Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 251: The disease which is imported into a country in which it does not occur is known as?

A. Exotic (Correct Answer)
B. Sporadic
C. Pandemic
D. Enzootic

Explanation: ### Explanation **Correct Option: A. Exotic** In epidemiology, an **exotic disease** is defined as a disease that is imported into a country or geographic region where it does not otherwise occur. A classic example is **Rabies in the UK** or **Yellow Fever in India**. These diseases are not endemic to the region; their presence is entirely dependent on introduction from an external source. **Analysis of Incorrect Options:** * **B. Sporadic:** Refers to cases that occur irregularly, haphazardly, and infrequently from time to time. The cases are so few and separated by space and time that there is no common source of infection (e.g., Tetanus, Herpes Zoster). * **C. Pandemic:** An epidemic that spreads over a very wide geographical area, usually crossing international boundaries and affecting a large number of people worldwide (e.g., COVID-19, Influenza). * **D. Enzootic:** This term refers to the constant presence of a disease in an **animal population** within a specific geographic area (the animal equivalent of "endemic"). Examples include Anthrax or Brucellosis in certain livestock regions. **High-Yield Clinical Pearls for NEET-PG:** * **Epizootic:** An outbreak (epidemic) of disease in an animal population (e.g., Bird Flu in poultry). * **Epornithic:** An epidemic occurring in a bird population. * **Enzootic vs. Endemic:** Always remember that "Zootic" refers to animals, while "Demic" refers to humans. * **Zoonosis:** A disease naturally transmitted from vertebrate animals to humans (e.g., Rabies, Plague).

Question 252: Insecticide-treated bed nets (ITBNs) are typically treated with which class of insecticides?

A. Pyrethroids (e.g., Deltamethrin, Cyfluthrin) (Correct Answer)
B. Diethyltoluamide (DEET)
C. Natural Pyrethrins
D. Organophosphates (e.g., Malathion)

Explanation: **Explanation:** **1. Why Pyrethroids are the Correct Choice:** Pyrethroids (e.g., **Deltamethrin, Cyfluthrin, Alpha-cypermethrin**) are the only class of insecticides currently recommended by the WHO for treating Long-Lasting Insecticidal Nets (LLINs). They are preferred because of their unique combination of **high efficacy** against mosquitoes, **rapid knockdown effect** (excito-repellent property), and **low mammalian toxicity**. They are stable enough to remain effective on the fabric even after multiple washes (usually up to 20 washes or 3 years). **2. Analysis of Incorrect Options:** * **B. Diethyltoluamide (DEET):** This is a chemical **repellent** applied to the skin or clothes to prevent bites; it does not kill insects and is not used for treating bed nets. * **C. Natural Pyrethrins:** While derived from the Chrysanthemum flower and insecticidal, they are highly **unstable** when exposed to light and heat (photo-labile), making them unsuitable for long-term use on bed nets. * **D. Organophosphates (e.g., Malathion):** These are primarily used for **Indoor Residual Spraying (IRS)** or space spraying (fogging). They are generally too toxic for the close, prolonged skin contact associated with sleeping under a bed net. **3. NEET-PG High-Yield Pearls:** * **LLIN vs. ITBN:** LLINs (Long-Lasting Insecticidal Nets) are the current standard; they are factory-treated and do not require re-treatment, unlike older ITBNs. * **The "Halo Effect":** Large-scale use of ITBNs provides community-wide protection by reducing the overall mosquito population, not just protecting the individual sleeper. * **Target:** ITBNs are a core strategy for **Malaria** and **Lymphatic Filariasis** control. * **Mechanism:** Pyrethroids work by prolonging the opening of sodium channels in the insect's nervous system, leading to paralysis and death.

Question 253: Secondary attack rate is a measure of?

