Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1111: According to international standards, what is the minimum fetal weight to define a stillbirth?

A. 500 gm
B. 850 gm
C. 1000 gm (Correct Answer)
D. 2000 gm

Explanation: ### Explanation The definition of **Stillbirth** varies between national and international reporting standards. According to the **World Health Organization (WHO)** and international standards for global comparison, a stillbirth is defined as a baby born with no signs of life at or after **28 completed weeks of gestation** or weighing **≥1000 grams**. **Why 1000 gm is the correct answer:** For the purpose of international comparison and ICD-10/ICD-11 reporting, the WHO sets the threshold at 1000 gm (or 28 weeks). This higher threshold ensures data consistency across countries with varying levels of neonatal intensive care capabilities. **Analysis of Incorrect Options:** * **A. 500 gm:** This is the threshold for **Fetal Death** (Abortion vs. Stillbirth) in many developed countries and for domestic reporting in some regions (often corresponding to 22 weeks). While used for local statistics, it is not the *international standard* for stillbirth comparison. * **B. 850 gm:** This value does not correspond to any standard epidemiological definition for stillbirth or viability. * **D. 2000 gm:** This weight is associated with "Low Birth Weight" (LBW is <2500g) but is far above the threshold for defining stillbirth. **High-Yield Clinical Pearls for NEET-PG:** * **International Standard (WHO):** ≥1000 gm or ≥28 weeks. * **National Standard (India/RCH):** In India, for domestic reporting, the threshold is often cited as **≥500 gm or ≥22 weeks**. Always read the question carefully to see if it asks for "International" vs. "National" standards. * **Stillbirth Rate:** Calculated as (Number of stillbirths / Total births) × 1000. * **Perinatal Mortality Rate (PMR):** Includes late fetal deaths (stillbirths) plus early neonatal deaths (first 7 days of life) per 1000 total births.

Question 1112: In a study, there were 35 cases of lung carcinoma and 82 controls. Of the cases, 33 had a positive history of smoking, and 55 of the controls had a positive history of smoking. What is the odds ratio?

A. 8 (Correct Answer)
B. 20
C. 50
D. 100

Explanation: ### Explanation **1. Understanding the Concept: Odds Ratio (OR)** The Odds Ratio is the measure of association used in **Case-Control studies**. It quantifies the relationship between an exposure (smoking) and an outcome (lung carcinoma). It is defined as the ratio of the odds of exposure among cases to the odds of exposure among controls. **2. Calculation Steps** To calculate the OR, we first arrange the data into a **2x2 Contingency Table**: | | Cases (Lung CA) | Controls (No CA) | | :--- | :---: | :---: | | **Exposed (Smokers)** | 33 (a) | 55 (b) | | **Non-Exposed (Non-smokers)** | 2 (c) | 27 (d) | * **Cases:** Total = 35. Smokers (a) = 33. Non-smokers (c) = 35 - 33 = 2. * **Controls:** Total = 82. Smokers (b) = 55. Non-smokers (d) = 82 - 55 = 27. **Formula:** $OR = \frac{a \times d}{b \times c}$ (Cross-product ratio) $OR = \frac{33 \times 27}{55 \times 2} = \frac{891}{110} = 8.1$ Rounding to the nearest whole number, the **Odds Ratio is 8**. This means smokers are 8 times more likely to develop lung carcinoma compared to non-smokers. **3. Analysis of Incorrect Options** * **B, C, and D:** These values are mathematically incorrect based on the provided data. They would only be reached if the number of non-exposed cases (c) was significantly lower or the exposed cases (a) were significantly higher. **4. NEET-PG High-Yield Pearls** * **Study Design:** Odds Ratio is the only measure of association for Case-Control studies because the incidence cannot be calculated (the researcher determines the number of cases/controls). * **Interpretation:** * OR > 1: Positive association (Risk factor). * OR = 1: No association. * OR < 1: Negative association (Protective factor). * **Rare Disease Assumption:** If a disease is rare, the Odds Ratio provides a good approximation of the **Relative Risk (RR)**.

