Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1401: A person's father had colon cancer. He had bloody stool. So, he just came for a check-up. Before he had not undergone any screening. He was recommended to do a colonoscopy for screening. Screening is which level of prevention?

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

Explanation: ***Secondary***- **Secondary prevention** involves measures like **screening** and early diagnosis to detect disease (e.g., **colorectal cancer**) in its earliest stages, allowing for timely intervention and reducing the burden of the disease. - A screening colonoscopy fits this definition perfectly, as it aims to identify **precancerous polyps** or early-stage asymptomatic cancer in an at-risk individual who has not yet been formally diagnosed. *Primordial*- **Primordial prevention** targets the underlying determinants of health and aims to prevent the establishment of risk factors themselves in the population (e.g., strict regulations on advertising unhealthy foods). - It operates at a societal level, preceding primary prevention, and is not applicable to an individual undergoing a specific medical screening test. *Primary*- **Primary prevention** aims to prevent disease onset by reducing risk factors or increasing protection *before* the disease process begins (e.g., **vaccination**, lifestyle modification, chemoprophylaxis). - Since this patient is already at high risk (family history) and presenting with an alarming symptom (**bloody stool**), the action is beyond preventing the initial exposure or onset. *Tertiary*- **Tertiary prevention** focuses on managing existing, established disease to prevent complications, reduce disability, and improve the quality of life (e.g., rehabilitation after a stroke, palliative care, or chemotherapy after a cancer diagnosis). - Screening is about early detection, whereas tertiary prevention is focused on minimizing the long-term impact of a disease that is already clinically apparent or diagnosed.

Question 1402: In a study based in Delhi, doctors followed 100 people who did exercise for a year and later they checked who all had developed coronary heart disease. Which type of study is this?

A. Case Control Study
B. Cohort study (Correct Answer)
C. Prospective Study
D. Cross Sectional Study

Explanation: ***Cohort study***- This study starts with a group of people free of the disease (**CHD**) and classifies them based on their exposure status (e.g., *exercise* vs. no exercise) and follows them forward in time (**prospectively**) to measure the incidence of the disease.- The study tracks the patients *forward* from exposure (**exercise**) to outcome (**CHD**) over a specified period (one year), which is the definitive characteristic of a **prospective cohort study**.*Case Control Study*- In this design, the study starts with the outcome (**CHD**) and retrospectively looks back (examining controls without CHD) to determine past exposure, making it unsuitable for this specific prospective tracking of exposure.- It is used primarily to estimate the **odds ratio** and is efficient for studying rare diseases; it does not measure incidence over time.*Prospective Study*- While this specific study is **prospective** (looking forward in time), this term describes the *timing* and direction of data collection, whereas **Cohort Study** is the most specific designation describing the fundamental design of following a defined exposed population.- A **prospective study** is a broad term, and the term **Cohort Study** most accurately describes the method of following an exposed group to measure disease incidence over time.*Cross Sectional Study*- This study type measures both the exposure (exercise) and the outcome (**CHD**) simultaneously at a **single point in time**, assessing prevalence rather than tracking incidence over one year.- It provides a **snapshot** and cannot establish the temporal relationship between exposure and outcome, failing to align with the follow-up design described.

Question 1403: Which of the following is the formula for sensitivity?

A. TP/(TP+FN)x100 (Correct Answer)
B. TN/(TN+FP)x100
C. TP/(TP+FP)x100
D. TN/(TN+FN)x100

Explanation: ***TP/(TP+FN)x100***- This is the formula for **sensitivity** (or True Positive Rate), which is the proportion of individuals who truly have the disease (**True Positives, TP**) who are correctly identified by the test.- The denominator $TP + FN$ accounts for all individuals who actually have the disease according to the **gold standard**, including those who tested negatively (**False Negatives, FN**).*TP/(TP+FP)x100*- This formula calculates the **Positive Predictive Value (PPV)**, which indicates the probability that a positive test result represents a true positive.- The denominator $TP + FP$ includes everyone who tested positive, regardless of their actual disease status (**True Positives** and **False Positives**).*TN/(TN+FP)x100*- This formula calculates **specificity** (or True Negative Rate), which is the proportion of individuals who are truly disease-free (**True Negatives, TN**) correctly identified by the test.- The denominator $TN + FP$ accounts for all individuals without the disease, including those who were incorrectly identified as positive (**False Positives, FP**).*TN/(TN+FN)x100*- This formula calculates the **Negative Predictive Value (NPV)**, which is the probability that a negative test result represents a true negative.- The denominator $TN + FN$ includes everyone who tested negative, reflecting the proportion of subjects with a negative test result who are truly disease-free.

Question 1404: A total of 60 patients were diagnosed with COVID-19. Out of which, 12 deaths were reported. Calculate the Case Fatality Rate?

