Epidemiology Practice Questions

Q: Which of the following conditions is characterized by low prevalence and high incidence?

A highly fatal disease. The relationship between incidence and prevalence is defined by the formula: **Prevalence (P) = Incidence (I) × Mean Duration of disease (D)**. ### Why Option A is Correct A **highly fatal disease** has a very short duration because the patient dies shortly after contracting the illness. Even if the **Incidence** (number of new cases) is high, the **Prevalence** (total number of existing cases at a given time) remains low because individuals are rapidly removed from the "pool" of cases due to death. Therefore, short duration (D) leads to low prevalence (P). ### Why Other Options are Incorrect * **B. An easily curable disease:** Similar to fatal diseases, these have a short duration. However, the question asks for a condition characterized by *high incidence*. While curable diseases have low prevalence, they do not inherently imply high incidence unless specified (e.g., common cold). * **C. A highly infectious disease with low mortality:** These typically result in high prevalence. If the disease is not fatal and not immediately cured, patients remain in the population pool for a longer period, increasing prevalence. * **D. A chronic disease:** Chronic conditions (e.g., Diabetes, Hypertension) have a long duration. Even with a low or stable incidence, the prevalence becomes high because cases accumulate over many years. ### NEET-PG High-Yield Pearls * **Prevalence** is a **snapshot** (point) or a **slice** (period); **Incidence** is a **flow** (rate). * **Factors increasing Prevalence:** Prolongation of life without a cure, increase in new cases (incidence), in-migration of cases. * **Factors decreasing Prevalence:** High fatality rate, rapid cure rate, out-migration of cases. * **Rule of Thumb:** If a disease is either **rapidly fatal** or **rapidly cured**, its prevalence will always be low relative to its incidence.

Q: Which of the following factors does not affect the Hardy-Weinberg equilibrium?

Random mating (panmixia). ### Explanation The **Hardy-Weinberg Principle** is a fundamental concept in population genetics which states that allele and genotype frequencies in a population will remain constant (in equilibrium) from generation to generation in the absence of evolutionary influences. #### Why "Random Mating" is the Correct Answer For a population to remain in equilibrium, **random mating (panmixia)** is a **requirement**, not a disruptive factor. In random mating, every individual has an equal chance of mating with any other individual of the opposite sex. This ensures that genes are shuffled without changing the overall frequency. Therefore, random mating **maintains** the equilibrium rather than affecting or disrupting it. #### Analysis of Incorrect Options (Factors that DO affect equilibrium) * **Small population size (Genetic Drift):** In small populations, chance events can cause allele frequencies to fluctuate unpredictably. Large population size is required to minimize this "sampling error." * **Mutations:** The sudden change in DNA sequences introduces new alleles into the gene pool, thereby altering existing frequencies. * **Gene flow (Migration):** The movement of individuals (and their genes) into or out of a population changes the local allelic makeup. * **Natural Selection:** If certain phenotypes have a survival advantage, those alleles will increase in frequency, breaking the equilibrium. #### NEET-PG High-Yield Pearls * **The Formula:** $p^2 + 2pq + q^2 = 1$ (where $p$ and $q$ are the frequencies of dominant and recessive alleles). * **Assumptions for Equilibrium:** Large population, Random mating, No mutation, No migration, and No selection. * **Clinical Application:** This principle is used in public health to calculate the **carrier frequency** of autosomal recessive diseases (e.g., Cystic Fibrosis, Phenylketonuria) in a population when only the disease prevalence ($q^2$) is known.

Q: Which one of the following diseases has the lowest incubation period?

