Viral structure and classification US Medical PG Practice Questions and MCQs
Practice US Medical PG questions for Viral structure and classification. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
Viral structure and classification US Medical PG Question 1: An 82-year-old woman is brought to the physician by her daughter because of a 3-day history of a runny nose, headache, and cough. The patient's grandson recently had similar symptoms. Her vital signs are within normal limits. Pulse oximetry on room air shows an oxygen saturation of 99%. Lungs are clear to auscultation. Testing of nasal secretions is performed to identify the viral strain. Electron microscopy shows a non-enveloped RNA virus with an icosahedral capsid. Binding to which of the following is responsible for the virulence of this virus?
- A. CD21
- B. Integrin
- C. ICAM-1 (Correct Answer)
- D. P antigen
- E. Sialic acid residues
Viral structure and classification Explanation: Both the clinical presentation (runny nose, headache, cough, viral spread) and the electron microscopy finding of a **non-enveloped RNA virus with an icosahedral capsid** are characteristic of a **rhinovirus** (a member of the Picornaviridae family).
***Correct: ICAM-1***
- **Rhinoviruses** primarily infect the upper respiratory tract and bind to **ICAM-1 (intercellular adhesion molecule-1)** on respiratory epithelial cells.
- Approximately **90% of rhinovirus serotypes** use ICAM-1 as their primary cellular receptor.
- This binding facilitates viral entry and is the key factor in the **virulence** of rhinovirus, leading to common cold symptoms.
*Incorrect: CD21*
- **CD21** is the receptor for **Epstein-Barr virus (EBV)**, which causes infectious mononucleosis, a condition distinct from the common cold.
- EBV is an **enveloped DNA virus**, unlike the non-enveloped RNA virus described in this case.
*Incorrect: Integrin*
- **Integrins** are cell adhesion receptors used by some viruses for entry (e.g., certain adenoviruses and enteroviruses).
- However, they are **not the primary receptor** for rhinovirus, which predominantly uses ICAM-1.
- The description of an **icosahedral non-enveloped RNA virus** with upper respiratory symptoms specifically points to rhinovirus and ICAM-1.
*Incorrect: P antigen*
- The **P antigen** (globoside) is the receptor for **parvovirus B19**, which causes Fifth disease (erythema infectiosum) and aplastic crisis.
- Parvovirus B19 is a **non-enveloped DNA virus** (not RNA), which is distinct from the virus described in this question.
*Incorrect: Sialic acid residues*
- **Sialic acid residues** are the receptors for **influenza viruses** (via hemagglutinin) for viral attachment and entry.
- Influenza viruses are **enveloped RNA viruses** and typically cause more severe systemic symptoms (fever, myalgias) than the mild upper respiratory symptoms described here.
Viral structure and classification US Medical PG Question 2: 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 structure and classification 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 structure and classification US Medical PG Question 3: An investigator is studying the replication of a virus in denucleated embryonic fibroblasts. After the fibroblasts are infected with the virus, viral proteins are directly translated from the virion's genetic material using fibroblast ribosomes. The resultant large polypeptides are then cleaved into smaller peptides by viral proteases to generate mature viral proteins. Finally, the virion's genetic material is replicated using a protein translated from the virion's genetic material. Which of the following is the most likely virus being evaluated in this study?
- A. Parvovirus
- B. Human immunodeficiency virus
- C. Measles virus
- D. Molluscum contagiosum virus
- E. Coxsackievirus (Correct Answer)
Viral structure and classification Explanation: ***Coxsackievirus***
- The description of **direct translation** from viral genetic material, **polyprotein cleavage by viral proteases**, and replication by a virally encoded protein is characteristic of **positive-sense single-stranded RNA viruses** like Coxsackievirus.
- This process mirrors the replication strategy of **picornaviruses**, which include Coxsackievirus, where the genomic RNA acts directly as mRNA.
- Critically, picornaviruses replicate entirely in the **cytoplasm**, allowing them to function in **denucleated cells** as described in the study.
*Parvovirus*
- Parvoviruses are **single-stranded DNA viruses** and require the host cell to be in **S-phase** to replicate their DNA, using host enzymes in the **nucleus**.
- They **cannot replicate in denucleated cells** as they depend on nuclear host cell machinery.
- They do not typically use **polyprotein cleavage** as their primary mechanism for generating mature viral proteins.