Question 1113: Out of 10 relatives exposed to a primary case of measles, 6 developed measles. What is the secondary attack rate of measles?

A. 60%
B. 66.6% (Correct Answer)
C. 40%
D. 16.6%

Explanation: ### Explanation **Secondary Attack Rate (SAR)** is a measure of the communicability of an infectious disease within a closed group (like a household). It represents the number of exposed persons who develop the disease within the incubation period following exposure to a **primary case**. **The Formula:** $$\text{SAR} = \frac{\text{Number of exposed persons developing the disease within the incubation period}}{\text{Total number of susceptible contacts exposed}} \times 100$$ **Calculation for this Question:** * **Numerator:** 6 (the relatives who developed measles). * **Denominator:** 9 (Total relatives exposed minus the primary case). * *Note:* The primary case is the source of infection and is **excluded** from the denominator because they cannot "catch" the disease from themselves. * **Calculation:** $(6 / 9) \times 100 = 66.6\%$. --- ### Analysis of Options: * **B (66.6%) - Correct:** Correctly excludes the primary case from the denominator (6/9). * **A (60%) - Incorrect:** This is a common trap where the primary case is mistakenly included in the denominator (6/10). * **C (40%) - Incorrect:** This represents the "escape rate" (those who did not get sick). * **D (16.6%) - Incorrect:** This value has no epidemiological relevance to this scenario. --- ### NEET-PG High-Yield Pearls: 1. **Denominator Rule:** Always subtract the primary case(s) from the total population exposed. 2. **Measles SAR:** Measles has one of the highest SARs (often >80-90% in unvaccinated populations), making it a benchmark for highly infectious diseases. 3. **Clinical Utility:** SAR is used to determine the effectiveness of prophylactic measures (like post-exposure vaccination) and to evaluate the infectivity of a specific pathogen. 4. **Primary vs. Index Case:** The **Primary case** is the first case to introduce the infection into the group; the **Index case** is the first case to come to the attention of the investigator. They are often, but not always, the same person.

Question 1114: What amount of bleaching powder is necessary to disinfect choleraic stools?

A. 50 gm/lit (Correct Answer)
B. 75 gm/lit
C. 90 gm/lit
D. 100 gm/lit

Explanation: **Explanation:** The disinfection of excreta, particularly in the context of highly infectious diseases like Cholera, is a critical component of **terminal disinfection** in Community Medicine. **Why 50 gm/lit is correct:** For the disinfection of stools and vomit in a cholera outbreak, the standard recommendation is to use **bleaching powder (Calcium Hypochlorite)** at a concentration of **50 grams per liter** (or 5% solution). This high concentration is necessary because organic matter (fecal material) rapidly neutralizes free chlorine. A lower dose would be insufficient to penetrate the organic load and achieve the required germicidal effect to kill *Vibrio cholerae*. The mixture should be allowed to stand for at least **2 hours** for complete disinfection. **Analysis of Incorrect Options:** * **75 gm/lit & 90 gm/lit:** These concentrations are unnecessarily high. While they would certainly disinfect the stool, they are not the standard public health recommendation and would lead to wastage of resources and increased chemical irritation. * **100 gm/lit:** This represents a 10% solution. While 10% bleach is sometimes used for large blood spills in laboratory settings, it is not the standard protocol for choleraic stool management in field epidemiology. **High-Yield Clinical Pearls for NEET-PG:** * **Contact Time:** Always remember that for stool disinfection, the contact time is **2 hours**. * **Urine Disinfection:** For disinfecting urine, the same concentration (50 gm/lit) is used, but the contact time can be reduced to **10 minutes**. * **Chlorine Demand:** The reason we use such a high dose (50g) compared to water disinfection (where we use milligrams) is the high **"Chlorine Demand"** of organic matter. * **Bleaching Powder Composition:** Fresh bleaching powder contains approximately **33% available chlorine**. It is unstable and loses chlorine content on exposure to air, light, or moisture.

