Epidemiology — MCQs

Epidemiology Indian Medical PG Practice Questions and MCQs

Question 1581: In a village of 100 children, 60% are immunized against measles. One child travelled outside and returned with measles and infected other 26 children. What is the secondary attack rate

A. 65%
B. 67% (Correct Answer)
C. 80%
D. 27%

Explanation: ***Correct: 67%*** - The **secondary attack rate** is calculated by: **(Number of Secondary Cases / Number of Susceptible Contacts) × 100** - There are 100 children total, with 60% immunized, meaning **60 children are immune** and **40 children are susceptible** - One child traveled with measles (the **index/primary case**), leaving **39 susceptible contacts** (40 - 1 = 39) - **26 new secondary cases** occurred among the 39 susceptible contacts - Calculation: **(26 / 39) × 100 = 66.67% ≈ 67%** *Incorrect: 65%* - This results from incorrectly including the index case in the denominator: (26/40) × 100 = 65% - The index case is the **primary case**, not a susceptible contact, so should be excluded from the denominator *Incorrect: 80%* - This percentage does not align with the given numbers - Would require either more secondary cases or fewer susceptible contacts than provided in the scenario *Incorrect: 27%* - This significantly underestimates the secondary attack rate - Likely results from using the **total population (100)** as denominator instead of susceptible contacts: (26/100) × 100 = 26%

Question 1582: Which of the following is the best parameter to predict virulence of acute infectious illness?

A. Incidence
B. Secondary attack rate
C. Case fatality rate (Correct Answer)
D. Crude death rate

Explanation: ***Case fatality rate*** - The **case fatality rate (CFR)** directly measures the **proportion of individuals diagnosed with a disease who die from it**, reflecting the pathogen's ability to cause death. - A higher CFR indicates greater **virulence** and a more severe disease outcome, making it the most direct predictor of an illness's ability to harm. *Incidence* - **Incidence** measures how often a disease occurs in a population over a specified period, indicating the **risk of contracting the disease**. - It does not provide information about the **severity** or **lethality** of the disease once contracted. *Secondary attack rate* - The **secondary attack rate** quantifies the probability that infection will occur among susceptible individuals within a particular group exposed to a primary case. - While it measures **transmissibility** and the potential for spread within a close group, it does not reflect the **virulence** or severity of the illness itself. *Crude death rate* - The **crude death rate** is the total number of deaths from all causes in a population over a given period, divided by the total population. - This parameter measures **overall mortality** in a population and is not specific to the deaths caused by a particular infectious illness, nor does it solely reflect its virulence.

Question 1583: Pig serves as an amplifier host for -

A. Japanese encephalitis (Correct Answer)
B. West Nile fever
C. Yellow fever
D. KFD

Explanation: ***Japanese encephalitis*** * **Pigs** serve as the primary **amplifying hosts** for the Japanese encephalitis virus (JEV), meaning the virus replicates to high titers in them, increasing the potential for mosquito transmission. * Mosquitoes, particularly *Culex tritaeniorhynchus*, feed on infected pigs and then transmit the virus to humans and other animals. *West Nile fever* * The main **amplifying hosts** for West Nile virus are **birds**, particularly corvids. * Humans and horses are **dead-end hosts**, meaning the virus does not replicate to high enough levels in them to sustain mosquito transmission. *Yellow fever* * The primary reservoir and **amplifying hosts** for yellow fever virus are **monkeys** in sylvatic cycles. * Humans are also infected, but mosquitoes primarily transmit the virus between monkeys or from monkeys to humans. *KFD* * **Kyasanur Forest Disease** is primarily maintained in a cycle involving **ticks** and small **rodents** and **monkeys**, which act as amplifying hosts. * Humans become infected through tick bites, particularly from *Haemaphysalis spinigera*.

Question 1584: In which of the following conditions is man the mammalian reservoir?