*Human immunodeficiency virus*
- HIV is a **retrovirus** (RNA virus) that requires **reverse transcriptase** to convert its RNA genome into DNA, which is then **integrated into the host genome in the nucleus**, before transcription and translation.
- **Cannot replicate in denucleated cells** due to its requirement for nuclear integration.
- While it does use **protease cleavage** of polyproteins, the initial steps of DNA synthesis and integration are distinct from the described mechanism.
*Measles virus*
- Measles is a **negative-sense single-stranded RNA virus**; its genome **cannot be directly translated** into proteins.
- It requires an **RNA-dependent RNA polymerase** to synthesize positive-sense mRNA from its genome before protein synthesis can occur.
- The question specifies that viral proteins are **directly translated** from the virion's genetic material, which is incompatible with negative-sense RNA viruses.
*Molluscum contagiosum virus*
- Molluscum contagiosum virus is a **poxvirus**, which is a **double-stranded DNA virus**.
- While poxviruses uniquely replicate entirely in the **cytoplasm** and could theoretically work in denucleated cells, they do not use **direct translation** of their genome.
- Instead, they employ a complex cascade of gene expression with early, intermediate, and late genes, not the direct genome translation and polyprotein cleavage described in the question.
Viral structure and classification 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 structure and classification 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 structure and classification US Medical PG Question 5: A 24-year-old college student presents to student health with 2 days of developing a sore throat, runny nose, and a cough that started today. He states that he has been getting mild fevers which began yesterday. On exam, his temperature is 102.0°F (38.9°C), blood pressure is 135/76 mmHg, pulse is 95/min, and respirations are 12/min. His physician recommends over-the-counter cold medications and reassures him that his symptoms are due to a viral infection that is self-limited. Which of the following best describes the most likely cause of his illness?
- A. Nonenveloped dsRNA virus
- B. Enveloped (+) ssRNA virus (Correct Answer)
- C. Nonsegmented, enveloped (-) ssRNA virus
- D. Segmented, enveloped (-) ssRNA
- E. Nonenveloped dsDNA virus
Viral structure and classification Explanation: ***Enveloped (+) ssRNA virus***
- The symptoms of **sore throat**, **runny nose**, **cough**, and **mild fevers** are characteristic of the **common cold**.
- While **rhinoviruses** (nonenveloped picornaviruses) are the most common cause overall, **coronaviruses** are **enveloped, positive-sense single-stranded RNA viruses** that frequently cause the common cold, accounting for approximately 15-20% of cases.
- Among the given viral classifications, **coronaviruses** best match this presentation of a self-limited upper respiratory infection in an otherwise healthy adult.
*Nonenveloped dsRNA virus*
- This describes **Rotavirus**, which causes **gastroenteritis** with severe diarrhea and vomiting, not upper respiratory symptoms.
- The patient's respiratory symptoms and lack of gastrointestinal complaints rule out this viral type.
*Nonsegmented, enveloped (-) ssRNA virus*
- This describes viruses like **Respiratory Syncytial Virus (RSV)** or **parainfluenza virus**.
- While these cause respiratory infections, they more commonly cause **bronchiolitis** in infants and young children, or **croup** in children.
- In adults, they typically cause more severe lower respiratory symptoms than the mild cold presented here.
*Segmented, enveloped (-) ssRNA*
- This describes **influenza viruses**, which typically present with **abrupt onset**, **high fevers**, **severe myalgias**, **headache**, and **profound fatigue**.
- Influenza is generally **more severe** than the mild, self-limited illness described here.
- The gradual onset over 2 days and reassurance of self-limited disease argue against influenza.
*Nonenveloped dsDNA virus*
- This describes **adenoviruses**, which can cause upper respiratory infections, **conjunctivitis**, **pharyngitis**, and sometimes **gastroenteritis**.
- While adenoviruses can cause cold-like symptoms, the typical mild common cold presentation is more consistent with **coronaviruses** among the enveloped RNA virus options.
Viral structure and classification US Medical PG Question 6: A 12-year-old boy is brought to his pediatrician with a high fever. He was feeling fatigued yesterday and then developed a high fever overnight that was accompanied by chills and malaise. This morning he also started complaining of headaches and myalgias. He has otherwise been healthy and does not take any medications. He says that his friends came down with the same symptoms last week. He is given oseltamivir and given instructions to rest and stay hydrated. He is also told that this year the disease is particularly infectious and is currently causing a global pandemic. He asks the physician why the same virus can infect people who have already had the disease and is told about a particular property of this virus. Which of the following properties is required for the viral genetic change that permits global pandemics of this virus?