Question 1115: Which of the following is NOT characteristic of a point source epidemic?

A. Person-to-person transmission (Correct Answer)
B. Clustering of cases within a short period of time
C. Epidemic curve rises and falls sharply
D. All cases usually develop within one incubation period

Explanation: ### Explanation In epidemiology, a **Point Source Epidemic** occurs when a group of susceptible individuals is exposed to a common infectious agent or toxin simultaneously or over a very short period. **Why Option A is the Correct Answer:** Point source epidemics are characterized by a **common vehicle** (e.g., contaminated food at a wedding or a specific water source). There is **no person-to-person (prospective) transmission**. If an epidemic involves person-to-person spread, it is classified as a **Propagated Epidemic**, which shows a gradual rise and multiple peaks. **Analysis of Incorrect Options:** * **Option B (Clustering of cases):** Because the exposure is simultaneous, cases appear almost all at once, leading to a concentrated "cluster" in time. * **Option C (Sharp rise and fall):** The epidemic curve is typically **unimodal** (single peak). It rises abruptly as people fall ill and falls sharply once the source is removed or the incubation period ends. * **Option D (One incubation period):** Since the exposure is a one-time event, all cases occur within the range of a single incubation period of the disease. **NEET-PG High-Yield Pearls:** 1. **Epidemic Curve:** Point source curves are "explosive" and positively skewed. 2. **Median Incubation Period:** Can be calculated from the peak of a point source epidemic curve. 3. **Continuous Common Source:** If the exposure is prolonged (e.g., a contaminated well not closed), the curve will have a plateau instead of a sharp peak (e.g., the Broad Street Pump cholera outbreak). 4. **Secondary Waves:** If you see a point source curve followed by a smaller second peak, it suggests secondary person-to-person spread.

Question 1116: Regarding poliovirus responsible for poliomyelitis, all are true except:

A. Type 1 is responsible for most epidemics
B. Type 1 is most common in India
C. Type 3 is most common in India (Correct Answer)
D. Type 2 is eradicated worldwide

Explanation: ### Explanation **1. Why Option C is the Correct Answer (The "Except" Statement):** In India, as well as globally, **Type 1 Poliovirus** has historically been the most common cause of both endemic poliomyelitis and major epidemics. Type 3 is the second most common but has never been the predominant strain in India. Therefore, the statement "Type 3 is most common in India" is factually incorrect, making it the right choice for this "except" question. **2. Analysis of Other Options:** * **Option A (Type 1 is responsible for most epidemics):** This is **True**. Type 1 is the most paralytogenic and highly infectious strain, historically responsible for the vast majority of outbreaks worldwide. * **Option B (Type 1 is most common in India):** This is **True**. Before eradication, Type 1 was the most frequently isolated serotype in the Indian subcontinent. * **Option C (Type 2 is eradicated worldwide):** This is **True**. The Global Commission for the Certification of Poliomyelitis Eradication declared Wild Poliovirus Type 2 (WPV2) eradicated in **September 2015** (last case detected in 1999). **3. High-Yield Clinical Pearls for NEET-PG:** * **Eradication Status:** * **WPV Type 2:** Eradicated (2015). * **WPV Type 3:** Eradicated (Declared in October 2019; last case seen in Nigeria, 2012). * **WPV Type 1:** Only type currently circulating (endemic in Afghanistan and Pakistan). * **India Status:** India was declared "Polio Free" by the WHO on **March 27, 2014** (Last case: Howrah, West Bengal, Jan 2011). * **Vaccine Shift:** Due to Type 2 eradication, the **trivalent OPV (tOPV)** was replaced by **bivalent OPV (bOPV)** containing only Types 1 and 3 to prevent Vaccine-Derived Poliovirus (VDPV) Type 2. * **Infectivity:** Polio is most infectious during the late incubation period and the first week of clinical illness. The virus is excreted in stools for 6–8 weeks.

