Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 91: What is the measure of infectivity of a disease?

A. Incidence rate
B. Prevalence rate
C. Secondary attack rate (Correct Answer)
D. Case fatality rate

Explanation: ### Explanation **Secondary Attack Rate (SAR)** is the correct measure of **infectivity**. Infectivity refers to the ability of an infectious agent to invade and multiply in a host. SAR specifically measures the number of exposed persons who develop the disease within the incubation period following exposure to a primary case. * **Formula:** (Number of exposed persons developing disease / Total number of exposed susceptible contacts) × 100. * It is the best indicator of how easily a disease spreads within a closed group (e.g., a household). **Analysis of Incorrect Options:** * **A. Incidence Rate:** Measures the number of **new cases** in a population over a specific period. It indicates the **rate of occurrence** and the risk of contracting the disease, but not specifically the ease of transmission from person to person. * **B. Prevalence Rate:** Measures the **total number of cases** (old + new) existing in a population at a given time. it is used to estimate the **burden of disease** and plan for health resources. * **C. Case Fatality Rate (CFR):** Measures the **virulence** or killing power of a disease. It is the proportion of deaths among diagnosed cases (Deaths / Total Cases × 100). **High-Yield Clinical Pearls for NEET-PG:** * **Virulence:** Measured by Case Fatality Rate. * **Pathogenicity:** The ability to produce clinical disease (Ratio of clinical cases to total infected). * **Generation Time:** The interval between receipt of infection and maximal infectivity (used to track spread instead of incubation period in diseases with subclinical stages). * **SAR for Measles:** Highly infectious (~80% in susceptible households).

Question 92: Under the National Poliomyelitis Elimination Programme, how is poliomyelitis diagnosed?

A. Antibody titre rise in blood
B. Viral isolation in stool (Correct Answer)
C. Viral microscopy in stool
D. Clinical examination

Explanation: ### Explanation **Correct Answer: B. Viral isolation in stool** Under the **Global Polio Eradication Initiative (GPEI)** and India’s National Programme, the gold standard for diagnosing poliomyelitis is the **isolation of wild poliovirus from stool samples**. This is part of **Acute Flaccid Paralysis (AFP) surveillance**. The virus is excreted in the feces for several weeks, making stool the most reliable specimen. For a "conclusive" diagnosis, **two "adequate" stool samples** must be collected 24–48 hours apart, within **14 days** of the onset of paralysis. The samples are processed via viral culture (isolation) in WHO-accredited laboratories using specific cell lines (e.g., RD or L20B cells). **Why other options are incorrect:** * **A. Antibody titre rise:** While serology can show a rise in antibodies, it cannot distinguish between antibodies from natural infection, oral polio vaccine (OPV), or prior immunization. It is not used for primary diagnosis in elimination programmes. * **C. Viral microscopy:** Polioviruses are too small (27–30 nm) to be seen under a light microscope. While electron microscopy can visualize them, it cannot differentiate between poliovirus and other enteroviruses. * **D. Clinical examination:** While AFP is detected clinically, the final diagnosis of "Poliomyelitis" requires virological confirmation to differentiate it from other causes of paralysis like Guillain-Barré Syndrome (GBS). **High-Yield Pearls for NEET-PG:** * **AFP Surveillance Criteria:** Includes all children **<15 years** with sudden onset flaccid paralysis or any person of any age where polio is suspected. * **Reverse Cold Chain:** Stool samples must be transported at **2–8°C** from the field to the lab to keep the virus viable. * **India Status:** India was declared "Polio Free" by the WHO on **March 27, 2014**. * **Last Case:** The last case of Wild Poliovirus in India was reported from Howrah, West Bengal (January 13, 2011).