- A. One virus that produces a non-functional protein
- B. Segmented genomic material (Correct Answer)
- C. Concurrent infection with 2 viruses
- D. Crossing over of homologous regions
- E. Point mutations in the viral genetic code
Viral structure and classification Explanation: ***Segmented genomic material***
- **Influenza viruses** have a **segmented RNA genome**, which allows for reassortment of genetic material when two different influenza strains co-infect the same host cell.
- This reassortment, known as **antigenic shift**, leads to novel viral strains with significantly altered surface antigens (hemagglutinin and neuraminidase), against which the human population has little to no pre-existing immunity, thereby enabling global pandemics.
*One virus that produces a non-functional protein*
- A single virus producing a **non-functional protein** would likely result in a less virulent or non-viable virus, not a new strain capable of causing a pandemic.
- This scenario describes a defect in viral replication or pathogenesis, not an evolutionary mechanism for immune escape.
*Concurrent infection with 2 viruses*
- While concurrent infection with two different influenza viruses is a prerequisite for **antigenic shift**, it is not the property of the virus itself that permits the genetic change.
- The critical viral property enabling this is its **segmented genome**, which allows genetic material exchange during co-infection.
*Crossing over of homologous regions*
- **Crossing over** typically involves recombination between homologous DNA sequences and is not the primary mechanism for major genetic shifts in influenza viruses, which have an **RNA genome**.
- While RNA recombination can occur, it is a less frequent and less significant driver of pandemic strains compared to reassortment of segmented genomes.
*Point mutations in the viral genetic code*
- **Point mutations** lead to **antigenic drift**, which causes gradual changes in existing influenza strains, requiring annual vaccine updates, but typically does not result in the dramatic antigenic changes needed for a global pandemic.
- Antigenic drift is responsible for seasonal epidemics but insufficient for the large-scale immune evasion seen in pandemics.
Viral structure and classification US Medical PG Question 7: 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 structure and classification 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 structure and classification US Medical PG Question 8: 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 structure and classification 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 structure and classification US Medical PG Question 9: A 6-year-old female from a rural village in Afghanistan presents with her mother to a local health center complaining of leg weakness. Her mother also reports that the patient had a fever, fatigue, and headache a week prior that resolved. The patient has not received any immunizations since being born. Her temperature is 98.6°F (37°C), blood pressure is 110/70 mmHg, pulse is 90/min, and respirations are 18/min. Physical examination reveals 1/5 strength in right hip and knee actions and 0/5 strength in left hip and knee actions. Tone is notably decreased in both lower extremities. Sensation to touch, temperature, and vibration is intact. Patellar and Achilles reflexes are absent bilaterally. The most likely cause of this patient’s condition has which of the following characteristics?
- A. Non-enveloped (+) ssRNA virus (Correct Answer)
- B. Enveloped (-) ssRNA virus
- C. Enveloped (+) ssRNA virus
- D. dsRNA virus
- E. Non-enveloped (-) ssRNA virus
Viral structure and classification Explanation: ***Non-enveloped (+) ssRNA virus***
- This describes **Poliovirus**, the causative agent of polio, which is characterized by **acute flaccid paralysis** and absent reflexes due to damage to **anterior horn cells**.
- The patient's presentation with **unvaccinated status**, **acute onset of asymmetric flaccid paralysis** following a febrile illness, and **areflexia** is highly suggestive of poliomyelitis.
*Enveloped (-) ssRNA virus*
- This describes viruses such as **Measles**, **Mumps**, or **Rabies**, which cause different clinical syndromes and are not typically associated with acute flaccid paralysis.
- While these can cause neurological symptoms, they manifest differently (e.g., encephalitis, specific rashes) and are not the primary cause of the described motor deficits.
*Enveloped (+) ssRNA virus*
- This describes viruses like **Dengue**, **Zika**, or **Coronaviruses (SARS-CoV-2)**, which are associated with various fever syndromes, rashes, or respiratory illness, but not typically the specific flaccid paralysis seen here.
