Epidemiology Practice Questions

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

Cohort study. **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).

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

Pneumonic plague. **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).

Q: 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?

Measurement bias. ### 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.

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

Incidence rates may be computed directly. ### 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**.

Q: 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?

60 days. ### 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.

Q: What is the vector for Yellow fever?

Aedes. **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.

Q: Which of the following diseases is spread by mosquitoes?

Dengue. **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.

Question 1

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

Accepted Answer

Cohort study

Answer

Case-control study

Answer

Ecological study

Answer

Cross-sectional study

Question 2

For how long is a temporary carrier of typhoid infective?

Accepted Answer

3 months to 1 year

Answer

Less than 3 weeks

Answer

3 weeks to 3 months

Answer

More than 1 year

Question 3

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)?

Accepted Answer

Follow-up of every case for one year and monthly blood smears

Answer

Presumptive treatment of all fever cases

Answer

Regular DDT spraying throughout the year

Answer

Epidemiological investigation of all malaria cases

Question 4

Isolation is strictly recommended for which of the following conditions?

Accepted Answer

Pneumonic plague

Answer

Mumps

Answer

Measles

Answer

Hepatitis A

Question 5

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?

Accepted Answer

Measurement bias

Answer

Hawthorne bias

Answer

Procedure bias

Answer

Sampling bias

Question 6

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

Accepted Answer

Coronary heart disease

Answer

Tuberculosis

Answer

Leprosy

Answer

None of the above

Question 7

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

Accepted Answer

Incidence rates may be computed directly

Answer

Definition of cases may be difficult

Answer

Assessment of past exposure may be biased

Answer

Cases with the disease are compared to controls without the disease

Question 8

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?

Accepted Answer

60 days

Answer

15 days

Answer

30 days

Answer

90 days

Question 9

What is the vector for Yellow fever?

Accepted Answer

Aedes

Answer

Culex

Answer

Anopheles

Answer

Mansonoides

Question 10

Which of the following diseases is spread by mosquitoes?

Accepted Answer

Dengue

Answer

Kala azar

Answer

Trypanosomiasis

Answer

Listeriosis

Epidemiology — MCQs

Epidemiology — MCQs

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