Cholera. **Explanation:** The incubation period (IP) is the interval between the invasion of an infectious agent and the appearance of the first sign or symptom of the disease. In epidemiology, understanding the IP is crucial for determining the source of infection and the period of surveillance. **Why Cholera is correct:** Cholera, caused by *Vibrio cholerae*, is characterized by a very short incubation period, typically ranging from **a few hours to 5 days** (commonly 1–2 days). This rapid onset occurs because the pre-formed toxins or the rapid multiplication of bacteria in the small intestine lead to immediate secretory diarrhea. Among the options provided, it has the shortest duration. **Analysis of Incorrect Options:** * **Diphtheria:** The IP is typically **2 to 5 days**. While short, the lower limit of Cholera (hours) makes Cholera the "lowest" among the two. * **Smallpox:** This viral illness has a much longer IP, averaging **7 to 17 days** (commonly 12 days). * **Dengue:** The IP for this mosquito-borne viral fever is generally **3 to 14 days** (commonly 4–7 days). **NEET-PG High-Yield Pearls:** * **Shortest IP overall:** Influenza (18–72 hours) and Staphylococcal food poisoning (1–6 hours) are often cited as having the shortest incubation periods in clinical medicine. * **Longest IP:** Leprosy (3–5 years or more) and Rabies (variable, usually 1–3 months but can be years). * **Median Incubation Period:** The time required for 50% of cases to occur following exposure. * **Clinical Utility:** The IP helps in identifying the **"Serial Interval"** (the gap between the onset of the primary case and the secondary case).

Q: In a village of 1 lakh population, among 2,000 exposed to smoking, 20 developed lung cancer and among 8,000 unexposed people, 8 developed lung cancer. What is the attributable risk?

90%. ### **Explanation** The question asks for the **Attributable Risk (AR)**, also known as Risk Difference. This epidemiological measure quantifies the excess risk of a disease in an exposed group that can be specifically attributed to the exposure. **Step-by-Step Calculation:** 1. **Incidence in Exposed ($I_e$):** $\frac{20}{2,000} = 0.01$ or $10$ per $1,000$. 2. **Incidence in Unexposed ($I_u$):** $\frac{8}{8,000} = 0.001$ or $1$ per $1,000$. 3. **Attributable Risk (AR) Formula:** $I_e - I_u$ * $10 - 1 = 9$ per $1,000$. 4. **Attributable Risk Proportion (AR%):** This is often what is implied when options are in percentages. It calculates the proportion of the disease in the exposed group that could be eliminated if the exposure were removed. * **Formula:** $\frac{I_e - I_u}{I_e} \times 100$ * $\frac{10 - 1}{10} \times 100 = \mathbf{90\%}$. --- ### **Analysis of Options** * **Option C (90%) is Correct:** As calculated above, 90% of lung cancer cases among smokers in this village are directly due to smoking. * **Option A (10%):** This represents the background risk (Incidence in unexposed relative to exposed), not the risk attributable to the factor. * **Option B (50%):** This would occur if the incidence in the exposed was only double that of the unexposed. * **Option D (100%):** This would only be possible if the incidence in the unexposed group was zero ($I_u = 0$), meaning the disease *only* occurs in the presence of the risk factor. --- ### **High-Yield NEET-PG Pearls** * **Relative Risk (RR):** $\frac{I_e}{I_u}$. In this case, $10/1 = 10$. It measures the **strength of association** and is best for investigating etiology. * **Attributable Risk (AR):** Measures the **public health impact**. It indicates how much of the disease can be prevented by removing the risk factor. * **Population Attributable Risk (PAR):** Extends AR to the entire population (including non-smokers). It helps prioritize national health policies.

Q: For which infection are the source and reservoir the same?

Tetanus. ### Explanation In epidemiology, the **reservoir** is the natural habitat (living or non-living) where an infectious agent lives and multiplies. The **source** is the immediate person, animal, or object from which the host acquires the infection. **Why Tetanus is the correct answer:** For **Tetanus**, the reservoir is the **soil**, where *Clostridium tetani* spores reside and persist. When a person sustains a wound contaminated with soil, the soil acts as both the natural habitat of the organism (reservoir) and the immediate vehicle of transmission (source). Therefore, the source and reservoir are identical. **Analysis of Incorrect Options:** * **Measles & Typhoid:** In these human-to-human diseases, the reservoir is a **human case or carrier**. However, the *source* can be different from the reservoir. For example, in Typhoid, the reservoir is a chronic carrier, but the source might be contaminated water or food handled by that carrier. * **Rabies:** This is a zoonotic disease. The **reservoir** is the animal population (e.g., dogs, bats), while the **source** is the specific rabid animal that bites the human. While they can overlap, in epidemiological terms, they are distinct because the human is a "dead-end host." **High-Yield NEET-PG Pearls:** * **Hookworm:** Another classic example where the source and reservoir are the same (**soil**). * **Source = Reservoir:** This usually occurs in infections where the agent is acquired directly from its natural habitat (often non-living environments like soil). * **Source ≠ Reservoir:** Common in food-borne illnesses (Reservoir: Human; Source: Contaminated food) or vector-borne diseases (Reservoir: Human/Animal; Source: Mosquito). * **Tetanus** is unique because it is a non-communicable infectious disease (no person-to-person spread).

