Epidemiology — MCQs

A group of sports-science researchers is trying to determine what physical characteristics are positive predictors for success at the quarterback position in high school sports. A thorough physical examination is planned for all high school participants, including ethnicity, height, weight, BMI, and other variables. One of the testing sites in the Midwest region places the measurement stick 4 inches off of the ground, generously adding to the height of each athlete in their training center. This mistake can be categorized as which kind of bias if not addressed by the researchers?

Q226Medium

Secondary level of prevention is important in all of the following conditions except?

Q227Easy

Which of the following is NOT a characteristic of a case-control study?

Q228Easy

Under the national polio eradication program, after how many days from the onset of paralysis is a confirmed case of acute flaccid paralysis considered a case of polio by surveillance?

Q229Easy

What is the vector for Yellow fever?

Q230Easy

Which of the following diseases is spread by mosquitoes?

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 221: Which epidemiological study design is best for testing the association between a risk factor and a disease?

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

Explanation: **Explanation** The **Cohort Study** is the gold standard among observational studies for testing associations because it establishes a clear **temporal relationship** (the cause precedes the effect). By starting with a group of disease-free individuals and following them over time, it allows for the direct calculation of **Relative Risk (RR)** and **Attributable Risk**, providing the strongest evidence of causality outside of experimental trials. **Why other options are incorrect:** * **Case-control study:** While efficient for rare diseases, it is retrospective. It starts with the effect (disease) and looks back for the cause, making it prone to recall bias. It can only estimate association using **Odds Ratio (OR)**, not direct risk. * **Ecological study:** These use populations or groups as the unit of study rather than individuals. They are prone to **"Ecological Fallacy,"** where observations at the group level may not apply to individuals. * **Cross-sectional study:** This provides a "snapshot" of a population, measuring exposure and outcome simultaneously. Because it cannot determine whether the exposure came before the disease, it is best for determining **prevalence**, not causation. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Cohort:** **P**rospective, **P**roceeds from cause to effect, calculates **P**revalence (incidence) and **P**robability (Relative Risk). * **Incidence:** Cohort studies are the only observational design that can directly calculate the incidence of a disease. * **Rare Exposures:** Cohort studies are best for rare exposures (e.g., a specific chemical leak), whereas Case-control studies are best for rare diseases (e.g., rare cancers).

Question 222: For how long is a temporary carrier of typhoid infective?

A. Less than 3 weeks
B. 3 weeks to 3 months
C. 3 months to 1 year (Correct Answer)
D. More than 1 year

Explanation: In the epidemiology of Typhoid fever (Enteric fever), carriers are classified based on the duration for which they shed *Salmonella typhi* in their stools or urine. ### **Explanation of the Correct Answer** **Option C (3 months to 1 year)** is correct. By definition, a **temporary carrier** is an individual who continues to excrete the bacilli for more than 3 weeks but **less than 12 months (1 year)** after the clinical recovery from the disease. This period represents a transitional phase where the body has not yet completely cleared the pathogen but has not yet reached the "chronic" threshold. ### **Analysis of Incorrect Options** * **Option A (< 3 weeks):** This period corresponds to the **convalescent stage** of the acute illness. Most patients shed the bacteria during this time, but they are not yet classified as carriers until shedding persists beyond 3 weeks. * **Option B (3 weeks to 3 months):** While this timeframe falls within the temporary carrier definition, it is incomplete. The established epidemiological cutoff for a temporary carrier extends up to 1 year. * **Option D (> 1 year):** This defines a **Chronic Carrier**. Chronic carriers often harbor the bacteria in the **gallbladder** (associated with gallstones) or the urinary tract (associated with *Schistosoma haematobium* infection). ### **High-Yield NEET-PG Pearls** * **Incubation Period:** Usually 10–14 days (Range: 3–21 days). * **Chronic Carrier Status:** More common in females (3:1 ratio) and individuals with biliary tract abnormalities. * **The "Typhoid Mary" Effect:** Chronic carriers are the most significant reservoirs for maintaining the disease in a community. * **Diagnostic Gold Standard for Carriers:** Repeated stool cultures (at least 3 negative samples) are required to declare a patient free of the carrier state. * **Drug of Choice for Carriers:** Ciprofloxacin (for 4–6 weeks) or Ampicillin/Amoxicillin + Probenecid. Cholecystectomy may be required if gallstones are present.