A. Communicability (Correct Answer)
B. Lethality
C. Strength of association
D. None of the above

Explanation: **Explanation:** **1. Why Option A is Correct:** The **Secondary Attack Rate (SAR)** is defined as the number of exposed persons developing the disease within the range of the incubation period following exposure to a primary case. It is a crucial epidemiological tool used to measure the **communicability** (infectiousness) of an agent. It specifically quantifies the spread of an infectious disease within a closed group (like a household or dormitory) where the primary case is known. **2. Why Other Options are Incorrect:** * **Option B (Lethality):** This is measured by the **Case Fatality Rate (CFR)**, which represents the proportion of cases that end in death. SAR measures spread, not severity. * **Option C (Strength of Association):** This is measured using **Relative Risk (RR)** or **Odds Ratio (OR)** in analytical studies (Cohort and Case-Control). SAR is a descriptive measure of transmission. **3. High-Yield NEET-PG Pearls:** * **Formula:** $SAR = \frac{\text{Number of exposed persons developing disease within one incubation period}}{\text{Total number of susceptible contacts}} \times 100$. * **Denominator Rule:** When calculating SAR, the **Primary Case** must always be excluded from the denominator. Only "susceptible" contacts are included (exclude those already immune via vaccination or prior infection). * **Application:** SAR is most useful for evaluating the effectiveness of control measures (like isolation or prophylactic treatment) and for determining the infectiousness of a new pathogen. * **Primary Attack Rate:** This refers to the initial wave of infection in a population, whereas SAR focuses on subsequent transmission from those initial cases.

Question 254: Chemoprophylaxis is not required in which of the following conditions?

A. Typhoid (Correct Answer)
B. Meningococcal meningitis
C. Bacterial conjunctivitis
D. Malaria

Explanation: **Explanation:** **Chemoprophylaxis** refers to the administration of drugs to prevent the development of an infection or its progression to clinical disease. **Why Typhoid is the Correct Answer:** Chemoprophylaxis is **not recommended** for Typhoid fever (Enteric fever). Prevention relies primarily on food and water sanitation, personal hygiene, and **vaccination** (e.g., Vi polysaccharide, Ty21a, or Typhoid Conjugate Vaccine). Using antibiotics as prophylaxis for typhoid is ineffective, promotes antimicrobial resistance, and does not prevent the carrier state. **Analysis of Incorrect Options:** * **Meningococcal Meningitis:** Chemoprophylaxis is mandatory for close contacts of a confirmed case to eliminate nasopharyngeal carriage. The drug of choice (DOC) is **Rifampicin** (Ciprofloxacin or Ceftriaxone are alternatives). * **Bacterial Conjunctivitis:** Topical antibiotic drops (e.g., Erythromycin or Sulfacetamide) are used prophylactically, especially in newborns (**Ophthalmia Neonatorum**), to prevent infection from birth canal pathogens. * **Malaria:** Chemoprophylaxis is a standard protocol for travelers moving from non-endemic to endemic areas. Common drugs include **Chloroquine** (if sensitive), **Doxycycline**, or **Mefloquine**. **High-Yield Clinical Pearls for NEET-PG:** * **DOC for Chemoprophylaxis:** * **Cholera:** Doxycycline (Adults), Tetracycline (Children). * **Pertussis/Diphtheria:** Erythromycin. * **Leptospirosis:** Doxycycline (200mg weekly). * **Rheumatic Fever:** Benzathine Penicillin G (every 3 weeks). * **Plague:** Doxycycline or Tetracycline. * **Note:** Chemoprophylaxis is generally used for diseases with a short incubation period and high secondary attack rates among contacts.

