Question 1

In medical virology, double-stranded RNA (dsRNA) viruses often refer to one strand as the “positive” strand. In this context, what does the positive strand represent functionally within the infected cell?

Accepted Answer

Introduction / Context: Understanding what the “positive” strand means in RNA virus biology is foundational for interpreting how viruses express genes and produce proteins. For double-stranded RNA (dsRNA) viruses, one strand is designated positive because of its direct relationship to protein synthesis. Given Data / Assumptions: We are dealing with dsRNA viruses. Terminology: positive strand, negative strand. Question asks what the positive strand stands for functionally. Concept / Approach: By convention, the positive (+) RNA strand has the same polarity as messenger RNA (mRNA). That means ribosomes can read it directly to synthesize viral proteins if it is accessible. In dsRNA viruses, the genome is double-stranded, but the positive strand corresponds to the coding sequence that can serve as a template for translation when separated and presented as single-stranded RNA. Step-by-Step Solution: Define positive strand: RNA capable of functioning as mRNA. Relate to translation: ribosomes bind mRNA to synthesize proteins. Apply to dsRNA viruses: the + strand equals the sense strand that can be translated once available. Therefore, positive strand stands for mRNA. Verification / Alternative check: In vitro translation systems translate purified +RNA into polypeptides, confirming that positive RNA behaves as mRNA. Why Other Options Are Wrong: rRNA: structural and catalytic component of ribosomes, not a coding template. tRNA: adapter molecule for amino acids, not a coding template. None of these: incorrect because positive strand clearly corresponds to mRNA. Common Pitfalls: Confusing positive-sense single-stranded RNA viruses (which are directly infectious RNA) with dsRNA viruses; in both cases, “positive” still refers to mRNA polarity. Final Answer: mRNA

Question 2

Segmented RNA genomes: Reovirus vs. influenza virus — identify the correct pairing of segment number and nucleic acid type used in classical virology descriptions.

Accepted Answer

Introduction / Context: Segmented viral genomes allow genetic reassortment and rapid evolution. Two exam favorites are the Reoviridae (dsRNA) and Orthomyxoviridae (influenza, negative-sense ssRNA), each with characteristic segment counts. Given Data / Assumptions: Reovirus: a prototypical dsRNA virus. Influenza virus: a negative-sense ssRNA virus. We compare segment numbers and nucleic acid types. Concept / Approach: Classically, orthoreoviruses are described as having 10 dsRNA genome segments (partitioned into L, M, S classes). Influenza A/B viruses possess 8 separate segments of negative-sense ssRNA, each encoding one or more proteins. These canonical numbers are staples in virology curricula. Step-by-Step Solution: Identify Reovirus genome: segmented dsRNA → 10 segments. Identify Influenza genome: segmented negative-sense ssRNA → 8 segments. Match the only option that pairs 10 dsRNA (reovirus) with 8 ssRNA (influenza). Select the correct pairing. Verification / Alternative check: Reference virology handbooks list Reoviridae with 10 segments (orthoreoviruses) and influenza A/B with 8 segments; lab RT-PCR panels target these segment counts. Why Other Options Are Wrong: 8/10, 5/7, 7/5: do not match standard segment counts and nucleic acid types for these viruses. Common Pitfalls: Confusing reovirus with rotavirus (another reovirid) which commonly has 11 segments; the stem names reovirus generically and influenza, whose textbook numbers are 10 and 8 respectively. Final Answer: 10 different segments of dsRNA and 8 different segments of ssRNA respectively

Question 3

Oncologic pathology terminology: The structural abnormality and loss of differentiation seen in malignant cells is termed what in histopathology?