Question 223: In the National Malaria Eradication Programme (NMEP), which of the following is NOT a recommended strategy for an area with high Annual Parasite Index (API)?

A. Presumptive treatment of all fever cases
B. Regular DDT spraying throughout the year
C. Epidemiological investigation of all malaria cases
D. Follow-up of every case for one year and monthly blood smears (Correct Answer)

Explanation: ### Explanation In the context of the National Malaria Eradication Programme (NMEP), the strategies employed depend heavily on the phase of the program and the local **Annual Parasite Index (API)**. **Why Option D is the correct answer:** Follow-up of every case for one year with monthly blood smears is a strategy reserved for the **Maintenance Phase** of the eradication program. In this phase, the API is very low (less than 0.1), and the goal is to prevent the re-introduction of malaria. In high API areas (Attack or Consolidation phases), the volume of cases is too high to make monthly follow-ups for a full year logistically feasible or epidemiologically necessary. **Analysis of Incorrect Options:** * **A. Presumptive treatment:** This is a cornerstone of the program in high-transmission areas. Every fever case is treated immediately with a single dose of Chloroquine to reduce the parasite reservoir before laboratory confirmation. * **B. Regular DDT spraying:** Residual insecticide spraying (like DDT) is the primary method of vector control in the **Attack Phase** (high API) to interrupt transmission. * **C. Epidemiological investigation:** While more intensive in the Consolidation phase, investigating clusters and cases is a standard epidemiological tool to identify focal outbreaks in high-burden areas. **High-Yield NEET-PG Pearls:** * **API (Annual Parasite Index):** The most sensitive index to differentiate between the Consolidation (<0.1) and Attack (>0.1) phases. * **Formula:** (Total confirmed cases in a year / Total population) × 1000. * **ABER (Annual Blood Examination Rate):** Must be at least **10%** for the surveillance to be considered effective. * **Radical Treatment:** Aimed at the complete clearance of parasites (including hypnozoites in *P. vivax*) to prevent relapse.

Question 224: Isolation is strictly recommended for which of the following conditions?

A. Mumps
B. Measles
C. Hepatitis A
D. Pneumonic plague (Correct Answer)

Explanation: **Explanation:** The core concept here is the distinction between **Isolation** (separation of infected persons) and **Quarantine** (separation of healthy contacts). While isolation is used for many infectious diseases, it is **strictly recommended** and legally mandated for diseases with high mortality and rapid transmission potential, such as **Pneumonic Plague**. **1. Why Pneumonic Plague is Correct:** Pneumonic plague is a highly infectious, fatal form of plague caused by *Yersinia pestis*. It is transmitted via respiratory droplets and has a near 100% fatality rate if untreated. Due to its potential for explosive outbreaks and its status as a "quarantinable" disease under International Health Regulations (IHR), strict isolation in a healthcare facility is mandatory until the patient has completed 48 hours of effective antibiotic therapy and shows clinical improvement. **2. Why other options are incorrect:** * **Mumps & Measles:** These are highly contagious viral infections. While "respiratory precautions" and staying home from school/work are advised to limit spread, they do not typically require the "strict" or mandatory clinical isolation protocols associated with plague. * **Hepatitis A:** This is transmitted via the feco-oral route. The period of maximum infectivity occurs *before* the onset of jaundice. By the time the disease is diagnosed, isolation is of limited public health value; instead, enteric precautions and hand hygiene are emphasized. **Clinical Pearls for NEET-PG:** * **Quarantinable Diseases (WHO):** Traditionally include Cholera, Plague, and Yellow Fever. * **Incubation Period of Plague:** 1–7 days (shortest for pneumonic). * **Chemoprophylaxis for Plague:** Doxycycline or Tetracycline is the drug of choice for contacts. * **Isolation vs. Quarantine:** Remember, we isolate the **S**ick (Isolation) and quarantine the **H**ealthy (Quarantine).