Question 255: The number of girls a woman would bear if she passes through her reproductive age with the same fertility rate gives:

A. Total fertility rate
B. Fertility rate
C. Gross reproductive rate (Correct Answer)
D. Net reproductive rate

Explanation: ### Explanation The correct answer is **Gross Reproductive Rate (GRR)**. #### 1. Why Gross Reproductive Rate (GRR) is correct: The **Gross Reproductive Rate** is defined as the average number of **female offspring** a woman would have if she survives to the end of her reproductive period (15–49 years) and experiences the current age-specific fertility rates. * **Key Concept:** Unlike the Total Fertility Rate, which counts all births, GRR focuses exclusively on female births because they represent the future reproductive potential of the population. * **Assumption:** It assumes **zero mortality**; it does not account for the possibility that the woman might die before completing her reproductive years. #### 2. Why other options are incorrect: * **Total Fertility Rate (TFR):** This is the average number of children (both boys and girls) a woman would have if she experiences current fertility patterns throughout her reproductive life. It is the best indicator of overall fertility but does not focus on female replacement. * **Fertility Rate:** This is a general term (e.g., General Fertility Rate) referring to the number of live births per 1,000 women of childbearing age in a year. It is a cross-sectional measure, not a cohort projection. * **Net Reproductive Rate (NRR):** This is the number of daughters a newborn girl will bear, **accounting for mortality**. It is the most relevant indicator for population stability. If NRR = 1, the population is at "replacement level." #### 3. High-Yield Clinical Pearls for NEET-PG: * **NRR vs. GRR:** NRR is always lower than or equal to GRR because NRR accounts for the risk of death before completing the reproductive cycle. * **Replacement Level Fertility:** Defined as an **NRR of 1**. This is the demographic goal of the National Health Policy in India. * **TFR Goal:** To achieve an NRR of 1, the corresponding TFR should be approximately **2.1**. * **Formula:** $GRR \times \text{Survival probability} = NRR$.

Question 256: Which of the following statements are true about the epidemiology of leprosy?

A. Leprosy bacilli cannot survive outside the human body and the relapse rate is an indicator of the efficacy of the drug regimen. (Correct Answer)
B. Leprosy bacilli cannot survive outside the human body and the bacterial load is high in the tuberculoid variety.
C. The bacterial load is high in the tuberculoid variety, and insects can transmit the disease.
D. The bacterial load is high in the tuberculoid variety, and the relapse rate is an indicator of the efficacy of the drug regimen.

Explanation: ### **Explanation** **1. Why Option A is Correct:** * **Survival:** *Mycobacterium leprae* is an obligate intracellular pathogen. While it can survive in moist soil or dried nasal secretions for a few days (up to 7–9 days), it cannot multiply outside a living host. For epidemiological purposes, the human reservoir is the only significant source of infection. * **Relapse Rate:** In leprosy, the **Relapse Rate** is the gold standard indicator for the **efficacy of a drug regimen** (MDT). A low relapse rate (currently <1% for MDT) confirms that the treatment is successfully killing the bacilli and preventing recurrence. **2. Why Other Options are Incorrect:** * **Bacterial Load:** In the **Tuberculoid (TT)** variety, the host has high cell-mediated immunity (CMI), resulting in very few bacilli (Paucibacillary). In contrast, the **Lepromatous (LL)** variety has low CMI and a very high bacterial load (Multibacillary). Options B, C, and D are incorrect because they state the load is high in the tuberculoid variety. * **Transmission:** While insects (like flies or mosquitoes) have been studied as mechanical vectors, there is **no scientific evidence** that they play a role in the actual transmission of leprosy to humans. The primary route is droplet infection. **3. High-Yield NEET-PG Pearls:** * **Incubation Period:** Average 3–5 years (longest for any bacterial disease). * **Most Sensitive Indicator:** **Annual New Case Detection Rate (ANCDR)** reflects the transmission of the disease in the community. * **Prevalence Rate:** Used to monitor the progress of leprosy elimination (Target: <1 case per 10,000 population). * **Infectivity:** Lepromatous cases are the most infectious; however, patients become non-infectious within days of starting MDT (specifically after the first dose of Rifampicin).

Question 257: Which of the following diseases shows bimodality?