Accepted Answer

Introduction / Context: Cancer pathology uses precise terms to describe cellular changes. Recognizing the word that captures structural abnormality and loss of differentiation is critical for interpreting biopsy reports and exam questions. Given Data / Assumptions: We focus on structural cellular abnormalities. Options include proliferation, dedifferentiation, and spread. Concept / Approach: Anaplasia refers to lack of differentiation and marked pleomorphism with abnormal nuclear morphology, atypical mitoses, and disordered architecture—classic features of malignant transformation. Hyperplasia is an increase in the number of normal cells; metastasis refers to spread of cancer to distant sites, not the structural abnormality itself. Step-by-Step Solution: Define anaplasia: loss of differentiation with structural atypia. Contrast with hyperplasia: increased cell numbers but preserved differentiation. Contrast with metastasis: a process of spread, not morphology. Select anaplasia as the term for structural abnormality. Verification / Alternative check: Histological criteria (nuclear-cytoplasmic ratio, pleomorphism, abnormal mitotic figures) align with anaplasia in standard pathology texts. Why Other Options Are Wrong: Hyperplasia: quantitative increase, architecture relatively normal. Metastasis: dissemination pathway rather than cell structural abnormality. All of these: incorrect because only anaplasia matches the definition. Common Pitfalls: Equating malignancy with metastasis; malignancy can be present without metastasis, and the morphological term is anaplasia. Final Answer: anaplasia

Question 4

Host antiviral response: Which type I interferons are produced by virus-infected human cells as an early innate immune defense?

Accepted Answer

Introduction / Context: Interferons are pivotal cytokines in the innate antiviral response. Early after infection, cells produce interferons that induce an antiviral state in neighboring cells and modulate immunity. Given Data / Assumptions: Question asks which interferons virus-infected cells produce. Options list interferon alpha (IFN-α) and interferon beta (IFN-β). Concept / Approach: Type I interferons include multiple IFN-α subtypes (often from plasmacytoid dendritic cells and other leukocytes) and IFN-β (commonly from fibroblasts and many infected cell types). Both are produced upon sensing viral nucleic acids via pattern recognition receptors and activate interferon-stimulated genes. Step-by-Step Solution: Identify type I interferons induced by viral infection: IFN-α and IFN-β. Note that both are produced by infected or sentinel cells to signal an antiviral state. Select the combined option indicating both α and β. Verification / Alternative check: Gene expression assays show upregulation of IFN-α and IFN-β following RIG-I/MDA5 or TLR signaling; downstream STAT1/STAT2 activation confirms functionality. Why Other Options Are Wrong: Only α or only β: incomplete; both are hallmark type I interferons. None of these: incorrect because type I interferons are indeed produced. Common Pitfalls: Confusing type I interferons (α/β) with type II interferon (IFN-γ), which is primarily produced by NK and T cells and has different roles. Final Answer: Both (a) and (b)

Question 5

Functional role of the positive strand in dsRNA viruses: Among the two strands, what is the primary function served by the positive-sense RNA inside host cells?

Accepted Answer

Introduction / Context: Viral replication depends on converting genetic information into proteins. For dsRNA viruses, understanding which strand can act directly in translation clarifies early steps of infection. Given Data / Assumptions: Genome is double-stranded RNA. One strand is positive-sense (same polarity as mRNA). We seek the function of that positive strand. Concept / Approach: The positive-sense RNA strand corresponds to mRNA. When accessible, it can be translated by host ribosomes to produce viral proteins. Ribosomes themselves are complex ribonucleoprotein machines assembled by the cell; viruses do not produce ribosomes de novo. Step-by-Step Solution: Define positive-sense RNA: has the same polarity as mRNA. Translation: ribosomes read mRNA to synthesize proteins. Apply to dsRNA viruses: the + strand serves as the coding template for protein synthesis once separated. Therefore, its function is protein synthesis, not ribosome production. Verification / Alternative check: In vitro translation of purified +RNA generates viral proteins; ribosome biogenesis uses host nucleolar pathways, independent of viral RNA sense. Why Other Options Are Wrong: Production of ribosomes: a host nuclear/nucleolar process; viruses do not encode complete ribosome assembly. Both (a) and (b): incorrect because only (a) applies. None of these: incorrect because (a) is true. Common Pitfalls: Assuming any viral RNA can create cellular organelles; viral RNAs program protein synthesis, not organelle biogenesis. Final Answer: Synthesis of protein

Question 6

Capsid symmetry in human and animal RNA viruses: Which group below correctly lists viruses whose nucleocapsids exhibit helical symmetry?