Question 225: A group of sports-science researchers is trying to determine what physical characteristics are positive predictors for success at the quarterback position in high school sports. A thorough physical examination is planned for all high school participants, including ethnicity, height, weight, BMI, and other variables. One of the testing sites in the Midwest region places the measurement stick 4 inches off of the ground, generously adding to the height of each athlete in their training center. This mistake can be categorized as which kind of bias if not addressed by the researchers?

A. Hawthorne bias
B. Measurement bias (Correct Answer)
C. Procedure bias
D. Sampling bias

Explanation: ### Explanation **Correct Answer: B. Measurement bias** **Why it is correct:** Measurement bias (also known as detection bias or information bias) occurs when there is a systematic error in the collection, recording, or analysis of data. In this scenario, the measurement stick is incorrectly placed 4 inches off the ground, leading to a **systematic overestimation** of height for all participants at that specific site. Because the error is built into the tool/method of data collection rather than occurring by chance, it is a classic example of measurement bias. **Why the other options are incorrect:** * **A. Hawthorne bias:** This occurs when study participants change their behavior because they are aware they are being observed. It relates to participant behavior, not faulty equipment. * **C. Procedure bias:** This occurs when subjects in different study groups are treated differently (e.g., one group receives more frequent follow-up than the other). Here, the error is a technical measurement flaw, not a difference in clinical protocol between groups. * **D. Sampling bias:** This occurs when the study population is not representative of the target population due to non-random selection (e.g., only recruiting athletes from elite schools). The error here is in *how* they were measured, not *who* was selected. **Clinical Pearls for NEET-PG:** * **Systematic Error vs. Random Error:** Measurement bias is a systematic error that affects **Validity**. Random error (chance) affects **Reliability/Precision**. * **Recall Bias:** A common subtype of measurement bias in Case-Control studies where cases remember exposures more clearly than controls. * **Lead-time Bias:** An illusion of increased survival time due to earlier detection by a screening test, without changing the actual outcome. * **Observer Bias:** When the investigator’s prior knowledge or expectations influence how they record the data. Use **Blinding** to minimize this.

Question 226: Secondary level of prevention is important in all of the following conditions except?

A. Coronary heart disease (Correct Answer)
B. Tuberculosis
C. Leprosy
D. None of the above

Explanation: ### Explanation The core concept in this question lies in the **natural history of disease** and the effectiveness of early diagnosis and treatment (Secondary Prevention) versus risk factor modification (Primary Prevention). **1. Why Coronary Heart Disease (CHD) is the Correct Answer:** Secondary prevention aims to halt disease progression through early detection and treatment. While secondary prevention (e.g., using aspirin or statins after a diagnosis) exists for CHD, it is **not the most important** level of prevention for this condition. CHD is a lifestyle-related, non-communicable disease where **Primary Prevention** (controlling hypertension, smoking cessation, diet) and **Primordial Prevention** (preventing the emergence of risk factors) are the most effective strategies to reduce the population burden. Once CHD is clinically detectable, significant irreversible damage to the coronary arteries has often already occurred. **2. Why the other options are incorrect:** * **Tuberculosis (TB):** Secondary prevention is the **mainstay** of TB control. The strategy (e.g., RNTCP/NTEP) focuses on early case finding (sputum microscopy) and prompt treatment (DOTS) to break the chain of transmission in the community. * **Leprosy:** Similar to TB, the global strategy for Leprosy relies heavily on **Secondary Prevention**. Early diagnosis and Multi-Drug Therapy (MDT) are crucial to prevent permanent nerve damage, physical deformities, and further transmission. **High-Yield Clinical Pearls for NEET-PG:** * **Primordial Prevention:** Best for chronic diseases (e.g., discouraging children from starting smoking). * **Secondary Prevention:** Synonymous with "Early Diagnosis and Treatment." It is the most vital level for most infectious diseases (TB, Leprosy, STIs). * **Tertiary Prevention:** Focuses on disability limitation and rehabilitation (e.g., physiotherapy after a stroke). * **Quaternary Prevention:** Actions taken to identify patients at risk of over-medicalization and to protect them from new medical invasions.