Question 93: The high stationary stage of the demographic cycle is characterized by:

A. High birth rate, low death rate
B. Low birth rate, high death rate
C. Low birth rate, low death rate
D. High birth rate, high death rate (Correct Answer)

Explanation: **Explanation:** The **Demographic Cycle** describes the stages of population growth based on the relationship between birth rates and death rates. **1. Why Option D is Correct:** The **High Stationary Stage (Stage 1)** is characterized by both a **high birth rate** and a **high death rate**. In this stage, the population remains stationary because the high number of births is cancelled out by high mortality due to poor sanitation, lack of medical facilities, and frequent epidemics. Historically, most of the world was in this stage before the industrial revolution. **2. Why the Other Options are Incorrect:** * **Option A (High birth rate, low death rate):** This describes the **Early Expanding Stage (Stage 2)**. Here, death rates begin to fall due to improved healthcare, but birth rates remain high, leading to a population explosion. * **Option B (Low birth rate, high death rate):** This is not a standard stage in the demographic cycle. A high death rate combined with a low birth rate would lead to rapid population extinction. * **Option C (Low birth rate, low death rate):** This describes the **Low Stationary Stage (Stage 4)**. The population becomes stable again, but at a much higher total level than in Stage 1. **Clinical Pearls & High-Yield Facts for NEET-PG:** * **India’s Current Status:** India is currently in the **Late Expanding Stage (Stage 3)**, where the birth rate is falling but still exceeds the death rate. * **Stage 5 (Declining):** Characterized by a birth rate lower than the death rate (e.g., Germany, Hungary, Japan), leading to a decreasing population. * **Key Indicator:** The transition from Stage 1 to Stage 2 is usually triggered by improvements in **public health and nutrition**, while the transition from Stage 3 to Stage 4 is driven by **socio-economic changes and family planning**.

Question 94: Which cancer is best suited for screening procedures?

A. Prostate cancer
B. Colon cancer (Correct Answer)
C. Gastric cancer
D. None of the above

Explanation: **Explanation:** The suitability of a cancer for screening depends on the **Wilson and Jungner criteria**, which state that the disease must have a recognizable latent or early symptomatic stage and a natural history that is well-understood. **Why Colon Cancer is the Correct Answer:** Colon cancer is the "ideal" candidate for screening because it follows a predictable **adenoma-to-carcinoma sequence**. This progression is slow (often taking 10–15 years), providing a wide "window of opportunity" for detection. Screening methods like Colonoscopy or Fecal Immunochemical Tests (FIT) can identify and remove precancerous polyps, not just detecting cancer early but actually **preventing** its occurrence. This significantly reduces both morbidity and mortality. **Analysis of Incorrect Options:** * **Prostate Cancer:** Screening (via PSA) remains controversial due to a high rate of **overdiagnosis**. Many prostate cancers are indolent and would never cause symptoms during a patient's lifetime; treating them can lead to unnecessary complications like impotence or incontinence. * **Gastric Cancer:** While screening (endoscopy) is performed in high-prevalence areas like Japan, it is not universally recommended. The natural history is less predictable than colon cancer, and the procedure is invasive and not cost-effective for general population screening in most regions. **High-Yield Clinical Pearls for NEET-PG:** * **Lead Time:** The period between early detection by screening and the time of usual clinical diagnosis. * **Length Bias:** Screening tends to detect slowly progressing cases more easily than rapidly progressing ones. * **Best Screening Tool:** Colonoscopy is the gold standard for colon cancer; Pap smear is the classic example for Cervical Cancer (another highly "screenable" cancer). * **Iceberg Phenomenon:** Screening aims to reveal the "submerged portion" of the iceberg (undiagnosed cases) in the community.

Question 95: What is Incidence Rate?

