Viral replication cycles US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Viral replication cycles. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Viral replication cycles US Medical PG Question 1: Part of the success of the Streptococcus pyogenes bacterium lies in its ability to evade phagocytosis. Which of the following helps in this evasion?
- A. Streptolysin S
- B. Streptolysin O
- C. Streptokinase
- D. M protein (Correct Answer)
- E. Pyrogenic toxin
Viral replication cycles Explanation: ***M protein***
- The **M protein** is a major virulence factor of *Streptococcus pyogenes* that **inhibits phagocytosis** by binding to factor H, a host complement regulatory protein, preventing C3b deposition.
- It also helps the bacterium adhere to host cells and resist killing by neutrophils.
*Streptolysin S*
- **Streptolysin S** is a **hemolysin** that causes beta-hemolysis on blood agar and contributes to tissue damage by lysing cells.
- While contributing to virulence, its primary role is not direct inhibition of phagocytosis but rather cell lysis.
*Streptolysin O*
- **Streptolysin O (SLO)** is another **hemolysin** that produces pore-forming toxins, leading to cell lysis and tissue destruction.
- It is highly antigenic and often used as a diagnostic marker (ASO titer) for past *S. pyogenes* infections, but it does not directly prevent phagocytosis.
*Streptokinase*
- **Streptokinase** is an enzyme that activates plasminogen, leading to the breakdown of fibrin clots, which helps in the **spread of infection** within tissues.
- Its main function is not to evade phagocytosis but rather to facilitate invasion by dissolving blood clots that would typically wall off the infection.
*Pyrogenic toxin*
- **Pyrogenic toxins** (also known as erythrogenic toxins) are superantigens that cause symptoms like fever and rash (e.g., in scarlet fever) by stimulating a massive, non-specific T-cell activation.
- These toxins contribute to the systemic manifestations of infection but do not directly interfere with the process of phagocytosis.
Viral replication cycles US Medical PG Question 2: A virology student is asked to identify a sample of virus. When subjected to a nonionic detergent, which disrupts lipid membranes, the virus was shown to lose infectivity. The student then purified the genetic material from the virus and subjected it to treatment with RNase, an enzyme that cleaves the phosphodiester linkages in the RNA backbone. A minute amount of the sample was then injected into a human cell line and was found to produce viral particles a few days later. Which of the following viruses was in the unknown sample?
- A. Togavirus
- B. Hepevirus
- C. Calicivirus
- D. Adenovirus
- E. Herpesvirus (Correct Answer)
Viral replication cycles Explanation: ***Herpesvirus***
- The loss of infectivity with nonionic detergents indicates the presence of a **lipid envelope**, a characteristic of herpesviruses.
- The genetic material survived **RNase treatment**, indicating it is **DNA** (not RNA), which is consistent with herpesviruses being DNA viruses.
- Under experimental conditions with **direct intracellular injection**, purified herpesvirus DNA can initiate viral replication by utilizing host cell transcription machinery, ultimately producing viral particles.
*Togavirus*
- Togaviruses are **enveloped RNA viruses**; they would lose infectivity with detergent treatment.
- However, their **RNA genome** would have been destroyed by RNase treatment, preventing any subsequent viral particle production.
*Hepevirus*
- Hepeviruses are **non-enveloped RNA viruses**; they would **not** lose infectivity with nonionic detergent, which contradicts the experimental observation.
- Additionally, their **RNA genome** would be destroyed by RNase, preventing viral replication.
*Calicivirus*
- Caliciviruses are **non-enveloped RNA viruses**, so they would not be inactivated by nonionic detergents.
- Their **RNA genome** would be susceptible to degradation by RNase, precluding viral production.
*Adenovirus*
- Adenoviruses are **non-enveloped DNA viruses**, meaning they would **not lose infectivity** when treated with nonionic detergent, which contradicts the first experimental result.
- Although they have a DNA genome that would survive RNase treatment, the lack of envelope rules them out.
Viral replication cycles US Medical PG Question 3: An outbreak of diphtheria has occurred for the third time in a decade in a small village in South Africa. Diphtheria is endemic to the area with many healthy villagers colonized with different bacterial strains. Vaccine distribution in this area is difficult due to treacherous terrain. A team of doctors is sent to the region to conduct a health campaign. Toxigenic strains of C. diphtheria are isolated from symptomatic patients. Which of the following best explains the initial emergence of a pathogenic strain causing such outbreaks?
