Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 1361: Which of the following viruses is classified as a poxvirus?

A. Variola (Correct Answer)
B. Coxsackie
C. ECHO
D. HSV

Explanation: ***Variola virus*** - **Variola virus** is the causative agent of **smallpox**, a historically significant and highly contagious disease. - It belongs to the **Orthopoxvirus** genus within the **Poxviridae family**, characterized by its large, brick-shaped virion and dsDNA genome. *Coxsackie virus* - **Coxsackieviruses** are part of the **Picornaviridae family**, specifically the **Enterovirus** genus. - They are known to cause a range of diseases including **hand-foot-and-mouth disease**, myocarditis, and aseptic meningitis, and are much smaller than poxviruses. *ECHO virus* - **Echoviruses** (Enteric Cytopathic Human Orphan viruses) are also members of the **Picornaviridae family** and the **Enterovirus** genus. - They are associated with diseases such as **aseptic meningitis**, gastroenteritis, and respiratory infections, completely distinct from poxviruses. *HSV (Herpes Simplex Virus)* - **Herpes Simplex Viruses** belong to the **Herpesviridae family**, characterized by a **linear dsDNA genome** and an enveloped icosahedral capsid. - They cause diseases like **oral and genital herpes** and are structurally and genetically distinct from poxviruses.

Question 1362: What is the primary function of the Nef gene in HIV?

A. Enhancing viral pathogenesis (Correct Answer)
B. Enhancing the expression of host genes
C. Decreasing viral replication
D. Promoting host cell apoptosis

Explanation: ***Enhancing viral pathogenesis*** - The **Nef protein** is critical for efficient viral replication and disease progression in HIV-infected individuals, significantly increasing **viral load** and contributing to immune evasion. - It achieves this by downregulating **CD4** and **MHC class I** molecules on the surface of infected cells, thereby preventing immune recognition and increasing viral infectivity. *Enhancing the expression of host genes* - While Nef interacts with various host proteins, its primary function is not to globally enhance the expression of host genes but rather to modulate specific host pathways to favor viral replication. - Instead, Nef often leads to **downregulation** of specific host proteins important for immune response, such as CD4 and MHC class I. *Decreasing viral replication* - Nef's role is to **increase**, not decrease, viral replication and spread within the host. - Viruses lacking a functional Nef gene show **attenuated replication** in vivo and often reduced pathogenicity. *Promoting host cell apoptosis* - While Nef can influence cellular processes, its direct and primary function is not to promote host cell apoptosis. - In fact, Nef can sometimes prevent early apoptosis of infected cells, allowing more time for virus production and release, thereby contributing to viral persistence.

Question 1363: Influenza virus culture is done on ?

A. Chorioallantoic membrane
B. Allantoic cavity (Correct Answer)
C. Yolk sac
D. All of the options

Explanation: ***Allantoic cavity*** - The **allantoic cavity** of embryonated chicken eggs is the standard and most effective site for isolating and propagating **influenza viruses** for vaccine production and research. - This cavity provides an optimal environment for viral replication, particularly yielding high titers of **hemagglutinin**, a key influenza antigen. *Chorioallantoic membrane* - While embryonated eggs are used for virus culture, the **chorioallantoic membrane (CAM)** is primarily used for cultivating viruses that produce **pocks** (visible lesions), such as Vaccinia and Herpes Simplex Virus. - Influenza virus growth on the CAM is less efficient and typically doesn't produce distinct pocks, making it unsuitable for high-yield propagation compared to the allantoic cavity. *Yolk sac* - The **yolk sac** of embryonated eggs is ideal for growing viruses or bacteria that require a **lipid-rich environment** and replicate intracellularly, such as Chlamydia or Rickettsia. - It is not the preferred site for influenza virus isolation or proliferation due to suboptimal conditions for viral replication and lower viral yields. *All of the options* - While all these sites are components of an embryonated chicken egg, each serves as a host for different types of microorganisms or for specific purposes in virology. - For **influenza virus culture**, the **allantoic cavity** is the specifically utilized site for optimal growth and high viral yield.