A. It represents the number of old cases in the community
B. It represents the number of new cases in the community (Correct Answer)
C. It represents the number of total cases in the community
D. It represents the number of cured cases in the community

Explanation: ### Explanation **Incidence Rate** is a fundamental measure of morbidity in epidemiology. It is defined as the number of **new cases** of a specific disease occurring in a defined population during a specific period of time. #### Why Option B is Correct: The core concept of incidence is the "rate of occurrence." It measures the speed at which a disease is spreading or developing. Mathematically, it is calculated as: $$\text{Incidence} = \frac{\text{Number of NEW cases during a specific period}}{\text{Population at risk during that period}} \times 1000$$ It is a longitudinal measure, meaning it requires a follow-up study (like a **Cohort Study**) to identify new events. #### Why Other Options are Incorrect: * **Option A & C:** These refer to **Prevalence**. Prevalence includes both old and new cases (Total cases) existing at a specific point or period in time. It is a cross-sectional measure. * **Option D:** The number of cured cases is not a standard epidemiological rate used to define incidence; rather, it relates to the prognosis or the effectiveness of a treatment intervention. #### High-Yield Clinical Pearls for NEET-PG: * **Study Design:** The best study design to calculate Incidence is a **Prospective Cohort Study**. * **Prevalence vs. Incidence:** Remember the formula: **Prevalence (P) = Incidence (I) × Mean Duration of disease (D)**. * **Utility:** Incidence is best for studying the **etiology (causation)** of a disease and for evaluating the efficacy of preventive programs. * **Attack Rate:** This is a type of incidence rate used specifically during **outbreaks** (expressed as a percentage). * **Denominator:** Note that the denominator for incidence only includes the "population at risk" (those who do not have the disease yet).

Question 96: A survey was conducted on 7,000 drivers using mobile phones and found that 700 met with an accident. In the same period, out of 3,000 drivers not using mobile phones, 30 met with an accident. Calculate the relative risk?

A. 5
B. 10 (Correct Answer)
C. 3.99
D. 8.339

Explanation: ### Explanation **1. Understanding the Correct Answer (B: 10)** Relative Risk (RR), also known as Risk Ratio, is the ratio of the incidence of a disease (or event) among an exposed group to the incidence among a non-exposed group. It is a key measure of association in **Cohort Studies**. * **Incidence in Exposed ($I_e$):** Drivers using mobile phones. $I_e = \frac{\text{Accidents}}{\text{Total Exposed}} = \frac{700}{7,000} = 0.1$ (or 10%) * **Incidence in Non-exposed ($I_ne$):** Drivers not using mobile phones. $I_ne = \frac{\text{Accidents}}{\text{Total Non-exposed}} = \frac{30}{3,000} = 0.01$ (or 1%) * **Relative Risk (RR):** $\frac{I_e}{I_ne} = \frac{0.1}{0.01} = \mathbf{10}$ This means drivers using mobile phones are 10 times more likely to meet with an accident compared to those who do not. **2. Why Other Options are Incorrect** * **Option A (5):** This would result if the incidence in the exposed group was only 0.05 (5%). * **Option C (3.99):** This is a distractor often seen in questions involving complex Odds Ratio calculations, but it is mathematically irrelevant here. * **Option D (8.33):** This might be reached if a student incorrectly uses the "Attributable Risk" formula or makes a calculation error in the denominator. **3. High-Yield Clinical Pearls for NEET-PG** * **RR = 1:** No association between exposure and outcome. * **RR > 1:** Positive association (Risk factor). * **RR < 1:** Negative association (Protective factor, e.g., vaccines). * **Study Design:** RR is calculated from **Cohort Studies** (Prospective), whereas Odds Ratio (OR) is typically calculated from **Case-Control Studies** (Retrospective). * **Attributable Risk (AR):** $I_e - I_ne$. In this case, $0.1 - 0.01 = 0.09$ (90 per 1000), representing the amount of disease prevented if the exposure is removed.

Question 97: To test the association between a risk factor and a disease, which of the following represents the weakest study design?