Question 227: Which of the following is NOT a characteristic of a case-control study?

A. Definition of cases may be difficult
B. Assessment of past exposure may be biased
C. Incidence rates may be computed directly (Correct Answer)
D. Cases with the disease are compared to controls without the disease

Explanation: ### Explanation **Why Option C is the Correct Answer:** In epidemiology, **Incidence** represents the number of *new* cases occurring in a population over a specific period. To calculate incidence, you must follow a disease-free group over time to see who develops the condition (as done in **Cohort studies**). In a **Case-Control study**, the researcher starts with people who already have the disease (Cases). Since the "denominator" (the total population at risk) is unknown and the study proceeds backwards from effect to cause, **Incidence rates cannot be computed directly.** Instead, Case-Control studies yield the **Odds Ratio (OR)** as a measure of association. **Analysis of Incorrect Options:** * **Option A:** Defining cases can indeed be difficult, especially for diseases with vague clinical boundaries or subclinical stages. Strict diagnostic criteria are required to ensure study validity. * **Option B:** Since these studies rely on interviews or medical records to determine past exposure, they are highly susceptible to **Recall Bias** (cases may remember past events more vividly than controls). * **Option D:** This is the fundamental design of a Case-Control study—it is a retrospective, analytical study that compares a group with the disease to a comparable group without it. **High-Yield NEET-PG Pearls:** * **Direction:** Case-Control studies move from **Effect to Cause** (Retrospective). * **Measure of Association:** Odds Ratio (OR). * **Best for:** Rare diseases or diseases with long latency periods. * **Nesting:** A "Nested Case-Control Study" is one conducted within a large cohort study, which helps minimize selection and information bias. * **Matching:** This technique is used in Case-Control studies to eliminate the effects of **Confounding factors**.

Question 228: Under the national polio eradication program, after how many days from the onset of paralysis is a confirmed case of acute flaccid paralysis considered a case of polio by surveillance?

A. 15 days
B. 30 days
C. 60 days (Correct Answer)
D. 90 days

Explanation: ### Explanation In the context of the Global Polio Eradication Initiative, the surveillance of **Acute Flaccid Paralysis (AFP)** is the gold standard for detecting polio. **Why 60 Days is Correct:** The definitive classification of a "confirmed polio case" relies on the persistence of paralysis. Under the WHO-recommended surveillance protocol, if two adequate stool samples fail to yield a virus but the patient has **residual paralysis at 60 days** from the onset of symptoms, the case is clinically confirmed as polio. This 60-day window is critical because non-polio AFP (like Guillain-Barré Syndrome) often shows signs of recovery or different progression, whereas paralytic poliomyelitis typically leaves permanent residual weakness. **Analysis of Incorrect Options:** * **15 Days:** This is the timeframe for "adequate stool collection." Two stool specimens must be collected 24–48 hours apart within 14 days of the onset of paralysis. * **30 Days:** While some clinical improvements in other neurological conditions may be noted here, it is not the standardized surveillance milestone for polio confirmation. * **90 Days:** This exceeds the standard surveillance window for primary follow-up and would delay the reporting and response activities (mop-up rounds). **High-Yield Clinical Pearls for NEET-PG:** * **AFP Surveillance Criteria:** Includes all children <15 years with sudden onset of flaccid paralysis or any person of any age where polio is suspected. * **Adequate Stool Samples:** 2 samples, 24 hours apart, within 14 days of onset, arriving at the lab in "good condition" (cold chain maintained). * **Virological Classification:** Since India is polio-free, any case is now classified via the **Virological Classification Path** (detection of Wild Poliovirus or VDPV) rather than just clinical residual paralysis. * **Non-Polio AFP Rate:** A key indicator of surveillance quality; it should be at least **2 per 100,000** children under 15 years.

