USMLE Step 1 Quiz
Practice Viral Structure And Replication in USMLE Step 1 with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.
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A 28-year-old woman has an abnormal Pap smear showing koilocytotic atypia. Further testing with DNA hybridization confirms an infection with a high-risk strain of human papillomavirus (HPV). The physician explains that this virus can lead to cervical cancer by interfering with the host cell cycle.
The oncogenic potential of this virus is primarily mediated by viral proteins that functionally inactivate which of the following host proteins?
This quiz focuses on Viral Structure And Replication, giving you a quick way to practice the rules, question types, and explanations that matter most for USMLE Step 1.
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A 28-year-old woman has an abnormal Pap smear showing koilocytotic atypia. Further testing with DNA hybridization confirms an infection with a high-risk strain of human papillomavirus (HPV). The physician explains that this virus can lead to cervical cancer by interfering with the host cell cycle.
The oncogenic potential of this virus is primarily mediated by viral proteins that functionally inactivate which of the following host proteins?
Explanation: 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.
A researcher is studying the replication of Poliovirus, a member of the Picornaviridae family. After infecting cultured cells, she observes that host cell protein synthesis is rapidly shut off, and the host ribosomes are used to translate a single, long viral mRNA molecule.
The translation of the poliovirus genome results in which of the following initial products?
Explanation: 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.
An outbreak of acute gastroenteritis occurs on a cruise ship, affecting over 100 passengers and crew members with vomiting and watery diarrhea. The illness is self-limited, resolving in 2-3 days. The public health investigation identifies Norovirus as the causative agent. The virus is known to be highly contagious and environmentally persistent.
Which structural feature of Norovirus most contributes to its stability and ease of transmission via fomites and contaminated food?
Explanation: 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.
A 7-year-old child presents to her pediatrician with a runny nose, sneezing, and a mild sore throat. The physician diagnoses a common cold, most likely caused by a Rhinovirus. The virus enters the cell and its genome is immediately used by the host machinery to synthesize viral proteins.
This ability for immediate translation implies which of the following characteristics about the Rhinovirus genome?
Explanation: Rhinovirus is a picornavirus. Its genome is a positive-sense, single-stranded RNA molecule. The term "positive-sense" means that the viral RNA sequence is equivalent to mRNA and can be directly recognized and translated by the host cell's ribosomes upon entry. This allows for the immediate synthesis of viral proteins without the need for prior transcription. A: Negative-sense RNA must first be transcribed into positive-sense mRNA by a viral polymerase. C: DNA must be transcribed into mRNA. D: Double-stranded RNA must be transcribed to produce mRNA.
A 23-year-old man presents with a painful blister on his upper lip. He reports that these "cold sores" recur in the same spot, especially when he is stressed or ill. The physician diagnoses recurrent herpes labialis, caused by Herpes Simplex Virus 1 (HSV-1).
After the replication of its double-stranded DNA genome, where does the assembly of the HSV-1 nucleocapsid primarily occur?
Explanation: 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.
A 40-year-old man from rural India presents with severe, fulminant hepatitis. Serologic testing reveals he is positive for hepatitis B surface antigen (HBsAg) and also has high titers of antibodies against the hepatitis D virus (HDV).
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?
Explanation: 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.
A 5-year-old girl is brought to the pediatrician with a bright red rash on her cheeks, giving her a "slapped cheek" appearance. Her mother reports the child had a low-grade fever and mild cold-like symptoms a week ago. The pediatrician suspects erythema infectiosum. The causative agent is a non-enveloped virus with a single-stranded DNA genome.
Which of the following best describes a key step in the replication of this virus's genetic material?
Explanation: 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).
A 25-year-old medical student presents with a one-week history of fever, sore throat, profound fatigue, and swollen lymph nodes in his neck. Physical examination reveals pharyngeal erythema and splenomegaly. A peripheral blood smear shows atypical lymphocytes. The patient is diagnosed with infectious mononucleosis caused by Epstein-Barr virus (EBV).
To initiate infection of B lymphocytes, the surface glycoproteins of EBV bind to which of the following host cell receptors?
Explanation: Epstein-Barr virus (EBV), a herpesvirus, primarily infects B lymphocytes and epithelial cells. The major viral glycoprotein, gp350/220, mediates attachment to the host cell by binding to CD21 (also known as the C3d complement receptor, CR2). This interaction is the primary determinant of the virus's tropism for B cells. A: CD4 is the primary receptor for HIV. C: ICAM-1 is the receptor for Rhinovirus. D: Sialic acid is the receptor for Influenza virus.
A 68-year-old man presents with a painful, vesicular rash in a dermatomal distribution on the right side of his chest. He reports having had chickenpox as a child. A diagnosis of herpes zoster (shingles) is made. This condition is caused by the reactivation of a latent virus.
Within which of the following structures does the Varicella-Zoster Virus (VZV) typically establish latency following the primary infection?
Explanation: 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.
A public health official is concerned about a potential smallpox bioterrorism event. Smallpox is caused by the variola virus, a large, complex DNA virus. The official reviews the unique replication strategy of this class of viruses to prepare for potential antiviral development.