Question 1364: Binding of gp120 causes:

A. Infection of target cell
B. Facilitation of co-receptor binding (Correct Answer)
C. Fusing of virus and target cell
D. None of the options

Explanation: ***Facilitation of co-receptor*** - **gp120** binding to the **CD4 receptor** on target cells induces a conformational change in gp120, which then exposes or creates a binding site for a **chemokine co-receptor** (CCR5 or CXCR4). - This interaction is crucial for the subsequent steps of viral entry, as it allows the virus to make further contact with the cell surface. *Infection of target cell* - While binding of gp120 is the *first step* in infection, it does not directly cause the infection itself. - Infection occurs after a series of events including co-receptor binding, membrane fusion, and reverse transcription. *Fusing of virus and target cell* - **Fusion** of the viral and cellular membranes is primarily mediated by **gp41**, which is part of the gp160 envelope glycoprotein complex alongside gp120. - This fusion event *follows* the binding of gp120 to CD4 and the co-receptor, as gp120 binding initiates the conformational changes that expose and activate gp41. *None of the options* - One of the provided options accurately describes a direct consequence of gp120 binding, making this option incorrect.

Question 1365: Virus quantification is done by-

A. Plaque assay (Correct Answer)
B. Studying virus-cell interactions
C. Visualizing virus particles
D. Virus isolation using eggs

Explanation: ***Plaque assay*** - The **plaque assay** is a widely used and quantitative method to determine the number of **infectious viral particles** in a sample. - It involves infecting a monolayer of host cells with serially diluted virus samples, leading to the formation of visible **plaques** (zones of lysed cells), which are then counted. *Studying virus-cell interactions* - Studying **virus-cell interactions** helps understand the mechanisms of viral entry, replication, and egress but does not directly quantify the number of viral particles. - While critical for understanding viral pathogenesis, this approach is qualitative rather than quantitative in nature. *Visualizing virus particles* - **Visualizing virus particles** (e.g., using electron microscopy) allows for counting individual virions, but it quantifies both infectious and non-infectious particles. - This method provides a total particle count rather than a measure of infectivity. *Virus isolation using eggs* - **Virus isolation using eggs** (e.g., embryonated chicken eggs) is a method for propagating and isolating certain viruses, particularly influenza. - While it can be optimized for relative comparisons, it is not primarily a direct quantitative method like the plaque assay for determining infectious viral titers.

Question 1366: What is the role of p24 in HIV?

A. Envelope antigen in HIV
B. Genome of HIV
C. Shell antigen
D. Core antigen of HIV (Correct Answer)

Explanation: ***Core antigen of HIV*** - **p24** is a **structural protein** that forms the viral capsid, which is the shell enclosing the genetic material of the virus. - Due to its abundance and immunogenicity, **p24** serves as a crucial **biomarker** for initial **HIV infection** and is targeted in diagnostic tests. *Envelop antigen in HIV* - The **envelope antigens** of HIV are typically **glycoproteins** like **gp120** and **gp41**, which are involved in viral attachment and entry into host cells. - These envelope proteins are distinct from **p24**, which is an internal core protein. *Genome of HIV* - The **genome of HIV** consists of **two copies of single-stranded RNA**, not a protein. - This **RNA genome** is contained within the viral capsid, which is primarily composed of **p24 protein**. *Shell antigen* - While **p24** does form a "shell" (the capsid) around the viral genome, the term **shell antigen** is not a standard specific classification in virology for p24. - The more precise term for its location and function is **core antigen**.

Question 1367: Which poxvirus does not grow in eggs or animal cells?

A. Cow pox
B. Vaccinia
C. Variola
D. Molluscum contagiosum (Correct Answer)

Explanation: ***Molluscum contagiosum*** - This poxvirus is unique among human poxviruses as it **cannot be propagated in cell culture** or embryonated eggs. - It specifically replicates in **human epidermal cells**, causing characteristic skin lesions. *Cow pox* - Cowpox virus can be readily grown in **cell cultures** (e.g., Vero cells) and in **embryonated eggs**, where it produces pocks on the chorioallantoic membrane (CAM). - It is known for its ability to **infect a wide range of host cells and animals**. *Vaccinia* - Vaccinia virus is a widely studied poxvirus that grows efficiently in various **mammalian cell lines** (e.g., Hela, Vero cells) and on the **chorioallantoic membrane of embryonated eggs**. - Its broad host range and ease of culture make it a popular **viral vector** for research and vaccine development. *Variola* - Variola virus, the causative agent of smallpox, was successfully propagated in **cell cultures** (e.g., primary human embryonic kidney cells) and in **embryonated chicken eggs**, producing characteristic pock lesions. - Its ability to grow in these systems was crucial for **vaccine production** and research before its eradication.