Accepted Answer

Introduction / Context: Viral capsids/nucleocapsids show either icosahedral or helical symmetry in most medically important viruses. Recognizing symmetry helps with identification, pathogenesis, and disinfection strategies. Given Data / Assumptions: Candidate viruses: measles, mumps (Paramyxoviridae), influenza (Orthomyxoviridae), rabies (Rhabdoviridae). We ask which have helical nucleocapsids. Concept / Approach: Paramyxoviruses (measles, mumps), orthomyxoviruses (influenza), and rhabdoviruses (rabies) are classic examples of enveloped viruses with helical nucleocapsids, though overall virion shapes vary (e.g., bullet-shaped rabies virions). Step-by-Step Solution: Recall paramyxoviruses: non-segmented negative-sense RNA with helical nucleocapsid. Recall influenza: segmented negative-sense RNA; ribonucleoprotein complexes are helical. Recall rabies: rhabdovirus with characteristic bullet-shaped helical nucleocapsid. All listed viruses therefore share helical nucleocapsid symmetry. Verification / Alternative check: Electron microscopy images and structural studies confirm helical ribonucleoprotein arrangements in these families. Why Other Options Are Wrong: Any single subset omits other correct examples, making them incomplete. Common Pitfalls: Confusing external virion shape with nucleocapsid symmetry; symmetry refers to the internal RNA-protein complex, not necessarily to the envelope contour. Final Answer: all of these

Question 7

Mechanism of antigenic shift in influenza: What process generates major antigenic changes in influenza viruses leading to pandemics?

Accepted Answer

Introduction / Context: Influenza epidemics and pandemics are driven by antigenic evolution. Two mechanisms are classically tested: antigenic drift (point mutations) and antigenic shift (major changes). Given Data / Assumptions: Influenza genome is segmented negative-sense ssRNA (8 segments). Antigenic shift causes abrupt, major antigenic changes. Concept / Approach: Antigenic shift arises when two different influenza viruses co-infect the same cell and exchange genome segments (reassortment). This can create novel hemagglutinin and/or neuraminidase combinations to which human populations lack immunity, precipitating pandemics. Step-by-Step Solution: Identify the unique property enabling shift: segmented genome. Describe event: co-infection and segment reassortment. Outcome: major antigenic novelty in HA/NA leading to pandemics. Select reassortment as the mechanism. Verification / Alternative check: Historical pandemics (e.g., 1957, 1968, 2009) show genomic evidence of segment reassortment events. Why Other Options Are Wrong: Somatic recombination of antibody chains: describes B-cell processes, not viral evolution. Capsule type expression: relevant to bacteria, not influenza viruses. Receptor canyon changes: a concept used in picornaviruses, not the primary mechanism of influenza shift. Common Pitfalls: Confusing antigenic drift (point mutations) with shift (reassortment). Both occur, but only shift produces large, abrupt changes. Final Answer: Reassortment of RNA segments from different influenza viruses

Question 8

Vaccine production substrate: The standard mumps vaccine (e.g., Jeryl Lynn strain) is propagated in which type of cell culture for mass production?

