** Under the National TB Elimination Program (NTEP), the treatment for drug-resistant TB has shifted toward shorter, all-oral regimens (e.g., BPaLM regimen: Bedaquiline, Pretomanid, Linezolid, and Moxifloxacin). * **Diagnostic Gold Standard:** Cartridge Based Nucleic Acid Amplification Test (**CBNAAT/GeneXpert**) is the initial investigation of choice to detect Rifampicin resistance.

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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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