A. Leiomyoma
B. Measles
C. Hodgkin's lymphoma
D. All the above (Correct Answer)

Explanation: **Explanation:** **Bimodality** in epidemiology refers to a frequency distribution with two distinct peaks. This indicates that the disease occurs more frequently in two specific age groups, genders, or populations, often suggesting two different underlying etiologies or risk factors. **Why "All the above" is correct:** * **Hodgkin’s Lymphoma:** This is the classic textbook example of bimodality. The first peak occurs in young adults (ages 15–35), and the second peak occurs in older adults (after age 50). * **Leiomyoma (Uterine Fibroids):** While most common in the late reproductive years, it shows a bimodal distribution when considering the age of diagnosis versus the age of symptomatic presentation, often peaking in the 30s and again in the perimenopausal period. * **Measles:** In populations with varying vaccination coverage, measles can show bimodality—peaking first in preschool children and later in adolescents or young adults who have waning immunity or were missed during primary immunization. **Other Examples of Bimodality:** * **Breast Cancer:** Shows peaks in the pre-menopausal and post-menopausal periods. * **Leukemia:** Peaks in early childhood (ALL) and again in the elderly (CLL/AML). * **Testicular Cancer:** Peaks in infancy and again in young adulthood. **High-Yield Clinical Pearls for NEET-PG:** * **Unimodal Curve:** Suggests a single source of infection or a uniform incubation period (e.g., a point-source epidemic like Food Poisoning). * **Bimodal Curve:** Suggests two different sets of causative factors or two distinct susceptible age groups. * **Key High-Yield Bimodal Diseases:** Hodgkin’s Lymphoma, Breast Cancer, Leukemia, and Female Schizophrenia.

Question 258: Which type of mosquito larvae lie horizontal on the water surface, resting parallel to it?

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

Explanation: ### Explanation The correct answer is **Anopheles**. The orientation of mosquito larvae at the water surface is a classic morphological feature used in entomological identification. **1. Why Anopheles is correct:** Anopheles larvae lack a long respiratory siphon. Instead, they possess **palmate hairs** on their abdominal segments and a pair of small respiratory spiracles on the 8th abdominal segment. To breathe, they must lie **parallel (horizontal)** to the water surface to keep these spiracles in contact with the air. **2. Why the other options are incorrect:** * **Culex:** These larvae possess a long, narrow respiratory siphon. They hang **at an angle (obliquely)** to the water surface, with only the tip of the siphon touching the surface film. * **Aedes:** Similar to Culex, Aedes larvae have a short, stout respiratory siphon and hang **at an angle** (head down) from the surface. * **Mansonides:** These are unique because they do not come to the surface to breathe. Their specialized siphon **pierces the underwater roots/stems** of aquatic plants (like *Pistia*) to extract oxygen. **3. High-Yield Clinical Pearls for NEET-PG:** * **Adult Resting Posture:** Anopheles adults rest at an angle (45°) to the surface, while Culex and Aedes rest parallel to it (the opposite of their larval stages). * **Eggs:** Anopheles eggs are laid singly and have **lateral floats**. Culex eggs are laid in "rafts," and Aedes eggs are cigar-shaped and laid singly. * **Disease Association:** * *Anopheles:* Malaria. * *Culex:* Japanese Encephalitis, Bancroftian Filariasis. * *Aedes:* Dengue, Chikungunya, Zika, Yellow Fever. * *Mansonides:* Brugian Filariasis (Malayan Filariasis).

Question 259: Which of the following is a risk factor for the development of active tuberculosis in individuals previously infected with tubercle bacilli?