- While some can cause neurological complications (e.g., Guillain-Barré syndrome with Zika), the direct neuronal damage leading to areflexic flaccid paralysis as seen in polio is not characteristic.
*dsRNA virus*
- This describes viruses like **Rotavirus**, which primarily cause **gastroenteritis** (nausea, vomiting, diarrhea), or **Reoviruses**.
- They are not known to cause acute flaccid paralysis or the specific neurological findings described in the patient.
*Non-enveloped (-) ssRNA virus*
- This type of virus is less common, but the description does not fit the typical etiologies for acute flaccid paralysis. Most common medically relevant (-) ssRNA viruses are enveloped.
- This classification does not align with any known human pathogen that presents with the classic symptoms of poliomyelitis.
Viral structure and classification US Medical PG Question 10: A 53-year-old man presents to an urgent care center with severe fever that began during the day along with muscle and joint pains. He states that he felt fine the day before but then developed a fever to 103°F (39.4°C) and had to leave work after which he developed a headache and body pains. The patient states that he was recently in South Asia for a business trip and was otherwise feeling well since returning 2 weeks ago. On exam, the patient’s temperature is 103.3°F (39.6°C), blood pressure is 110/84 mmHg, pulse is 94/min, and respirations are 14/min. On physical exam, the patient appears flushed and has a rash that blanches when touched. On laboratory workup, the pathogen was identified as an enveloped virus with an icosahedral capsid and had positive-sense, single-stranded linear RNA. Which of the following is the most likely cause of this patient's presentation?
- A. Dengue virus (Correct Answer)
- B. Norovirus
- C. Coronavirus
- D. Marburg virus
- E. Saint Louis encephalitis virus
Viral structure and classification Explanation: ***Dengue virus***
- The patient's presentation with **acute onset of high fever**, severe **muscle and joint pains** ("breakbone fever"), headache, and a **blanching rash** after recent travel to **South Asia** is highly characteristic of dengue fever.
- The description of the pathogen as an **enveloped virus** with an **icosahedral capsid** and **positive-sense, single-stranded linear RNA** perfectly matches the **Flaviviridae family** to which the dengue virus belongs.
- Among the options, only dengue virus and Saint Louis encephalitis virus have these exact structural characteristics (both are flaviviruses), but the **clinical presentation** with severe myalgia/arthralgia and travel to South Asia clearly points to dengue.
*Norovirus*
- Norovirus typically causes **gastroenteritis**, characterized primarily by **vomiting, diarrhea**, and abdominal cramps, which are not the dominant symptoms in this patient.
- While fever can occur, it's usually **mild** and not as prominent as the high fever and severe myalgia/arthralgia seen in dengue.
- **Structurally**, norovirus is **non-enveloped** (naked capsid), which does not match the pathogen description.
*Coronavirus*
- Coronaviruses are associated with **respiratory illnesses** (e.g., common cold, SARS, MERS, COVID-19) causing symptoms like cough, shortness of breath, and sore throat.
- While fever and body aches can occur, the **severe joint pains** and typical rash are not hallmarks of coronavirus infections.
- **Structurally**, coronaviruses have **helical nucleocapsid symmetry**, not icosahedral, which excludes this option based on the pathogen description.
*Marburg virus*
- Marburg virus causes a severe **hemorrhagic fever** with symptoms including high fever, severe headache, malaise, followed by gastrointestinal symptoms, and eventually **hemorrhagic manifestations** (e.g., bleeding from orifices, petechiae, purpura).
- The patient's presentation does not describe any hemorrhagic signs, and the rash is blanching, not petechial or purpuric.
- **Structurally**, Marburg is a filovirus with **helical symmetry** and **negative-sense ssRNA**, not positive-sense with icosahedral capsid, which excludes this option.
*Saint Louis encephalitis virus*
- Saint Louis encephalitis virus causes a **neuroinvasive disease** characterized by encephalitis, presenting with altered mental status, seizures, and focal neurological deficits, although some patients may have milder fever and headache.
- While it shares the **same viral structure** as dengue (both are flaviviruses with enveloped, icosahedral, (+)ssRNA), the **clinical presentation** differs significantly—this patient lacks neurological symptoms.
- The prominent **severe myalgia, arthralgia**, typical blanching rash, and **travel history to dengue-endemic South Asia** distinguish dengue from Saint Louis encephalitis.
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