Question 229: What is the vector for Yellow fever?

A. Aedes (Correct Answer)
B. Culex
C. Anopheles
D. Mansonoides

Explanation: **Explanation:** **Yellow Fever** is a viral hemorrhagic disease caused by a **Flavivirus**. The correct answer is **Aedes** because the primary vector for transmitting the virus to humans is the **Aedes aegypti** mosquito (urban cycle). In the jungle cycle, other species like *Haemagogus* and *Sabethes* are involved. **Analysis of Options:** * **Aedes (Correct):** Specifically *Aedes aegypti*, which breeds in artificial containers and is a day-biter. It is also the vector for Dengue, Chikungunya, and Zika. * **Culex:** This mosquito is the primary vector for **Japanese Encephalitis**, West Nile Virus, and **Bancroftian Filariasis**. It typically breeds in dirty, stagnant water. * **Anopheles:** This is the well-known vector for **Malaria**. It breeds in clean, moving, or stagnant water. * **Mansonoides:** This mosquito is the vector for **Brugian Filariasis** (Malayan filariasis). It is unique because its larvae attach to the roots of aquatic plants like *Pistia*. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period:** 3 to 6 days. * **Vaccine:** The **17D vaccine** is a live attenuated vaccine. It provides immunity for life (as per WHO 2014 guidelines), though international travel regulations may still reference a 10-year validity. * **Contraindications:** The vaccine is contraindicated in infants <6 months, pregnant women (unless during an outbreak), and immunocompromised individuals (e.g., symptomatic HIV, thymus disorders). * **India Status:** India is "Yellow Fever receptive" (the vector is present, but the disease is not), which is why strict quarantine and vaccination certificates are required for travelers from endemic zones.

Question 230: Which of the following diseases is spread by mosquitoes?

A. Dengue (Correct Answer)
B. Kala azar
C. Trypanosomiasis
D. Listeriosis

Explanation: **Explanation:** **Dengue** is the correct answer because it is a viral infection transmitted to humans through the bite of infected female mosquitoes, primarily the **_Aedes aegypti_** species (and to a lesser extent, *Aedes albopictus*). These mosquitoes are "day-biters" and typically breed in stagnant clean water in urban settings. **Analysis of Incorrect Options:** * **Kala-azar (Visceral Leishmaniasis):** This is caused by the protozoan parasite *Leishmania donovani* and is transmitted by the bite of the infected female **Sandfly** (*Phlebotomus argentipes*). * **Trypanosomiasis:** African Trypanosomiasis (Sleeping Sickness) is transmitted by the **Tsetse fly** (*Glossina* species). American Trypanosomiasis (Chagas disease) is transmitted by **Triatomine bugs** (kissing bugs). * **Listeriosis:** This is a bacterial infection caused by *Listeria monocytogenes*. It is primarily a **foodborne illness** contracted by consuming contaminated dairy products (unpasteurized cheese), deli meats, or soil-contaminated vegetables. **High-Yield Clinical Pearls for NEET-PG:** * **Aedes aegypti** is also the vector for Yellow Fever, Zika virus, and Chikungunya. * **Extrinsic Incubation Period:** In Dengue, the virus requires 8–12 days of replication inside the mosquito before it can be transmitted to another human. * **Dengue Triad:** Fever, rash, and severe headache (retro-orbital pain). It is often called "Breakbone fever" due to intense joint and muscle pain. * **Key Diagnostic:** NS1 antigen is the marker of choice in the first 5 days; IgM antibodies appear after day 5.

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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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