- A. Infection with a lytic phage
- B. Conjugation between the toxigenic and non-toxigenic strains of C. diphtheriae
- C. Suppression of lysogenic cycle
- D. Lysogenic conversion (Correct Answer)
- E. Presence of naked DNA in the environment
Viral replication cycles Explanation: ***Lysogenic conversion***
- **Lysogenic conversion** occurs when a temperate bacteriophage infects a bacterium and integrates its DNA into the bacterial genome, carrying genes that confer new properties, such as **toxin production**.
- The **diphtheria toxin** gene is encoded by the *tox* gene carried by the **beta-phage**, which integrates into *Corynebacterium diphtheriae* via lysogeny, converting a non-pathogenic strain into a pathogenic one.
*Infection with a lytic phage*
- A **lytic phage** infects a bacterium, replicates rapidly, and then lyses the host cell, releasing new phage particles; it typically does not integrate into the host genome to confer new stable properties like toxin production.
- Lytic phages are primarily responsible for bacterial destruction, not for conferring new stable virulence factors like the **diphtheria toxin**.
*Conjugation between the toxigenic and non-toxigenic strains of C. diphtheriae*
- **Conjugation** involves the direct transfer of genetic material via a pilus between bacteria, usually involving plasmids. While it can transfer virulence factors, the **diphtheria toxin gene** is chromosomally integrated via a **phage**, not typically transferred through conjugation in this manner.
- *C. diphtheriae* toxin production is specifically associated with the presence of the **toxin gene from a lysogenic bacteriophage**, not plasmid-mediated transfer between strains.
*Suppression of lysogenic cycle*
- **Suppression of the lysogenic cycle** means the phage exits the dormant lysogenic state and enters the lytic cycle, leading to host cell lysis. This would not explain the *initial emergence* or stable acquisition of toxin production.
- If the lysogenic cycle were suppressed, the integrated phage (and thus the **toxin gene**) might be lost or the host cell destroyed, rather than stably expressing a new pathogenic trait.
*Presence of naked DNA in the environment*
- The presence of **naked DNA** in the environment leads to **transformation**, where bacteria take up free DNA from their surroundings. While this can transfer genes, the **diphtheria toxin gene** is specifically introduced into *C. diphtheriae* by a **lysogenic bacteriophage**, not typically by free environmental DNA.
- Transformation is a mechanism for acquiring genetic material, but the origin and mechanism of acquisition for the **diphtheria toxin gene** are well-established as phage-mediated.
Viral replication cycles US Medical PG Question 4: An investigator studying viral replication isolates the genetic material of an unidentified virus strain. After exposing a cell culture to the isolated, purified viral genetic material, the cells begin to produce viral polymerase and subsequently replicate the viral genome. Infection with the investigated strain is most likely to cause which of the following conditions?
- A. Rotavirus infection
- B. Poliomyelitis (Correct Answer)
- C. Hepatitis B
- D. Rabies
- E. Influenza
Viral replication cycles Explanation: ***Poliomyelitis***
- The isolation of **purified viral genetic material** directly leading to viral protein production (polymerase) and genome replication indicates the virus has an **RNA genome that can directly serve as mRNA**.
- **Poliovirus** is a **positive-sense single-stranded RNA (+ssRNA) virus**, meaning its genome can immediately be translated by host ribosomes upon entry, acting like mRNA.
*Rotavirus infection*
- Rotavirus is a **double-stranded RNA (dsRNA) virus** and requires its own **RNA-dependent RNA polymerase** to synthesize mRNA before protein production and genome replication can occur.
- Its purified genetic material alone would not directly lead to viral protein synthesis in the absence of viral enzymes.
*Hepatitis B*
- Hepatitis B virus (HBV) is a **DNA virus** and replicates through an **RNA intermediate** via **reverse transcriptase**.
- Its genetic material cannot directly initiate the production of viral polymerase or genome replication without complex cellular machinery and viral enzymes.
*Rabies*
- Rabies virus is a **negative-sense single-stranded RNA (-ssRNA) virus**, which means its genome cannot be directly translated into protein.
- It requires its own **RNA-dependent RNA polymerase** to first synthesize complementary positive-sense mRNA strands.
*Influenza*
- Influenza virus is also a **negative-sense single-stranded RNA (-ssRNA) virus**.
- Like rabies, it carries its own **RNA-dependent RNA polymerase** to transcribe its genome into mRNA before protein synthesis can begin.