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

Explanation: To test the association between a risk factor and a disease, the strength of evidence depends on the study's ability to link individual exposure to individual outcomes. ### **Why Ecological Study is the Correct Answer** An **Ecological Study** is considered the weakest design for testing associations because the unit of observation is a **population or group** (e.g., countries, cities) rather than individuals. Because it uses aggregate data, it cannot guarantee that the individuals who developed the disease were the same ones exposed to the risk factor. This leads to the **"Ecological Fallacy"**—an error where an association observed at the group level is incorrectly assumed to apply to individuals. ### **Explanation of Incorrect Options** * **Cohort Study (C):** This is the strongest observational design. it starts with exposed and non-exposed individuals and follows them forward in time to see who develops the disease, allowing for the direct calculation of Relative Risk (RR). * **Case-Control Study (A):** Stronger than ecological studies because it compares individuals with the disease (cases) to those without (controls) to retrospectively determine exposure. It allows for the calculation of Odds Ratio (OR). * **Cross-sectional Study (D):** While it only provides a "snapshot" of prevalence and cannot establish temporal sequences, it still uses **individual-level data**, making it more robust for association than an ecological study. ### **High-Yield Clinical Pearls for NEET-PG** * **Hierarchy of Evidence (Descending order):** Meta-analysis > Systematic Review > RCT > Cohort > Case-Control > Cross-sectional > Ecological > Case Series/Report. * **Ecological Fallacy:** The hallmark limitation of ecological studies. * **Unit of Study:** * Ecological: Populations/Groups * Cross-sectional/Case-Control/Cohort: Individuals * Community Trial: Communities * **Best study for rare diseases:** Case-control. * **Best study for rare exposures:** Cohort.

Question 98: Which of the following is NOT a strategy in preventing HIV transmission?

A. Education
B. Treatment of sexually transmitted diseases
C. Anti-retroviral treatment (Correct Answer)
D. Condoms

Explanation: ### Explanation The core objective of this question is to distinguish between **preventive strategies** (aimed at the general or high-risk population to stop acquisition) and **clinical management** (aimed at infected individuals). **Why Option C is the Correct Answer:** In the context of traditional public health strategies for HIV, **Anti-retroviral treatment (ART)** is primarily classified as a **Tertiary Prevention** measure (or clinical management) aimed at reducing morbidity and mortality in those already infected. While modern concepts like "Treatment as Prevention" (TasP) exist, in standard epidemiological frameworks for exams like NEET-PG, ART is categorized as a treatment modality rather than a primary prevention strategy for the community. **Analysis of Incorrect Options:** * **A. Education:** This is a cornerstone of **Primary Prevention**. Behavior change communication (BCC) regarding safe practices and needle sharing is the most cost-effective way to prevent transmission. * **B. Treatment of STDs:** This is a vital preventive strategy because the presence of an untreated STD (especially ulcerative ones like Syphilis or Chancroid) causes mucosal breaks, increasing the biological vulnerability to HIV transmission by 3–10 fold. * **D. Condoms:** These are the primary "barrier" method of prevention. Consistent and correct use of condoms is the most effective intervention for preventing sexual transmission of HIV. **High-Yield Clinical Pearls for NEET-PG:** * **Window Period:** The time between infection and the appearance of detectable antibodies (usually 2–12 weeks). * **Post-Exposure Prophylaxis (PEP):** Must be started within **72 hours** of exposure; the preferred regimen is TDF + 3TC (or FTC) + DTG for 28 days. * **Indicator Disease:** Tuberculosis is the most common opportunistic infection and cause of death in HIV patients in India. * **Target:** NACO’s current goal follows the **95-95-95** target to be achieved by 2025.

Question 99: Berkesonian bias refers to which of the following?