Accepted Answer

Introduction / Context: Live attenuated viral vaccines are grown in specific cell substrates to ensure safety, yield, and regulatory compliance. Mumps vaccine, part of the MMR formulation, has a well-established production history. Given Data / Assumptions: Focus is on the routine production substrate for licensed mumps vaccine strains. Options list several animal cell sources. Concept / Approach: The Jeryl Lynn mumps strain is classically manufactured in chick embryo fibroblast cultures. This substrate supports viral replication and meets safety standards; it is also used for other licensed vaccines. While some older vaccines used primate cells, current standard mumps vaccine production uses chick embryo fibroblasts rather than monkey or mammalian livestock tissues. Step-by-Step Solution: Identify licensed mumps vaccine strain: Jeryl Lynn and derivatives. Recall standard substrate: chick embryo fibroblasts. Exclude primate and livestock options not used for routine mumps vaccine production. Select chick fibroblasts. Verification / Alternative check: Regulatory documents and manufacturer product information list chick embryo fibroblast culture as the substrate for many MMR components including mumps. Why Other Options Are Wrong: African green monkey cells: used historically for some vaccines, but not the standard for mumps. Healthy calves and sheep: not used as primary cell substrates for the licensed mumps vaccine. Common Pitfalls: Assuming all live vaccines are grown in the same primate lines; substrates vary by virus and by regulatory approvals. Final Answer: chick fibroblasts

Question 9

Entry and uncoating of non-enveloped viruses: Viropexis occurs in endocytic (phagocytic) vacuoles and uncoating is facilitated primarily by which lysosomal enzyme activity?

Accepted Answer

Introduction / Context: Many non-enveloped (naked) viruses enter cells via receptor-mediated endocytosis (viropexis). Acidification and lysosomal enzymes then promote capsid disassembly (uncoating) to release the genome. Given Data / Assumptions: Viropexis occurs in endocytic vesicles. Question asks which enzyme class is central to uncoating. Concept / Approach: Lysosomal proteases degrade protein components of the capsid or trigger conformational changes that allow pore formation and genome release. DNA gyrase is a bacterial enzyme (topoisomerase II) and irrelevant to viral uncoating. Lipases target lipids, more relevant to enveloped-virus membrane changes than naked capsid breakdown. Step-by-Step Solution: Locate process: endosome/lysosome with decreasing pH. Identify key catalysts: proteases attacking capsid proteins. Exclude bacterial DNA gyrase and lipases as primary factors for naked capsid uncoating. Choose lysosomal protease. Verification / Alternative check: Inhibiting endosomal acidification or protease activity blocks uncoating for several naked viruses, confirming protease dependence. Why Other Options Are Wrong: DNA gyrase: bacterial enzyme; not present in human lysosomes. Lysosomal lipase: targets lipids; capsid is proteinaceous. All of these: incorrect because only lysosomal protease is appropriate. Common Pitfalls: Assuming all lysosomal enzymes contribute equally; for protein capsids, proteases are the key drivers. Final Answer: lysosomal protease

Question 10

Diagnostic virology: Negri bodies are characteristic intracytoplasmic inclusions observed in neurons infected by which virus?

Accepted Answer

Introduction / Context: Histopathological inclusion bodies are classic diagnostic clues in virology. Negri bodies are among the most well-known, historically used to confirm a lethal zoonotic infection. Given Data / Assumptions: Negri bodies are eosinophilic intracytoplasmic inclusions in neurons. Question asks which virus produces them. Concept / Approach: Negri bodies are pathognomonic for rabies virus infection (Rhabdoviridae). They are found particularly in hippocampal pyramidal cells and Purkinje cells of the cerebellum and represent sites of viral replication and nucleocapsid accumulation. Step-by-Step Solution: Identify inclusion type: Negri bodies in neuronal cytoplasm. Associate with etiologic agent: rabies virus. Exclude other poxviruses and paramyxoviruses that have different inclusion patterns. Select rabies virus. Verification / Alternative check: Immunofluorescence or RT-PCR for rabies virus in brain tissue corroborates histologic findings; animal models show identical inclusions. Why Other Options Are Wrong: Paramyxoviruses: can form syncytia and inclusions but not classic Negri bodies. Vaccinia and fowlpox: poxviruses replicate in cytoplasm with different inclusion types. Common Pitfalls: Confusing general inclusion bodies with Negri bodies; exact morphology and location matter. Final Answer: Rabies virus

Question 11

Oncogenic RNA (sarcoma) retroviruses — In classic cell-culture experiments, which mammalian cell types can these strongly transforming viruses convert into continuously proliferating transformed cells?