Viral replication cycles US Medical PG Question 5: A 3-year-old boy presents to an urgent care clinic with his mother. She states that his behavior has been lethargic for the past 3 days. She also notes that he has had a runny nose, mild cough, and sore throat during this time. She does not believe that he has been febrile. His temperature is 99.1°F (37.2°C), blood pressure is 105/67 mmHg, pulse is 100/min, respirations are 18/min, and SpO2 97% on room air. Which nucleic acid structure most accurately describes the most likely virus responsible for this boy’s clinical condition?
- A. Double-stranded DNA
- B. Single-stranded, negative-sense RNA
- C. Double-stranded RNA
- D. Single-stranded DNA
- E. Single-stranded, positive-sense RNA (Correct Answer)
Viral replication cycles Explanation: ***Single-stranded, positive-sense RNA***
- The constellation of a runny nose, mild cough, sore throat, and lethargy in a 3-year-old child strongly suggests a **common cold** (viral upper respiratory infection).
- The most frequent causes of the common cold are **rhinoviruses** and **coronaviruses**, both of which possess **single-stranded, positive-sense RNA genomes**.
*Double-stranded DNA*
- Viruses with double-stranded DNA genomes include **adenoviruses** and **herpesviruses**, which can cause respiratory infections but often present with more severe or distinct symptoms (e.g., adenoviral conjunctivitis, herpetic stomatitis).
- While adenoviruses can cause common cold-like symptoms, rhinoviruses and coronaviruses are statistically more prevalent in this clinical picture.
*Single-stranded, negative-sense RNA*
- Viruses with single-stranded, negative-sense RNA genomes include **influenza viruses**, **respiratory syncytial virus (RSV)**, and **paramyxoviruses**.
- While these can cause respiratory symptoms, influenza often presents with more significant **fever** and myalgia, and RSV more commonly causes **bronchiolitis** in young children.
*Double-stranded RNA*
- **Rotaviruses**, which have double-stranded RNA genomes, are primarily associated with **gastroenteritis**, causing severe diarrhea and vomiting, not typical respiratory symptoms.
- Though some reoviruses (a family of double-stranded RNA viruses) can cause respiratory illness, it is less common in this context than the positive-sense RNA viruses.
*Single-stranded DNA*
- **Parvovirus B19**, a single-stranded DNA virus, is known for causing **erythema infectiosum** (fifth disease) and aplastic crises, not general upper respiratory symptoms associated with the common cold.
- There are very few human pathogens with single-stranded DNA genomes that would cause this specific set of symptoms.
Viral replication cycles US Medical PG Question 6: Two viruses, X and Y, infect the same cell and begin to reproduce within the cell. As a result of the co-infection, some viruses are produced where the genome of Y is surrounded by the nucleocapsid of X and vice versa with the genome of X and nucleocapsid of Y. When the virus containing genome X surrounded by the nucleocapsid of Y infects another cell, what is the most likely outcome?
- A. Virions containing genome Y and nucleocapsid Y will be produced
- B. No virions will be produced
- C. Virions containing genome X and nucleocapsid Y will be produced
- D. Virions containing genome Y and nucleocapsid X will be produced
- E. Virions containing genome X and nucleocapsid X will be produced (Correct Answer)
Viral replication cycles Explanation: ***Virions containing genome X and nucleocapsid X will be produced***
- The virus containing **genome X** surrounded by **nucleocapsid Y** is a pseudotype. During the infection of a new cell, the **genome X** will direct the synthesis of new viral components, including **nucleocapsid X**.
- Since the genetic material (genome X) dictates the production of viral proteins, the new virions will be genetically identical to virus X, thus containing its own genome and nucleocapsid.
*Virions containing genome Y and nucleocapsid Y will be produced*
- This is incorrect because the infecting particle carried **genome X**, not genome Y.
- The genetic information encoded in the genome determines the type of progeny viruses produced.
*No virions will be produced*
- This is unlikely as the pseudotyped virus is capable of infection and delivery of a functional genome into the host cell.
- The cell is presumed to be permissive for virus replication.
*Virions containing genome X and nucleocapsid Y will be produced*
- This would only happen if the **nucleocapsid Y** was somehow replicated independently of its original genome, which is not how viral replication works.
- The progeny nucleocapsids are always encoded by the genome that is replicating within the cell.