A. 40%
B. 10%
C. 30%
D. 20% (Correct Answer)

Explanation: ***20%*** - The **Case Fatality Rate (CFR)** is calculated by dividing the number of deaths from a specific disease by the total number of confirmed cases of that disease, multiplied by **100** to get the percentage. - Based on the data: (12 deaths / 60 cases) * 100 = **0.20** * 100 = **20%**. *30%* - This result is incorrect; it would correspond to **18 deaths** out of 60 cases, not the reported 12 deaths. - A calculation error yielding **30%** does not align with the standard formula for CFR using the given figures. *40%* - This percentage represents a CFR where **24 deaths** were reported among 60 cases, which is double the actual number of fatalities. - Such a high error suggests a significant misapplication of the **CFR formula**. *10%* - This value is incorrect; **10%** CFR would be obtained if only **6 deaths** occurred (6/60 * 100). - This result significantly underestimates the **Case Fatality Rate** as it is half of the calculated actual rate.

Question 1405: In a class of 100 students, 80% students were immunised with measles, 12 were affected. What is the primary attack rate?

A. 80%
B. 70%
C. 60% (Correct Answer)
D. 50%

Explanation: ***60%*** - The **Primary Attack Rate** measures the number of new cases among the susceptible population during an outbreak; the susceptible population must first be determined by excluding the immunized students. - Calculation: The total susceptible population is 100 students - 80 immunized students = **20 susceptible contacts**. Primary Attack Rate = (12 affected / 20 susceptible) × 100 = **60%**. *80%* - This figure represents the percentage of students in the class who were **immunised** (80 out of 100), not the attack rate among the susceptible population. - Using 80 as the denominator would incorrectly calculate the rate among the protected group (12/80 = 15%). *70%* - This option is mathematically incorrect and does not result from the standard calculation of **Primary Attack Rate** using the given data (12 cases among 20 susceptible individuals). - It is likely derived from an incorrect calculation or failure to correctly identify the **susceptible population** for the denominator. *50%* - This value is incorrect, as the observed number of affected students (12) leads to a higher rate than 50% when calculated against the susceptible population (20). - A **Primary Attack Rate** of 50% would only account for 10 affected students (50% of 20 susceptible individuals).

Question 1406: A study was conducted in 3 communities (across 3 states) to measure the mean blood pressure in each community. Health workers were assigned to visit each house in the 3 communities. The mean blood pressure of each community was then compared. What is the study design called?

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

Explanation: ***Cross-sectional***- This design takes a **snapshot** of the population (the 3 communities) at a specific time, simultaneously assessing the current status of the outcome (mean **blood pressure**) in each house.- The goal is to determine the **prevalence** of a characteristic (mean blood pressure) within the defined population by studying individuals (each house) within them.*Case-control*- This design requires comparing individuals who have the outcome (**cases**) to those who do not (**controls**) by looking **retrospectively** for past exposure differences.- The current study does not involve selecting groups based on outcome status (e.g., high BP vs. normal BP) to investigate an antecedent exposure.*Cohort*- A **cohort** study follows groups based on their **exposure status** over a period of time to calculate the **incidence** (rate of new cases) of a specific outcome.- This study measures current blood pressure status in a single visit; it does not track individuals longitudinally to see who develops hypertension later.*Ecological study*- This type of study correlates aggregate data (mean outcomes) across different population groups (e.g., states or countries), where the units of analysis are **populations**, not individuals.- Although the final comparison involves community means (ecological data), the design phase involving detailed collection of individual BP data by visiting **each house** is characteristic of a primary **cross-sectional** survey.

Question 1407: A gym owner observes that individuals who drink iced tea during their workouts tend to lose more weight. What is the nature of this relationship?

A. Indirect (Correct Answer)
B. Direct
C. Spurious
D. Relative

Explanation: ***Indirect*** - This relationship is considered **indirect (mediated)** because the iced tea consumption operates through an intermediary mechanism to produce the observed outcome - The proposed pathway: iced tea (A) → improved hydration/sustained energy during workout (B) → enhanced exercise performance (B) → increased weight loss (C) - In an **indirect relationship**, the exposure influences the outcome through one or more **mediating variables** rather than acting alone - While **confounding** (spurious association) is also plausible in this observational scenario, the question assumes a mediated causal pathway exists *Spurious* - A **spurious association** occurs when two variables appear related only because both are independently caused by a **third confounding variable** - Example: If highly motivated individuals both drink iced tea AND exercise more intensely, the tea itself may not cause weight loss—both behaviors are driven by motivation - This is actually a **very plausible alternative explanation** for this observational finding - However, if we accept that iced tea has a true physiological effect on workout quality (hydration/performance), then the relationship becomes indirect rather than spurious *Relative* - **"Relative"** is not a type of epidemiological relationship - This term describes **measures of association** (relative risk, relative rate, odds ratio) used to quantify relationships - It does not classify the nature or causal structure of an association *Direct* - A **direct relationship** means the exposure directly causes the outcome without any intermediary steps (A → C) - Weight loss fundamentally results from **caloric deficit** (energy expenditure > intake), primarily driven by physical activity and diet - Iced tea alone, without the mechanism of improved workout performance, would not directly cause significant weight loss - Since the weight loss depends on the workout as an intermediary step, this is not a direct relationship