A. Epidemic typhus (Correct Answer)
B. Endemic typhus
C. Scrub typhus
D. Rickettsial pox

Explanation: **Epidemic typhus** - In **epidemic typhus**, caused by *Rickettsia prowazekii*, humans are the primary reservoir for the pathogen. - The **human body louse** (*Pediculus humanus corporis*) acts as the vector, transmitting the infection from one human to another. *Endemic typhus* - **Endemic typhus**, caused by *Rickettsia typhi*, primarily has **rodents** (like rats) as its mammalian reservoir. - The disease is transmitted to humans through the **flea** (*Xenopsylla cheopis*). *Scrub typhus* - **Scrub typhus**, caused by *Orientia tsutsugamushi*, has **rodents** (e.g., wild rats and mice) as its reservoir. - It is transmitted to humans by the bite of infected **larval trombiculid mites** (chiggers). *Rickettsial pox* - **Rickettsial pox**, caused by *Rickettsia akari*, maintains a reservoir primarily in **mice**. - The infection is transmitted to humans through the bite of the **mouse mite** (*Liponyssoides sanguineus*).

Question 1585: If a new effective treatment is initiated for a communicable disease and all the other factors remain the same, which of the following is most likely to happen?

A. Neither incidence nor prevalence will change
B. Incidence will not change
C. Incidence and prevalence both will change
D. Prevalence will change but incidence will not change (Correct Answer)

Explanation: ***Prevalence will change but incidence will not change*** - **Incidence** refers to the rate of NEW cases occurring in a population over time - Treatment does NOT prevent new cases from occurring; it only treats existing cases - Therefore, **incidence remains unchanged** when treatment is introduced (assuming all other factors like exposure, transmission, and susceptibility remain constant) - **Prevalence** = Incidence × Duration of disease - Effective treatment **shortens disease duration** by curing patients faster or preventing progression - This directly **reduces the number of existing cases**, thereby **decreasing prevalence** - This is the classic epidemiological principle: **treatment affects prevalence, prevention affects incidence** *Incidence and prevalence both will change* - This would be incorrect for treatment alone - While effective treatment might have secondary effects on transmission in some communicable diseases, the **primary and direct effect is on prevalence only** - To reduce incidence, you need **primary prevention measures** (vaccination, vector control, reducing exposure) not treatment - The question specifically states "all other factors remain the same," meaning transmission dynamics are unchanged *Incidence will not change* - This statement alone is incomplete as it doesn't address prevalence - While correct that incidence won't change, this option fails to indicate what happens to prevalence *Neither incidence nor prevalence will change* - This is completely incorrect - Effective treatment must reduce prevalence by shortening disease duration and curing existing cases - If neither changed, the treatment would be ineffective by definition

Question 1586: The time interval between the receipt of infection by a host and the time when the host becomes infectious is known as–

A. latent period (Correct Answer)
B. serial interval
C. quarantine period
D. generation time

Explanation: ***latent period*** - The **latent period** refers to the time from infection until the individual **first becomes infectious** to others. - During this period, the pathogen replicates within the host but has not yet reached a level where it can be transmitted. - This is distinct from the incubation period, which is the time from infection to onset of symptoms. - An individual may be infectious before, during, or after becoming symptomatic depending on the disease. *serial interval* - The **serial interval** is the duration between the onset of symptoms in a primary case and the onset of symptoms in a secondary case who was infected by the primary case. - It describes the time course of disease transmission from one person to another in a chain of transmission. *quarantine period* - The **quarantine period** is the time during which an individual suspected of being exposed to a communicable disease is isolated to prevent potential transmission. - It aims to monitor for symptoms and ensure that the person does not spread the disease if they become ill. *generation time* - In epidemiology, the **generation time** is the time between infection in a primary case and infection in a secondary case. - This differs from serial interval which measures time between symptom onsets rather than infection times.