A. Different rates of admission to the hospital (Correct Answer)
B. Interviewer's bias
C. Reduces bias
D. Systematic difference in characteristics between cases and controls

Explanation: ### Explanation **Berksonian Bias** (also known as Admission Rate Bias) is a type of selection bias that occurs specifically in **hospital-based case-control studies**. **1. Why Option A is Correct:** The bias arises because hospital patients differ from the general population. It occurs when the **probability of admission** to a hospital is different for cases and controls, often because they have multiple diseases (comorbidities). For example, if a study examines the link between respiratory disease and bone fractures using hospitalized patients, the results may show a false association because people with *both* conditions are more likely to be admitted than those with only one. This distorts the true relationship between the exposure and the outcome. **2. Why the Other Options are Incorrect:** * **Option B (Interviewer’s Bias):** This is a type of **Information/Measurement bias** where the investigator asks questions differently or probes more deeply with cases than with controls. * **Option C (Reduces bias):** Berksonian bias *introduces* systematic error; it does not reduce it. * **Option D (Systematic difference in characteristics):** While this is the general definition of **Selection Bias**, Berksonian bias is a *specific subtype* defined by the hospital admission mechanism. **3. High-Yield Clinical Pearls for NEET-PG:** * **Key Setting:** Always look for "Hospital-based study" in the question stem. * **Prevention:** The best way to avoid Berksonian bias is to select controls from the **community** rather than the hospital. * **Neyman Bias (Prevalence-Incidence Bias):** Often confused with Berksonian, this occurs when very severe or very mild cases are excluded (e.g., patients who die before reaching the hospital). * **Recall Bias:** Common in case-control studies where cases remember past exposures more vividly than controls.

Question 100: Which of the following is NOT included in Chapter XXI (Z00-Z99) of the ICD-10 classification?

A. Factors influencing health status and contact with health services
B. Symptoms, signs and abnormal clinical and laboratory findings, not elsewhere classified
C. Certain infectious and parasitic diseases (Correct Answer)
D. External causes of morbidity

Explanation: ### Explanation The **International Classification of Diseases (ICD-10)** is a standardized diagnostic tool used for epidemiological tracking and health management. It is organized into chapters based on body systems or types of conditions. **Why the correct answer is right:** * **Option C (Certain infectious and parasitic diseases):** This is categorized under **Chapter I (A00–B99)**. Since the question asks for what is **NOT** included in Chapter XXI (Z00–Z99), this is the correct choice. Chapter XXI is specifically reserved for "Factors influencing health status and contact with health services," which covers encounters with healthcare providers for reasons other than a current illness (e.g., vaccinations, screenings, or family planning). **Analysis of incorrect options:** * **Option A:** This is the literal title of **Chapter XXI (Z00–Z99)**. It includes codes for healthy individuals seeking preventive care or people with conditions that influence their health but are not current diseases. * **Option B:** This belongs to **Chapter XVIII (R00–R99)**. While it is not Chapter XXI, in the context of multiple-choice questions regarding ICD-10 structure, Option C is the most distinct "classic" chapter (Chapter I) often used to test knowledge of the sequence. * **Option D:** This belongs to **Chapter XX (V01–Y98)**. **High-Yield Clinical Pearls for NEET-PG:** * **ICD-10 Structure:** It uses an **alphanumeric** coding scheme (e.g., A00.0). It consists of 21 chapters. * **Chapter I (A00-B99):** Infectious and Parasitic diseases. * **Chapter II (C00-D48):** Neoplasms. * **Chapter XV (O00-O99):** Pregnancy, childbirth, and the puerperium (High yield for OBG). * **Chapter XXI (Z codes):** Crucial for Community Medicine as it includes **immunization status (Z23-Z26)**, contraceptive management, and medical exams. * **ICD-11:** The latest version (effective 2022) has 26 chapters and uses a different coding structure.

Practice by Chapter

Principles of Epidemiology

Practice Questions

Measures of Disease Frequency

Practice Questions

Epidemiological Study Designs

Practice Questions

Descriptive Epidemiology

Practice Questions

Analytical Epidemiology

Practice Questions

Experimental Epidemiology

Practice Questions

Screening for Disease

Practice Questions

Surveillance Systems

Practice Questions

Investigation of an Epidemic

Practice Questions

Association and Causation

Practice Questions

Modern Epidemiological Methods

Practice Questions

Critical Appraisal of Epidemiological Studies

Practice Questions

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