Question 1117: Attributable risk is an important parameter for whom?

A. Clinicians
B. Epidemiologists (Correct Answer)
C. Public health program managers
D. Microbiologists

Explanation: **Explanation:** **Attributable Risk (AR)**, also known as Risk Difference, is defined as the difference in the incidence of a disease between an exposed group and a non-exposed group. It indicates the magnitude of the disease risk that can be directly attributed to a specific exposure. **Why Epidemiologists?** Epidemiologists are primarily concerned with identifying the **causative factors** and the strength of association between an exposure and an outcome. Attributable Risk is the best measure to determine the **etiological role** of a factor in causing a disease. It answers the question: "How much of the disease is due to the exposure?" This makes it a vital tool for researchers and epidemiologists to establish causality. **Analysis of Incorrect Options:** * **Clinicians:** Clinicians are more interested in **Relative Risk (RR)**, which helps in predicting the likelihood of an individual developing a disease based on risk factors. * **Public Health Program Managers:** They prioritize **Population Attributable Risk (PAR)**. PAR indicates how much of the disease burden can be eliminated from the *entire population* if the exposure is removed, which is essential for prioritizing resources and policy-making. * **Microbiologists:** Their focus is on the isolation, identification, and characteristics of pathogens rather than population-level risk statistics. **NEET-PG High-Yield Pearls:** * **Attributable Risk (AR) Formula:** $\text{Incidence among exposed} (I_e) - \text{Incidence among non-exposed} (I_o)$. * **Relative Risk (RR):** Measures the *strength* of association (best for clinicians). * **Attributable Risk (AR):** Measures the *impact* of an exposure (best for epidemiologists). * **Population Attributable Risk (PAR):** Measures the *benefit* to the community if a risk factor is removed (best for public health administrators).

Question 1118: Benzathine Penicillin prophylaxis for rheumatic fever falls under which category of prevention?

A. Primary prevention
B. Primordial Prevention
C. Secondary prevention (Correct Answer)
D. Tertiary prevention

Explanation: ### Explanation **Correct Answer: C. Secondary Prevention** **Why it is Secondary Prevention:** Secondary prevention aims to halt the progress of a disease at its incipient stage and prevent complications. In the context of Rheumatic Heart Disease (RHD), the "disease" (Rheumatic Fever) has already occurred. Benzathine Penicillin prophylaxis is administered to patients who have already suffered an initial attack of Acute Rheumatic Fever (ARF). The goal is to prevent recurrent streptococcal infections that would trigger further episodes of ARF, thereby preventing the development or worsening of permanent valvular damage (RHD). Since it involves **early diagnosis and prompt treatment** to prevent complications in an already affected individual, it is classified as secondary prevention. **Why other options are incorrect:** * **Primordial Prevention:** This involves preventing the emergence of risk factors (e.g., improving socio-economic conditions and housing to prevent overcrowding). * **Primary Prevention:** This aims to prevent the *first* occurrence of a disease. In this context, treating a sore throat (Streptococcal pharyngitis) with antibiotics to prevent the initial onset of Rheumatic Fever is primary prevention. * **Tertiary Prevention:** This focuses on limiting disability and rehabilitation after the disease has caused significant damage (e.g., cardiac surgery/valve replacement for established RHD). **High-Yield Clinical Pearls for NEET-PG:** * **Primary Prevention of RF:** Prompt treatment of Group A Streptococcal (GAS) pharyngitis. * **Secondary Prevention of RF:** Continuous prophylaxis with Benzathine Penicillin G (1.2 million units IM every 3–4 weeks). * **Duration of Prophylaxis:** * *RF without carditis:* 5 years or until age 21 (whichever is longer). * *RF with carditis but no persistent valvular disease:* 10 years or until age 21 (whichever is longer). * *RF with persistent valvular disease:* 10 years or until age 40 (sometimes lifelong).