Question 1587: All are monitoring indices of leprosy control project except -

A. New case detected per year
B. Rate of new cases with disability
C. Treatment completion rate
D. Lepromin test positive population (Correct Answer)

Explanation: ***Lepromin test positive population*** - A positive **lepromin test** indicates cellular immunity to *Mycobacterium leprae* in previously exposed individuals and is used in classifying leprosy, but not as a **monitoring index** for control projects. - The lepromin test primarily reflects an individual's immune response to the bacteria rather than the overall **disease burden** or **effectiveness of control measures** in a population. *New case detected per year* - This is a crucial **epidemiological indicator** to track the incidence of leprosy within a population and assess the **effectiveness of detection and control efforts**. - A decrease in new cases detected per year indicates progress in reducing **disease transmission**. *Rate of new cases with disability* - This index is vital for monitoring the **severity of leprosy** and the success of early detection and treatment programs. - A reduction in the rate of new cases presenting with **disabilities** signifies improved access to prompt diagnosis and treatment. *Treatment completion rate* - This indicator measures the proportion of patients who successfully complete their prescribed **multidrug therapy (MDT)**. - A high **treatment completion rate** is essential for effective disease control, preventing drug resistance, and reducing the reservoir of infection.

Question 1588: Modes of disease transmission by vectors include all except?

A. Regurgitation
B. Ingestion (Correct Answer)
C. Rubbing of infected excrement
D. Biting

Explanation: ***Ingestion*** - While **ingestion** can be a mode of disease acquisition (e.g., contaminated food/water), it is not a direct mode of **disease transmission by a vector**. - Vectors primarily transmit pathogens through other mechanisms, such as biting or depositing infectious material. *Regurgitation* - This is a common mode of **vector-borne transmission** where the vector, often an insect, regurgitates partially digested blood containing pathogens during a subsequent meal. - Examples include the transmission of **Leishmania** by sandflies. *Rubbing of infected excrement* - This mode involves a vector depositing **infected feces** on the host's skin, which can then be rubbed into a bite wound or mucous membranes. - An example is the transmission of **Trypanosoma cruzi** (Chagas disease) by triatomine bugs. *Biting* - **Biting** is the most direct and common mode of vector-borne transmission, where the vector injects pathogens into the host's bloodstream during feeding. - This includes diseases like **malaria** (mosquitoes), **dengue** (mosquitoes), and **Lyme disease** (ticks).

Question 1589: All of the following diseases may be acquired by ingestion except -

A. Leishmaniasis (Correct Answer)
B. Taeniasis
C. Guinea worm
D. Toxoplasmosis

Explanation: ***Leishmaniasis*** - Leishmaniasis is typically acquired through the bite of an infected **sandfly vector**, not by ingestion. - The parasite lives and multiplies in the sandfly's midgut and is transmitted to humans during a blood meal. *Taeniasis* - **Taeniasis** is acquired by ingesting undercooked meat containing **Taenia larvae** (cysticerci). - Humans can get infected by eating raw or undercooked beef (for *Taenia saginata*) or pork (for *Taenia solium*). *Guinea worm* - **Guinea worm disease** (*Dracunculus medinensis*) is acquired by ingesting water contaminated with water fleas (copepods) that harbor the infective larvae. - The larvae mature in the human host after ingestion of the infected water. *Toxoplasmosis* - **Toxoplasmosis** can be acquired by ingesting undercooked meat containing **Toxoplasma cysts** or by consuming food/water contaminated with cat feces (containing oocysts). - It can also be transmitted congenitally from mother to fetus.

Question 1590: Communicability is best measured by

A. Generation time
B. Case fatality rate
C. Secondary attack rate (Correct Answer)
D. Serial Interval

Explanation: ***Secondary attack rate*** - The **secondary attack rate** directly measures the proportion of susceptible individuals who develop a disease after being exposed to a primary case. - It quantifies the **spread of infection** within a defined contact group, thus reflecting communicability. *Generation time* - **Generation time** is the period between infection of a primary case and infection of secondary cases. - While related to transmission dynamics, it doesn't directly quantify the **likelihood of transmission** upon exposure. *Case fatality rate* - The **case fatality rate** measures the proportion of individuals with a disease who die from it. - It reflects **disease severity** and lethality, not its ability to spread from person to person. *Serial Interval* - The **serial interval** is the time between symptom onset in a primary case and symptom onset in a secondary case. - Similar to generation time, it describes the **temporal spread**, but not the intrinsic communicability or transmissibility rate.

Practice by Chapter

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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