Viral Structure And Replication - USMLE Step 1

1. For the replication of this virus to proceed upon entering a host cell, which of the following enzymes must be carried within the virion?
   Answer: RNA-dependent RNA polymerase The clinical presentation is classic for measles, caused by the measles virus, which is a negative-sense single-stranded RNA virus. Host cells lack the machinery to create mRNA from an RNA template. Therefore, negative-sense RNA viruses must carry their own RNA-dependent RNA polymerase within the virion. Upon entry into the host cell, this enzyme transcribes the negative-sense genome into positive-sense mRNA, which can then be translated by host ribosomes to produce viral proteins. A: Reverse transcriptase is carried by retroviruses. B: DNA-dependent RNA polymerase is a host cell enzyme used for transcription. D: Proteases are carried by some viruses (like HIV) to cleave polyproteins, but the essential enzyme for initiating replication in negative-sense RNA viruses is the polymerase.
2. Within which of the following structures does the Varicella-Zoster Virus (VZV) typically establish latency following the primary infection?
   Answer: Dorsal root ganglia Varicella-Zoster Virus (VZV), a member of the herpesvirus family, causes chickenpox (varicella) as a primary infection. Following this, the virus travels up sensory nerves and establishes a lifelong latent infection in the neurons of the dorsal root or trigeminal ganglia. Reactivation later in life, often due to waning cell-mediated immunity, results in herpes zoster (shingles). A: EBV establishes latency in B lymphocytes. B: VZV replicates in epithelial cells during active infection but does not establish latency there. D: CMV establishes latency in mononuclear cells.
3. The oncogenic potential of this virus is primarily mediated by viral proteins that functionally inactivate which of the following host proteins?
   Answer: p53 and Rb High-risk HPV strains (e.g., 16 and 18) promote carcinogenesis through the actions of two main oncoproteins: E6 and E7. The E6 protein binds to the tumor suppressor protein p53, targeting it for ubiquitin-mediated degradation. The E7 protein binds to the retinoblastoma tumor suppressor protein (Rb), disrupting its ability to inhibit the E2F transcription factor. The inactivation of these two key cell cycle regulators (p53 and Rb) leads to uncontrolled cell proliferation and malignant transformation. A, C, and D are all important proteins in cell cycle regulation and cancer, but they are not the primary targets of HPV oncoproteins E6 and E7.
4. Unlike most other DNA viruses, where does the replication of the variola virus genome occur?
   Answer: Within the host cell cytoplasm Poxviruses, including the variola virus (smallpox) and vaccinia virus, are unique among DNA viruses because their entire replication cycle occurs within the cytoplasm of the host cell. They are large and complex enough to encode their own machinery for DNA replication and transcription (e.g., DNA-dependent DNA polymerase and DNA-dependent RNA polymerase), making them independent of the host cell's nucleus. A: Most DNA viruses (e.g., Herpesviruses, Adenoviruses, Papillomaviruses) replicate in the nucleus. B and D are not primary sites of viral genome replication.
5. The function of which of the following viral proteins would be the most logical target for this therapeutic strategy?
   Answer: Neuraminidase The influenza virus surface protein hemagglutinin (HA) binds to sialic acid receptors on the host cell to initiate infection. When new virions bud from the cell surface, their HA proteins would also bind to sialic acid on the same cell, causing them to clump and preventing their release. The viral neuraminidase (NA) protein is an enzyme that cleaves sialic acid residues from the surface of the host cell and the virion. This action is crucial for releasing the newly formed progeny viruses, allowing them to infect other cells. Therefore, inhibiting neuraminidase (as drugs like oseltamivir and zanamivir do) traps the viruses on the cell surface and halts the spread of infection.
6. Which characteristic of the influenza virus genome is most responsible for its ability to undergo this type of major genetic change?
   Answer: It has a segmented genome. Influenza virus has a segmented genome, consisting of eight separate pieces of negative-sense RNA. When a single host cell (e.g., in a pig) is co-infected with two or more different strains of influenza virus, the RNA segments from the different viruses can mix and be repackaged into new progeny virions. This process, known as genetic reassortment or antigenic shift, can create a novel virus with a new combination of surface antigens (hemagglutinin and neuraminidase), to which the human population has little or no immunity, potentially leading to a pandemic. A, B, and D do not describe the feature that allows for reassortment.
7. Which structural feature of Norovirus most contributes to its stability and ease of transmission via fomites and contaminated food?
   Answer: Absence of a lipid envelope Norovirus is a non-enveloped (naked) virus. Non-enveloped viruses are generally more resistant to environmental stressors such as drying, acid, detergents, and heat compared to enveloped viruses. This stability allows them to persist on surfaces (fomites) and in contaminated food and water, facilitating fecal-oral transmission. A: A segmented genome (like in influenza virus) facilitates reassortment but does not directly contribute to environmental stability. B: Capsid symmetry (helical vs. icosahedral) does not determine environmental resistance. C: A lipid envelope is sensitive to detergents, acid, and drying, making enveloped viruses less stable in the environment.