A. Peptic ulcer
B. Silicosis (Correct Answer)
C. Leprosy
D. Acute renal failure

Explanation: **Explanation:** The progression from latent tuberculosis infection (LTBI) to active tuberculosis (TB) depends on the host's immune status and specific co-morbidities. **Why Silicosis is the Correct Answer:** Silicosis is one of the most potent risk factors for developing active TB (often termed **Silicotuberculosis**). The underlying mechanism involves the inhalation of silica particles, which are toxic to **alveolar macrophages**. These macrophages ingest silica, leading to phagolysosomal rupture and cell death. Since macrophages are the primary defense against *Mycobacterium tuberculosis*, their impairment allows dormant bacilli to multiply unchecked. Patients with silicosis have a 3 to 30-fold increased risk of developing TB. **Analysis of Incorrect Options:** * **A. Peptic Ulcer:** While gastrectomy (surgical treatment for ulcers) is a risk factor due to subsequent malnutrition, a simple peptic ulcer does not significantly impair the cell-mediated immunity required to keep TB in check. * **C. Leprosy:** Both are mycobacterial diseases, but leprosy itself does not predispose a patient to TB. In fact, historically, it was hypothesized that some cross-immunity might exist, though they can occasionally occur as co-infections. * **D. Acute Renal Failure (ARF):** While **Chronic Kidney Disease (CKD)** and end-stage renal disease requiring dialysis are major risk factors due to chronic immunosuppression, ARF is a transient state and is not classically associated with the reactivation of latent TB. **High-Yield Clinical Pearls for NEET-PG:** * **Strongest Risk Factor:** HIV infection is the most potent risk factor for the progression of LTBI to active TB. * **Other Key Risk Factors:** Diabetes Mellitus, prolonged corticosteroid therapy, TNF-alpha inhibitors, malnutrition, and excessive alcohol consumption. * **Screening:** In patients with silicosis, a Tuberculin Skin Test (TST) induration of **≥5 mm** is considered positive (similar to HIV-infected individuals).

Question 260: The most potent risk factor for TB among infected individuals is which of the following conditions?

A. Silicosis
B. Diabetes
C. HIV infection (Correct Answer)
D. Chronic renal failure

Explanation: **Explanation:** The progression from latent Tuberculosis (TB) infection to active disease is primarily governed by the host's cell-mediated immunity (CMI). **HIV infection** is the most potent risk factor because it selectively destroys CD4+ T-lymphocytes, which are essential for forming granulomas that sequester *Mycobacterium tuberculosis*. In an immunocompetent individual, the lifetime risk of developing active TB is approximately 5–10%. In contrast, an HIV-positive individual faces a **5–10% annual risk**, making them 20 to 30 times more likely to develop active disease. **Analysis of Incorrect Options:** * **Silicosis (Option A):** While silica particles are toxic to alveolar macrophages and significantly increase TB risk (about 30-fold), HIV remains the stronger biological driver of progression globally. * **Diabetes Mellitus (Option B):** DM triples the risk of TB due to impaired chemotaxis and cytokine production, but its potency is significantly lower than that of HIV. * **Chronic Renal Failure (Option D):** Uremia suppresses CMI, increasing TB risk by 10–25 times, but it does not reach the extreme vulnerability levels seen in advanced HIV/AIDS. **High-Yield Clinical Pearls for NEET-PG:** * **Most common opportunistic infection** in HIV patients in India: Tuberculosis. * **Strategy:** Under the National TB Elimination Program (NTEP), there is a policy of "bidirectional screening" (screening all TB patients for HIV and all HIV patients for TB). * **Risk Hierarchy:** HIV > Silicosis > Chronic Renal Failure > Diabetes > Smoking/Malnutrition. * **Gold Standard for Latent TB:** Interferon-Gamma Release Assay (IGRA) or Mantoux test (though HIV patients may show 'anergy').

Practice by Chapter

Principles of Epidemiology

Practice Questions

Measures of Disease Frequency

Practice Questions

Epidemiological Study Designs

Practice Questions

Descriptive Epidemiology

Practice Questions

Analytical Epidemiology

Practice Questions

Experimental Epidemiology

Practice Questions

Screening for Disease

Practice Questions

Surveillance Systems

Practice Questions

Investigation of an Epidemic

Practice Questions

Association and Causation

Practice Questions

Modern Epidemiological Methods

Practice Questions

Critical Appraisal of Epidemiological Studies

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free