Question 1408: Calculate the relative risk for the given situation: Developed Malaria | Did Not Develop Malaria | Total Vaccinated 6 | 94 | 100 Non-vaccinated 12 | 88 | 100

A. 1.5
B. 2
C. 1.7
D. 0.5 (Correct Answer)

Explanation: ***Correct: 0.5*** **Relative Risk (RR)** is calculated as: RR = Risk in exposed group / Risk in unexposed group **Step-by-step calculation:** - Risk in **vaccinated group** = 6/100 = 0.06 - Risk in **non-vaccinated group** = 12/100 = 0.12 - **RR = 0.06 / 0.12 = 0.5** **Interpretation:** An RR of **0.5 indicates a protective effect** of vaccination. Vaccinated individuals have **half the risk** (50% reduced risk) of developing malaria compared to non-vaccinated individuals. *Incorrect: 2* This is the **inverse** of the correct RR, calculated as 0.12/0.06 = 2 (risk in non-vaccinated / risk in vaccinated). This would incorrectly suggest vaccination **doubles the risk** of malaria, which contradicts the data showing vaccination is protective. *Incorrect: 1.5* This value does not result from the correct RR formula using the given incidence rates (0.06 vs 0.12). This may arise from incorrect formula application or confusion with other epidemiological measures like the **Odds Ratio**. *Incorrect: 1.7* This is not the result of standard RR calculation based on the incidence rates of 0.06 and 0.12. It represents a **calculation error** and has no epidemiological meaning in this context.

Question 1409: In the context of a new onset of a morbid disease, how does the change in incidence affect the prevalence of the disease?

A. Prevalence is not related to incidence
B. Incidence will increase, and prevalence will decrease
C. Incidence and prevalence will increase (Correct Answer)
D. Prevalence will increase with a decrease in the incidence

Explanation: ***Incidence and prevalence will increase*** - **Incidence** is the rate of new cases arising in a population; a "new onset" inherently implies that the occurrence of **new cases** is rising or starting. - Since **prevalence** is the total number of existing cases (P ≈ I × D, where D is duration), a rise in new cases (**incidence**) directly contributes to and increases the total existing burden of the disease. *Prevalence is not related to incidence* - This is incorrect because **incidence** (the inflow of new cases) is the primary determinant, along with duration and mortality/cure, of the overall total number of existing cases (**prevalence**). - Prevalence is mathematically linked to incidence; if incidence rises, prevalence typically rises, and if incidence approaches zero, prevalence will eventually fall (assuming cases are cleared). *Incidence will increase, and prevalence will decrease* - When **incidence** increases (more new cases), it leads to an increased rate of accumulation of cases, which consequently increases **prevalence**. - Prevalence only decreases despite increasing incidence if the removal rate (due to death or cure) drastically exceeds the rate of new cases, which is highly unlikely in a scenario described as a "new onset" morbid disease. *Prevalence will increase with a decrease in the incidence* - A decrease in **incidence** (fewer new cases) leads to a decrease in **prevalence** over time, assuming the duration of the disease remains stable. - Prevalence can increase with decreasing incidence only if the **duration** of the disease or survival time increases significantly (e.g., effective palliative treatment without cure), trapping existing cases in the prevalent pool.

Question 1410: How does the World Health Organization (WHO) define blindness?

A. VA < 6/60
B. VA > 3/60
C. VA < 3/60 (Correct Answer)
D. VA > 6/60

Explanation: ***VA < 3/60*** - This visual acuity (VA) level in the better eye, with the best possible correction, meets the World Health Organization (WHO) definition of **blindness** (Visual Impairment Category 3 or worse). - It signifies that the patient cannot count fingers at a distance of 3 meters (CF < 3m), representing a profound loss of useful vision. *VA < 6/60* - A visual acuity of less than 6/60 but equal to or better than 3/60 is classified by the WHO as **severe visual impairment** (Category 2), not clinical blindness. - This means the patient can still appreciate the largest optotype on the Snellen chart, but their vision is severely compromised. *VA > 3/60* - Visual acuity *greater* than 3/60 (e.g., 6/60, 6/18) indicates better visual function and therefore does not satisfy the criteria for WHO **blindness**. - Depending on the exact VA, this level of vision may fall into the categories of **moderate** or **mild visual impairment**. *VA > 6/60* - Visual acuity greater than 6/60 (e.g., 6/12) is well above the threshold defined for both **severe visual impairment** and blindness. - An acuity of 6/60 is often the benchmark for severe impairment; vision better than this suggests residual useful vision.

Practice by Chapter

Principles of Epidemiology

Practice Questions

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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