8. Which of the following enzymes plays an essential role in the replication of the HBV genome, a feature it shares with retroviruses?
   Answer: Viral reverse transcriptase Hepatitis B virus (HBV) is a hepadnavirus with a partially double-stranded circular DNA genome. Its replication cycle is unique. After entering the nucleus, the viral DNA is repaired to form a covalently closed circular DNA (cccDNA). This cccDNA is transcribed by host RNA polymerase II into a pre-genomic RNA. This RNA molecule is then transported to the cytoplasm, where it is reverse-transcribed back into the DNA genome by a virally encoded reverse transcriptase. This is the key step targeted by many anti-HBV drugs. B: Unlike retroviruses, HBV does not typically integrate its genome into the host chromosome. C: Viral protease is critical for viruses like HIV that produce polyproteins, but not for HBV. A: Host RNA polymerase II is involved, but the reverse transcriptase is the unique and essential viral enzyme in this context.
9. Which viral surface protein is primarily responsible for the attachment of the mumps virus to host cell receptors?
   Answer: Hemagglutinin-neuraminidase (HN) protein Paramyxoviruses, such as mumps, measles, and parainfluenza virus, possess two major surface glycoproteins. The hemagglutinin-neuraminidase (HN) protein (or just H protein in measles) is responsible for binding to sialic acid on host cell surface glycoproteins, mediating viral attachment. A: The fusion (F) protein is responsible for mediating the fusion of the viral envelope with the host cell membrane after attachment has occurred. C and D are internal structural proteins not involved in initial host cell attachment.
10. In which of the following cell types does HPV complete its productive replication cycle, including assembly and release of new virions?
    Answer: Terminally differentiated keratinocytes The HPV life cycle is synchronized with the differentiation of epithelial keratinocytes. The virus infects the mitotically active basal cells of the epithelium, where it establishes its genome as a low-copy-number plasmid. As the infected basal cell divides and its progeny migrate towards the surface and differentiate, the viral life cycle progresses. Late gene expression, viral DNA amplification, and capsid protein synthesis occur only in the upper, terminally differentiated layers of the epithelium. New virions are assembled and shed from the surface with the dead keratinocytes. Productive replication does not occur in the basal layer or in non-epithelial cells.
11. Which of the following best describes a key step in the replication of this virus's genetic material?
    Answer: It depends on host cell DNA polymerase during the S phase. The causative agent of erythema infectiosum (fifth disease) is Parvovirus B19, a single-stranded DNA virus. Because it is one of the simplest DNA viruses, it lacks its own DNA polymerase and is dependent on the host cell's machinery for replication. Specifically, it must infect mitotically active cells (like erythrocyte precursors) and utilizes the host's DNA polymerase during the S phase of the cell cycle to replicate its genome. A: Poxviruses are the main DNA viruses that replicate in the cytoplasm; most, including Parvovirus B19, replicate in the nucleus. B: RNA-dependent RNA polymerase is used by RNA viruses, not DNA viruses. D: Reverse transcriptase is used by retroviruses (like HIV) and Hepadnaviruses (like HBV).
12. Which of the following accurately describes the genetic material of Rotavirus?
    Answer: Double-stranded, segmented RNA Rotavirus, a member of the Reoviridae family, has a unique genome consisting of 11 segments of double-stranded RNA (dsRNA). The virus is non-enveloped and has a characteristic triple-layered icosahedral capsid. The dsRNA genome remains within the core of the virus during replication, where a viral RNA polymerase synthesizes mRNA transcripts from each segment. This segmented nature, similar to influenza virus, allows for genetic reassortment.
13. The malignant transformation of T-cells by HTLV-1 is primarily driven by a virally encoded protein that acts as a transcriptional transactivator, altering the expression of numerous host cell genes involved in proliferation. What is this viral protein?
    Answer: Tax HTLV-1 is an oncogenic retrovirus. Unlike many other cancer-causing viruses, it does not integrate near a proto-oncogene or carry a captured oncogene. Instead, its transforming ability is mainly attributed to the viral protein Tax. Tax is a potent transcriptional activator that does not bind DNA directly but interacts with host transcription factors to dysregulate the expression of many cellular genes, including those for cytokines (like IL-2) and their receptors, promoting T-cell proliferation and survival, and inhibiting apoptosis, ultimately leading to malignant transformation. A, B, and D are essential retroviral proteins (polymerase, integration, structural protein) but are not the primary drivers of oncogenesis in HTLV-1.
14. The causative virus has one of the simplest genomes among all viruses that infect humans. Which of the following describes its genome?