Accepted Answer

Introduction / Context: Sarcoma-inducing RNA viruses (historically called RNA tumor viruses; modern classification: oncogenic retroviruses) are classic tools in cancer biology. In vitro, they can transform multiple mammalian cell types, producing hallmarks such as focus formation, loss of contact inhibition, anchorage-independent growth, and altered morphology. Given Data / Assumptions: The question refers to transformation (not mere infection) leading to persistent, neoplastic-like growth in culture. Cell types listed are fibroblasts, myoblasts (muscle precursors), and iris epithelial cells. “Strongly transforming” retroviruses typically carry viral oncogenes (v-onc) that drive rapid transformation. Concept / Approach: Retroviral transformation depends primarily on the presence of a susceptible receptor and a permissive intracellular environment for v-onc expression, not on a single unique lineage. Numerous historical experiments have demonstrated transformation across diverse primary and continuous cell cultures, including fibroblasts (e.g., NIH/3T3), myoblasts, and various epithelial cells. Step-by-Step Solution: Recognize that “strongly transforming” retroviruses encode oncogenes and rapidly induce transformed foci in culture.Note that fibroblasts, myoblasts, and iris epithelial cells have all been transformed in standard assays.Therefore the most inclusive and correct choice is “all of these.” Verification / Alternative check: Transformation has been observed using focus-formation assays, soft agar colony formation, and altered growth kinetics across many mammalian cell types when infected with acutely transforming retroviruses. Why Other Options Are Wrong: Fibroblasts only: too narrow; other lineages are transformable. Myoblasts only: excludes well-documented fibroblast and epithelial transformation. Iris epithelial only: likewise too restrictive. Common Pitfalls: Confusing tropism (receptor usage) with absolute lineage specificity; many cell types are transformable if receptors and intracellular conditions are present. Final Answer: all of these

Question 12

Virus composition — What are the two fundamental components found in all complete virions (across diverse families)?

Accepted Answer

Introduction / Context: Despite their vast diversity, all viruses share a minimal structural plan: a genome packaged by a protective protein shell. Understanding this universal blueprint helps differentiate essential elements from optional features like envelopes. Given Data / Assumptions: “Complete virion” means an infectious particle. Viral genomes may be DNA or RNA (single-stranded or double-stranded, segmented or non-segmented). The protein shell (capsid) provides protection and facilitates delivery to host cells. Concept / Approach: The minimal viral unit contains nucleic acid (genetic material) and capsid protein. Some viruses additionally acquire a host-derived lipid envelope containing viral glycoproteins, but envelopes are not universal. Carbohydrates may decorate glycoproteins but are not obligatory. Thus, among offered pairs, only “nucleic acid and protein” is universally required. Step-by-Step Solution: Identify indispensable genome: DNA or RNA is mandatory.Identify protective shell: capsid protein is mandatory.Exclude non-essential features (lipid envelope, carbohydrates) that are not present in all virions. Verification / Alternative check: Non-enveloped viruses (e.g., adenovirus, picornavirus) demonstrate infectivity with only nucleic acid and capsid proteins, confirming that lipids and polysaccharides are not universal requirements. Why Other Options Are Wrong: Fat and protein: lipids are absent in many non-enveloped viruses. Carbohydrate and nucleic acid: no universal carbohydrate component. Fat and carbohydrate: omits the essential capsid protein and genome pairing. Common Pitfalls: Assuming all viruses are enveloped; many clinically important viruses lack envelopes entirely. Final Answer: nucleic acid and protein

Question 13

Animal virus entry — The uptake of whole virions by host cells through a phagocytic/endocytic process is termed:

Accepted Answer

Introduction / Context: Animal viruses exploit host cell endocytic pathways for entry. A classical term used in virology for engulfment of intact virions into the cell is “viropexis,” conceptually similar to receptor-mediated endocytosis or phagocytosis depending on virus and cell type. Given Data / Assumptions: The question emphasizes engulfment of whole virions. We are focusing on terminology rather than a specific molecular mechanism. Alternative answer choices are distractors not used in standard virology nomenclature. Concept / Approach: “Viropexis” historically denotes the entry by which virions are taken up en bloc, after attachment to specific receptors. Acidification and uncoating typically follow, releasing the genome into the cytosol or via endosomal escape. Step-by-Step Solution: Identify that the described process equals virion engulfment.Recall classical term “viropexis” for this endocytic uptake.Select “viropexis.” Verification / Alternative check: Standard virology texts use “viropexis” in describing uptake of non-enveloped viruses (e.g., adenoviruses) as well as certain enveloped viruses via clathrin-mediated endocytosis. Why Other Options Are Wrong: Vitropexis/vivopexis: nonstandard or incorrect terms in this context. Ectodesmata: not a recognized animal virus entry term. Common Pitfalls: Confusing membrane fusion (direct entry used by many enveloped viruses) with viropexis (endocytic uptake). The stem specifies engulfment of whole virions. Final Answer: viropexis

Question 14

Viral gene expression — In which group is prior transcription of the viral genome into mRNA generally NOT required because the genome itself can function directly as mRNA after entry?

Accepted Answer

Introduction / Context: Viruses must express proteins using host ribosomes. Whether transcription is needed before translation depends on the chemical nature and polarity of the viral genome. Positive-sense RNA genomes can be translated immediately as mRNA. Given Data / Assumptions: Positive-sense single-stranded RNA [(+)ssRNA] acts as mRNA upon cytoplasmic release. Negative-sense RNA [(−)ssRNA] requires viral RNA-dependent RNA polymerase to synthesize complementary mRNA first. DNA viruses require transcription by host or viral DNA-dependent RNA polymerase to make mRNA. Concept / Approach: The only class that can bypass an initial transcription step is the (+)ssRNA viruses (e.g., picornaviruses), whose genomes serve directly as mRNA. The umbrella option “RNA viruses” is correct in common MCQ framing because the exception (direct translation) exists within RNA viruses, whereas dsDNA and ssDNA viruses must be transcribed into RNA first. Step-by-Step Solution: Recognize that mRNA is required for protein synthesis.Identify genome types able to serve as mRNA directly: (+)ssRNA → within RNA viruses.Select “RNA viruses” as the group containing members that do not require a prior transcription step. Verification / Alternative check: Classic examples include poliovirus and other picornaviruses where genomic RNA is infectious and directly translated. Why Other Options Are Wrong: dsDNA/ssDNA viruses must generate mRNA via transcription before translation. “All of these” is incorrect because DNA viruses cannot skip transcription. Common Pitfalls: Forgetting polarity: not all RNA viruses behave the same; only (+)ssRNA genomes are immediately translatable. Final Answer: RNA viruses

Question 15

Virus structure — The nucleocapsid of certain animal viruses is externally covered by a membrane-like layer called the:

Accepted Answer

Introduction / Context: Many viruses acquire an outer lipid bilayer during budding through host membranes. This membrane contains viral glycoproteins essential for entry and immune evasion and is termed the viral envelope. Given Data / Assumptions: “Nucleocapsid” = genome + capsid. Some viruses are enveloped (lipid bilayer) and some are non-enveloped (naked). The question asks for the standard term for the outer membrane-like covering. Concept / Approach: The accepted term is “envelope.” The envelope derives from host membranes (plasma membrane, ER, or Golgi) but is studded with viral proteins that determine host range and antigenicity. Step-by-Step Solution: Recognize the definition of nucleocapsid.Recall that additional lipid membrane = envelope.Select “envelope.” Verification / Alternative check: Influenza, herpesviruses, and coronaviruses are textbook examples of enveloped virions. Why Other Options Are Wrong: “Covering” and “membronocapsid” are not standard terms in virology. “All of these” is invalid because only “envelope” is accepted nomenclature. Common Pitfalls: Equating envelope presence with fragility in all environments; while many envelopes are detergent-sensitive, stability varies by virus. Final Answer: envelope

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