Question 1119: Soil may act as a reservoir for all except:

A. Brucellosis (Correct Answer)
B. Anthrax
C. Tetanus
D. Coccidiomycosis

Explanation: **Explanation:** The core concept in this question is the distinction between **soil-borne pathogens** (which can survive as saprophytes or spores in the environment) and **obligate animal pathogens**. **1. Why Brucellosis is the Correct Answer:** Brucellosis is a zoonotic infection caused by *Brucella* species. The primary **reservoir** for Brucellosis is **infected animals** (cattle, goats, sheep, and pigs). The bacteria are intracellular pathogens that do not form spores and cannot survive indefinitely in the soil. While soil can be contaminated by the urine or placental products of infected animals, it acts only as a temporary vehicle for transmission, not a reservoir where the organism naturally lives or multiplies. **2. Analysis of Incorrect Options:** * **Anthrax (*Bacillus anthracis*):** This bacterium forms highly resilient spores that can persist in the soil for decades. Soil is a major environmental reservoir. * **Tetanus (*Clostridium tetani*):** The spores are ubiquitous in soil and the intestinal tracts of animals. Soil is the primary reservoir from which human infection (via wounds) occurs. * **Coccidioidomycosis:** This is a fungal infection (Valley Fever). The fungus *Coccidioides immitis* lives naturally in the soil of endemic arid regions, making soil its definitive reservoir. **3. NEET-PG High-Yield Pearls:** * **Reservoir vs. Source:** A reservoir is the natural habitat where an infectious agent lives and multiplies. A source is the immediate object/person from which the agent passes to the host. * **Other Soil Reservoirs:** Mycobacteria (non-tuberculous), Gas gangrene (*C. perfringens*), and various Helminths (Hookworm, Ascariasis - though these often require soil for maturation, the soil acts as a developmental site). * **Brucellosis Transmission:** Most commonly via unpasteurized dairy products or direct contact with animal tissues (occupational hazard for veterinarians/butchers).

Question 1120: All of the following are eradicable diseases EXCEPT?

A. Tuberculosis (Correct Answer)
B. Guinea worm
C. Polio
D. Measles

Explanation: **Explanation** The concept of **disease eradication** refers to the permanent reduction to zero of the worldwide incidence of an infection caused by a specific agent. For a disease to be eradicable, it must typically have no animal reservoir, an effective intervention (like a vaccine), and a simple diagnostic tool. **Why Tuberculosis is the correct answer:** Tuberculosis (TB) is considered **non-eradicable** with current technology. The primary reasons include: * **Latent Infection:** *M. tuberculosis* can remain dormant in the body for decades without causing symptoms, making it impossible to identify and clear all carriers. * **Long Treatment Duration:** Unlike a single-dose vaccine, TB requires months of multi-drug therapy, leading to compliance issues and drug resistance. * **Environmental Persistence:** The bacteria can survive in the environment under specific conditions. Therefore, the global goal for TB is **Elimination** (defined as <1 case per million population), not Eradication. **Analysis of Incorrect Options:** * **Guinea Worm (Dracunculiasis):** Targeted for eradication. It has no significant animal reservoir and can be prevented by simple water filtration. It is on the verge of being the second human disease eradicated. * **Polio:** Targeted for eradication. It has no animal reservoir and highly effective vaccines (OPV/IPV) exist. * **Measles:** Theoretically eradicable because humans are the only reservoir, an accurate diagnostic test exists, and a highly effective vaccine is available. **NEET-PG High-Yield Pearls:** 1. **Only Eradicated Diseases:** Smallpox (1980) is the only human disease eradicated. Rinderpest (2011) is the only animal disease eradicated. 2. **Eliminated in India:** Smallpox, Guinea Worm (2000), Polio (2014), Maternal & Neonatal Tetanus (2015), and Yaws (2016). 3. **Criteria for Eradication:** Human-only reservoir, short incubation period, easily identifiable clinical cases, and effective intervention.

Practice by Chapter

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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