Q: What is the first step in managing an epidemic?

Verification of diagnosis. ### Explanation In epidemiology, the management of an outbreak follows a systematic, step-by-step approach. The **first and most crucial step** is the **Verification of Diagnosis**. **1. Why "Verification of Diagnosis" is correct:** Before mobilizing resources or implementing control measures, it is essential to confirm that the reported cases are indeed the disease they are suspected to be. This prevents "false alarms" caused by misinterpretation of signs/symptoms or laboratory errors. Verification involves clinical examination of a sample of cases and confirming them through specific laboratory tests. Once the diagnosis is verified, the next step is to confirm the existence of an epidemic (comparing current incidence with previous years). **2. Why other options are incorrect:** * **Notification (D):** While notification to local health authorities is a legal requirement, it typically occurs *after* the diagnosis is verified and the existence of an epidemic is confirmed. * **Isolation (B):** This is a control measure. You cannot justify isolating individuals or restricting movement until you have confirmed what pathogen you are dealing with. * **Immunization (C):** This is a specific prevention strategy (if a vaccine is available). It is implemented much later in the investigation process as part of the "Control Measures" phase. **3. NEET-PG High-Yield Pearls:** * **Sequence of Investigation:** 1. Verification of diagnosis → 2. Confirmation of existence of epidemic → 3. Defining the population at risk → 4. Rapid search for all cases. * **Definition of Epidemic:** The occurrence of cases of an illness in a community or region clearly in excess of normal expectancy. * **Primary Case:** The first case of a disease to be introduced into a population. * **Index Case:** The first case that comes to the attention of the investigator (not necessarily the primary case).

Question 1

According to the CDC, what is the definition of a past smoker?

Accepted Answer

A person who has smoked more than 100 cigarettes previously and is not smoking currently.

Answer

A person who has smoked more than 10 cigarettes previously and is not smoking currently.

Answer

A person who has smoked more than 100 cigarettes previously and reduces to 10 cigarettes per day.

Answer

A person who has smoked more than 100 cigarettes in their lifetime, who smokes now, but does not smoke every day.

Question 2

Which of the following conditions is characterized by low prevalence and high incidence?

Accepted Answer

A highly fatal disease

Answer

An easily curable disease

Answer

A highly infectious disease with low mortality

Answer

A chronic disease

Question 3

Which of the following factors does not affect the Hardy-Weinberg equilibrium?

Accepted Answer

Random mating (panmixia)

Answer

Small population size (genetic drift)

Answer

Mutations

Answer

Gene flow (migration)

Question 4

Part I of the 'death certificate' deals with which of the following?

Accepted Answer

The immediate cause, and the direct underlying cause which started the sequence of events leading to death.

Answer

Any significant associated diseases that contributed to the death but did not directly lead to it.

Answer

The approximate interval between the onset of the cause of death and death.

Answer

The mode of death.

Question 5

Which one of the following diseases has the lowest incubation period?

Accepted Answer

Cholera

Answer

Small pox

Answer

Diphtheria

Answer

Dengue

Question 6

In a village of 1 lakh population, among 2,000 exposed to smoking, 20 developed lung cancer and among 8,000 unexposed people, 8 developed lung cancer. What is the attributable risk?

Accepted Answer

90%

Answer

10%

Answer

50%

Answer

100%

Question 7

For which infection are the source and reservoir the same?

Accepted Answer

Tetanus

Answer

Rabies

Answer

Typhoid

Answer

Measles

Question 8

What is the mode of transmission for a propagative epidemic?

Accepted Answer

Person to person

Answer

Animal to person

Answer

Water to person

Answer

Fomite to person

Question 9

What is the first step in managing an epidemic?

Accepted Answer

Verification of diagnosis

Answer

Isolation

Answer

Immunization

Answer

Notification

Question 10

In a population of 5000, 90 new cases of myopia were found over a year. Calculate the incidence of myopia per 100 people.

Accepted Answer

2

Answer

1.8

Answer

5

Answer

18

Epidemiology — MCQs

Epidemiology — MCQs

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