Unlike most other DNA viruses, where does the replication of the variola virus genome occur?
Explanation: 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.
During the winter, a new strain of influenza virus emerges, causing a worldwide pandemic. Scientists determine that the new strain contains a combination of RNA segments from human, swine, and avian influenza viruses. This rapid evolution of a novel, highly transmissible virus is a major public health concern.
Which characteristic of the influenza virus genome is most responsible for its ability to undergo this type of major genetic change?
Explanation: 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.
A 45-year-old woman is brought to the emergency department with confusion, agitation, and hydrophobia after being bitten by a raccoon one month ago. The diagnosis of rabies is suspected. The causative agent is an RNA virus with a distinctive morphology.
Which of the following describes the characteristic structure of the rabies virus?
Explanation: 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.
A 19-year-old military recruit develops fever, sore throat, and a persistent cough. A chest X-ray shows bilateral infiltrates, and a diagnosis of acute respiratory disease is made. The outbreak on the base is traced to Adenovirus.
Which of the following correctly describes the structure and genome of Adenovirus?
Explanation: Adenovirus is a non-enveloped (naked) virus with an icosahedral capsid and a linear, double-stranded DNA genome. Its non-enveloped nature makes it very stable in the environment, contributing to its transmission in crowded settings like military barracks. The distinctive fibers protruding from its vertices are involved in attachment to host cells. A: Describes a paramyxovirus or rhabdovirus. C: Describes a herpesvirus or poxvirus. D: Describes a parvovirus.
A 55-year-old man with a history of a blood transfusion in the 1980s is diagnosed with chronic hepatitis C. His physician explains that the virus is difficult for the immune system to clear, leading to persistent infection and chronic liver damage. This is partly due to the error-prone nature of its replication enzyme.
The high rate of mutation in Hepatitis C virus (HCV) is primarily due to the lack of proofreading activity in which of its essential enzymes?
Explanation: Hepatitis C virus (HCV) is a positive-sense single-stranded RNA virus. It replicates its genome using its own virally encoded RNA-dependent RNA polymerase. Unlike DNA polymerases, viral RNA polymerases lack proofreading exonuclease activity. This leads to a high mutation rate during replication, creating a population of slightly different viral variants known as quasispecies. This rapid antigenic variation allows the virus to evade the host immune response, contributing to the establishment of chronic, persistent infection. A, B, and D are not the primary replication enzyme for HCV.
A 6-month-old infant is hospitalized with wheezing, coughing, and respiratory distress. A rapid antigen test on a nasal swab is positive for Respiratory Syncytial Virus (RSV). A characteristic feature of RSV infection in cell culture is the formation of large, multinucleated cells.
This cytopathic effect, known as syncytia formation, is primarily mediated by which of the following viral components?
Explanation: Respiratory Syncytial Virus (RSV), a paramyxovirus, has a surface glycoprotein called the fusion (F) protein. This protein is responsible for fusing the viral envelope with the host cell membrane to allow entry. Critically, the F protein can also be expressed on the surface of an infected host cell, causing it to fuse with neighboring uninfected cells. This cell-to-cell fusion creates large, multinucleated giant cells called syncytia, a hallmark cytopathic effect of RSV and other paramyxoviruses. A and B are surface proteins of the influenza virus involved in attachment and release, respectively. D: The matrix protein is an internal structural protein.
A 9-month-old infant with severe, dehydrating watery diarrhea is admitted to the pediatric ward. Stool analysis is positive for Rotavirus. This virus is a major cause of gastroenteritis in young children worldwide.
Which of the following accurately describes the genetic material of Rotavirus?
Explanation: 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.
A 4-year-old boy with sickle cell disease is brought to the hospital with severe fatigue, pallor, and a dangerously low hemoglobin level. His reticulocyte count is near zero. The physician diagnoses a transient aplastic crisis, a known complication of infection with Parvovirus B19 in patients with underlying hemolytic anemia.
The causative virus has one of the simplest genomes among all viruses that infect humans. Which of the following describes its genome?
Explanation: 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.
A 45-year-old kidney transplant recipient on immunosuppressive therapy develops fever, shortness of breath, and blurred vision. Bronchoalveolar lavage and retinal examination lead to a diagnosis of Cytomegalovirus (CMV) pneumonitis and retinitis. This illness is due to the reactivation of a latent virus.
The reactivation of CMV in this patient most likely originated from a latent viral reservoir established in which of the following cell types?
Explanation: 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.
A 3-year-old child presents with fever and painful swelling in front of his ear and over the angle of his jaw. The pediatrician diagnoses mumps, caused by a Paramyxovirus. This family of viruses initiates infection by attaching to host cells and fusing the viral envelope with the cell membrane.
Which viral surface protein is primarily responsible for the attachment of the mumps virus to host cell receptors?
Explanation: 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.
A 22-year-old woman is found to have anogenital warts during a routine gynecological exam. The physician identifies the cause as a low-risk strain of Human Papillomavirus (HPV). The life cycle of this virus is tightly linked to the differentiation program of the host cell it infects.
In which of the following cell types does HPV complete its productive replication cycle, including assembly and release of new virions?
Explanation: 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.