*Virions containing genome Y and nucleocapsid X will be produced*
- This is incorrect. The infecting virus introduced **genome X** into the cell, not genome Y.
- The genetic material delivered determines the type of viral particles that will be synthesized.
Viral replication cycles US Medical PG Question 7: A 16-year-old Mexican female presents with symptoms of the common cold after the patient's respiratory epithelial cells were infected with Rhinovirus. Due to the presence of the virus, her respiratory epithelial cells begin producing interferon. Which of the following is LEAST likely to be an outcome of the activation of the interferon response?
- A. Interferon binding to nearby uninfected epithelial cells
- B. A rhinovirus-specific, cell-mediated immune response (Correct Answer)
- C. Decreased viral replication within the cell
- D. Activation of NK cells
- E. Upregulation of NK cell ligands on the infected cell
Viral replication cycles Explanation: ***A rhinovirus-specific, cell-mediated immune response***
- The **interferon response** is part of the **innate immune system** and acts as a first line of defense against viral infections.
- While interferons can modulate adaptive immunity, they do not directly trigger a **pathogen-specific B or T cell-mediated immune response**; that function belongs to antigen-presenting cells and lymphocytes.
*Interferon binding to nearby uninfected epithelial cells*
- This is a primary function of interferons, which are secreted by infected cells to warn neighboring cells.
- Upon binding through a **receptor-ligand interaction**, interferons induce an antiviral state in these uninfected cells, making them resistant to viral replication.
*Decreased viral replication within the cell*
- Interferons induce the expression of **antiviral proteins (AVPs)** like PKR and 2',5'-OAS, which inhibit viral protein synthesis and degrade viral RNA, respectively.
- This leads to a significant reduction in the virus's ability to replicate within the infected cell and subsequently in neighboring cells.
*Activation of NK cells*
- Interferons, particularly **Type I interferons (IFN-α/β)**, can directly activate **natural killer (NK) cells**.
- Activated NK cells then play a crucial role in the **innate immune response** by recognizing and killing virus-infected cells.
*Upregulation of NK cell ligands on the infected cell*
- Viruses often downregulate MHC class I molecules to evade cytotoxic T cells, but this can make infected cells more susceptible to NK cell mediated killing (missing self hypothesis).
- Interferons can induce the expression of ligands that are recognized by NK cells, thereby enhancing the ability of NK cells to detect and eliminate infected cells.
Viral replication cycles US Medical PG Question 8: A 24-year-old man presents to the clinic with the complaint of a new rash. The lesions are not bothersome, but he is worried as he has never seen anything like this on his body. Upon further questioning the patient states has been generally healthy except for a one time "horrible" flu-like episode two months ago in June. He has since gotten better. On physical exam the following rash is observed (Figure 1). What is the cause of this patient's rash?
- A. Molluscum contagiosum virus (Correct Answer)
- B. Varicella zoster virus (VZV)
- C. Human immunodeficiency virus (HIV)
- D. Human papilloma virus (HPV)
- E. Staphylococcus aureus cellulitis
Viral replication cycles Explanation: ***Molluscum contagiosum virus***
- The image displays characteristic **umbilicated papules**, which are pathognomonic for **molluscum contagiosum**, a viral skin infection.
- The history of a "horrible" flu-like episode two months prior could suggest recent **immunocompromise** or an acute retroviral syndrome, making the patient more susceptible to or exacerbating molluscum contagiosum.
*Varicella zoster virus (VZV)*
- VZV typically causes **vesicles on an erythematous base** that evolve into **crusted lesions** in a dermatomal distribution (shingles) or widespread (chickenpox), which is not consistent with the described rash.
- While VZV can reactivate due to immunocompromise, the morphology of the rash does not fit a typical VZV presentation like chickenpox or shingles.
*Human immunodeficiency virus (HIV)*
- While **HIV infection** can lead to various skin manifestations, including increased susceptibility to molluscum contagiosum, it is not the direct cause of the rash itself.
- The flu-like episode could represent acute retroviral syndrome, but HIV itself does not cause this specific papular rash, rather it creates an environment for opportunistic infections or conditions like molluscum.
*Human papilloma virus (HPV)*
- HPV causes **warts** (verrucae) which are typically **rough, hyperkeratotic papules** or lesions with a cauliflower-like appearance, a different morphology than seen in the image.
- While HPV infections are common, the characteristic smooth, umbilicated papules seen here are not consistent with typical HPV-induced lesions.