Question 1368: Colorado Tick fever is caused by:

A. Coronaviridae
B. Filoviridae
C. Caliciviridae
D. Reoviridae (Correct Answer)

Explanation: ***Reoviridae*** - Colorado Tick Fever (CTF) is caused by the **Colorado Tick Fever Virus (CTFV)**, which belongs to the genus **Coltivirus** within the family **Reoviridae**. - Reoviridae viruses are **non-enveloped**, double-stranded RNA viruses. *Filoviridae* - This family includes viruses like **Ebola virus** and **Marburg virus**, which cause severe hemorrhagic fevers. - They are **enveloped**, negative-sense single-stranded RNA viruses, distinct from the CTFV. *Coronaviridae* - This family includes viruses like **SARS-CoV-2 (COVID-19)** and SARS-CoV, which cause respiratory illnesses. - They are **enveloped**, positive-sense single-stranded RNA viruses, structurally different from CTFV. *Caliciviridae* - This family includes viruses like **Norovirus**, a common cause of acute gastroenteritis (viral stomach flu). - They are **non-enveloped**, positive-sense single-stranded RNA viruses and do not cause tick-borne illnesses.

Question 1369: A patient with sore throat has a positive Paul Bunnell test, indicating infectious mononucleosis. The causative organism is?

A. EBV (Correct Answer)
B. Adenovirus
C. CMV
D. HSV (Herpes Simplex Virus)

Explanation: ***Correct: EBV*** - The **Paul-Bunnell test** (monospot test) detects **heterophile antibodies**, which are characteristic of acute **Epstein-Barr virus (EBV)** infection. - **EBV** is the primary causative agent of **infectious mononucleosis**, commonly known as "mono." *Incorrect: Adenovirus* - **Adenoviruses** can cause various infections, including **pharyngitis** and **conjunctivitis**, but are not associated with a positive **Paul-Bunnell test** or heterophile antibodies. - While it can cause sore throat, the presence of a **positive Paul-Bunnell test** differentiates it from EBV. *Incorrect: CMV* - **Cytomegalovirus (CMV)** can cause a mononucleosis-like syndrome, but it typically results in a **negative Paul-Bunnell test** (i.e., it is heterophile antibody-negative). - CMV mononucleosis is often seen in individuals who are **immunocompromised** or in infants as a congenital infection. *Incorrect: HSV (Herpes Simplex Virus)* - **Herpes simplex virus (HSV)** causes infections such as **oral herpes (cold sores)** and **genital herpes**, and in some cases, **pharyngitis**. - HSV infection is not associated with a positive **Paul-Bunnell test** or the production of heterophile antibodies.

Question 1370: Coxsackie virus is classified as which type of virus?

A. Pox virus
B. Enterovirus (Correct Answer)
C. Myxovirus
D. Herpes virus

Explanation: ***Enterovirus*** - **Coxsackie virus** belongs to the genus *Enterovirus* within the family *Picornaviridae*. - Enteroviruses are characterized as **non-enveloped, positive-sense, single-stranded RNA viruses** and typically infect the gastrointestinal tract. *Herpes virus* - Herpes viruses are **enveloped, double-stranded DNA viruses** known for causing latent infections. - Examples include HSV-1 (oral herpes) and VZV (chickenpox), which are distinct from Coxsackie. *Pox virus* - Pox viruses are **large, enveloped, double-stranded DNA viruses** that replicate in the cytoplasm of infected cells. - Smallpox and molluscum contagiosum are caused by pox viruses, which have different genetic and structural characteristics than Coxsackie virus. *Myxovirus* - **Myxovirus** is an older classification that once included viruses now categorized into *Orthomyxoviridae* (e.g., influenza) and *Paramyxoviridae* (e.g., measles, mumps). - These are **enveloped, negative-sense, single-stranded RNA viruses**, a different viral structure and replication strategy compared to Coxsackie virus.

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Virology — MCQs

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