    Answer: Single-stranded DNA Parvovirus B19 is the smallest of the human DNA viruses and is unique in that it has a single-stranded DNA (ssDNA) genome. It is a non-enveloped virus with an icosahedral capsid. Its dependence on dividing cells (like erythrocyte precursors) for replication machinery explains its ability to cause aplastic crises in patients with high red blood cell turnover. A: Rotavirus. B: Influenza, measles. C: Herpesviruses, Adenovirus.
15. The translation of the poliovirus genome results in which of the following initial products?
    Answer: A single large polyprotein that is subsequently cleaved Poliovirus, like other picornaviruses, has a positive-sense single-stranded RNA genome that functions directly as an mRNA upon entering the host cell. The entire genome is translated by host ribosomes into a single, large polyprotein. This polyprotein is then cleaved by viral proteases (also part of the polyprotein) into the final, functional structural and non-structural viral proteins. A: This describes viruses with segmented genomes like influenza. C: This is characteristic of retroviruses. D: The positive-sense genome is translated directly; an antisense template is made for genome replication, not for initial protein synthesis.
16. Which of the following enzymes is responsible for this critical step in the HIV replication cycle?
    Answer: Reverse transcriptase HIV is a retrovirus, meaning it has a positive-sense single-stranded RNA genome. A key feature of its replication cycle is the conversion of this RNA genome into double-stranded DNA. This process is called reverse transcription and is catalyzed by the virally encoded enzyme reverse transcriptase (an RNA-dependent DNA polymerase). This DNA copy is then integrated into the host genome. A: RNA-dependent RNA polymerase is used by RNA viruses (e.g., influenza, measles) to replicate their RNA genome. B: DNA-dependent DNA polymerase is used by host cells and most DNA viruses to replicate DNA. D: Integrase is another crucial HIV enzyme, but it acts after reverse transcriptase to insert the newly made viral DNA into the host chromosome.
17. The reactivation of CMV in this patient most likely originated from a latent viral reservoir established in which of the following cell types?
    Answer: Myeloid lineage cells (monocytes) Cytomegalovirus (CMV), a member of the Herpesviridae family, establishes a lifelong latent infection after the primary exposure. The principal site of CMV latency is in hematopoietic progenitor cells of the myeloid lineage, such as monocytes and their precursors in the bone marrow. When the host's cell-mediated immunity is suppressed (e.g., in transplant recipients or AIDS patients), the virus can reactivate from these cells, differentiate, and cause systemic disease. A: This is the site of latency for HSV-1. B: HBV and HCV cause chronic infections in hepatocytes but don't establish true latency in the same manner as herpesviruses. C: EBV can replicate in oropharyngeal epithelial cells.
18. Replication of the hepatitis D virus is entirely dependent on the co-infection with hepatitis B virus. What essential component does HBV provide for the HDV life cycle?
    Answer: The surface antigen for viral packaging and entry Hepatitis D virus (HDV), also known as the delta agent, is a defective virus. It has a small, circular single-stranded RNA genome and its own antigen (delta antigen), but it cannot produce its own envelope protein. To be infectious, HDV must co-opt the hepatitis B surface antigen (HBsAg) from a co-infecting Hepatitis B virus (HBV) to serve as its outer envelope. This allows the HDV particle to attach to and enter new hepatocytes. Therefore, HDV can only infect and replicate in individuals who are also infected with HBV. The other options describe functions that HDV does not require from HBV.
19. Which of the following describes the characteristic structure of the rabies virus?
    Answer: Bullet-shaped and enveloped The rabies virus, a member of the Rhabdoviridae family, has a very distinct morphology. It is an enveloped virus with a helical nucleocapsid that is coiled into a cylindrical, bullet-like shape. This morphology is characteristic and often noted in descriptions of the virus. A: This describes viruses like Adenovirus or Picornavirus. C: This describes Influenza virus. D: This describes viruses like Ebola or Marburg virus.
20. After the replication of its double-stranded DNA genome, where does the assembly of the HSV-1 nucleocapsid primarily occur?
    Answer: Within the nucleus Herpesviruses, including HSV-1, are DNA viruses that replicate their genome inside the host cell nucleus. Following DNA replication and synthesis of capsid proteins (which are translated in the cytoplasm and imported into the nucleus), the viral genome is packaged into newly formed capsids. This assembly of the nucleocapsid occurs within the nucleus. The completed nucleocapsid then buds through the inner nuclear membrane to acquire its initial envelope, which is later modified as it traverses the Golgi and is released from the cell. A: Poxviruses assemble in the cytoplasm. B: Assembly for some viruses occurs at the plasma membrane during budding. D: The Golgi is involved in modifying viral glycoproteins and transport, but not primary nucleocapsid assembly.

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