*Staphylococcus aureus cellulitis*
- **Cellulitis** is a **bacterial skin infection** characterized by localized areas of **redness, warmth, swelling, and pain**, often with poorly defined borders, which are absent in this presentation.
- **Staphylococcus aureus** can cause various skin infections, but not the distinct umbilicated papules observed in the image; these lesions are viral, not bacterial.
Viral replication cycles US Medical PG Question 9: A scientist performed an experiment to produce hybrid viruses by mixing two different serotypes of influenza virus, H1N1 and H2N2, in a respiratory epithelium cell line. Several days later, the scientist collected the media and analyzed the viral progeny. She found the following serotypes of virus: H1N1, H2N2, H1N2, and H2N1. Which of the following terms best explains the appearance of new serotypes?
- A. Phenotypic mixing
- B. Complementation
- C. Reassortment (Correct Answer)
- D. Transformation
- E. Recombination
Viral replication cycles Explanation: ***Reassortment***
- **Reassortment** occurs in viruses with **segmented genomes**, like influenza, when a host cell is co-infected with two different viral strains.
- During replication, the progeny viruses can package segments from either parent, leading to novel combinations of surface proteins (**H** and **N** antigens), as seen with the emergence of H1N2 and H2N1.
*Phenotypic mixing*
- **Phenotypic mixing** involves the packaging of the genome of one virus into the capsid or envelope proteins derived from another virus, without genetic exchange.
- While progeny viruses might temporarily display characteristics of both parents, their genetic material remains unchanged, so subsequent generations would revert to the original serotype unless continuous co-infection occurs.
*Complementation*
- **Complementation** happens when one virus provides a functional protein that is deficient in another co-infecting virus, allowing the deficient virus to replicate.
- This process does not lead to the formation of new hybrid serotypes but rather allows a 'defective' virus to replicate alongside a 'helper' virus.
*Transformation*
- **Transformation** in microbiology typically refers to the uptake of foreign genetic material (DNA) by a bacterial cell, altering its genetic makeup.
- In virology, it can also refer to the process by which some viruses induce uncontrolled cell growth in eukaryotic cells, leading to oncogenesis, which is unrelated to the formation of hybrid serotypes.
*Recombination*
- **Recombination** involves the exchange of genetic material between two homologous chromosomes or DNA molecules, leading to a new genetic sequence.
- While it results in new genetic combinations, the term **reassortment** is specifically used for the exchange of entire genome segments characteristic of viruses like influenza.
Viral replication cycles US Medical PG Question 10: When hepatitis D was injected into an immunocompromised mouse, there was no detectable hepatitis D RNA in the blood at any time point during the next several months. When co-injected with hepatitis B, hepatitis D RNA was detected in the blood. Which of the following best describes this phenomenon?
- A. Complementation (Correct Answer)
- B. Transduction
- C. Recombination
- D. Reassortment
- E. Phenotypic mixing
Viral replication cycles Explanation: ***Complementation***
* **Hepatitis D virus (HDV)** is a **defective RNA virus** that requires co-infection with **hepatitis B virus (HBV)** to replicate and produce infectious virions.
* This symbiotic relationship highlights **complementation**, where HBV provides the necessary **surface antigen (HBsAg)** components for HDV assembly and budding, enabling its detectable presence in the blood.
*Transduction*
* **Transduction** is a process where **bacterial DNA** is transferred from one bacterium to another by a **bacteriophage**.
* This mechanism is specific to **bacterial gene transfer** and is not applicable to the replication cycle of hepatitis viruses.
*Recombination*
* **Recombination** involves the **exchange of genetic material** between two different viral genomes, leading to new genetic combinations.
* While recombination can occur in viruses, HDV's reliance on HBV is due to its need for specific structural proteins, not simply genetic exchange.
*Reassortment*
* **Reassortment** occurs when viruses with **segmented genomes** exchange entire gene segments during co-infection, leading to significant genetic shifts.
* HDV has a single-stranded circular RNA genome, which is not segmented, making reassortment an irrelevant mechanism for its replication.
*Phenotypic mixing*
* **Phenotypic mixing** involves the packaging of the genome of one virus into the capsid or envelope proteins of another virus during co-infection.
* While HDV *uses* the HBsAg of HBV, the phenomenon described is not just a transient mixing of proteins but a fundamental requirement for HDV's entire replication cycle and infectivity, which is better described as complementation.
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