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?
Q122
A 7-year-old boy is brought to the physician by his father because of a 1-day history of a pruritic rash on his trunk and face. Five days ago, he developed low-grade fever, nausea, and diarrhea. Physical examination shows a lace-like erythematous rash on the trunk and face with circumoral pallor. The agent most likely causing symptoms in this patient has selective tropism for which of the following cells?
Q123
A 17-year-old boy comes to the physician because of body aches and sore throat for 1 week. He has no history of serious illness and takes no medications. He lives with his parents; they recently adopted a cat from an animal shelter. He is sexually active with one female partner, and they use condoms consistently. His temperature is 38.7°C (101.7°F), pulse is 99/min, and blood pressure is 110/72 mm Hg. Examination shows bilateral posterior cervical lymphadenopathy. The pharynx is red and swollen. Laboratory studies show:
Hemoglobin 15 g/dL
Leukocyte count 11,500/mm3
Segmented neutrophils 48%
Band forms 2%
Basophils 0.5%
Eosinophils 1%
Lymphocytes 45%
Monocytes 3.5%
When the patient's serum is added to a sample of horse erythrocytes, the cells aggregate together. Which of the following is the most likely causal pathogen?
Q124
A 35-year-old man is brought to the emergency department by his wife because of a 1-week history of progressive confusion, myalgia, and nausea. His wife says that he first reported headaches and fatigue 10 days ago, and since then “he has not been himself”. He has refused to drink any liquids for the last day. Two months ago, he helped his neighbor remove a raccoon's den from her backyard. He appears agitated. His temperature is 100.8°F (38.2°C). Examination shows excessive drooling. Muscle tone and deep tendon reflexes are increased bilaterally. Administration of which of the following is most likely to have prevented this patient's condition?
Q125
A 13-year-old girl is brought to the physician because of worsening fever, headache, photophobia, and nausea for 2 days. One week ago, she returned from summer camp. She has received all age-appropriate immunizations. Her temperature is 39.1°C (102.3°F). She is oriented to person, place, and time. Physical examination shows a maculopapular rash. There is rigidity of the neck; forced flexion of the neck results in involuntary flexion of the knees and hips. Cerebrospinal fluid studies show:
Opening pressure 120 mm H2O
Appearance Clear
Protein 47 mg/dL
Glucose 68 mg/dL
White cell count 280/mm3
Segmented neutrophils 15%
Lymphocytes 85%
Which of the following is the most likely causal organism?
Q126
A 17-year-old boy comes to the physician because of fever, fatigue, and a sore throat for 12 days. He was prescribed amoxicillin at another clinic and now has a diffuse rash all over his body. He was treated for gonorrhea one year ago. He has multiple sexual partners and uses condoms inconsistently. He appears lethargic and thin. His BMI is 19.0 kg/m2. His temperature is 38.4°C (101.1°F), pulse 94/min, blood pressure 106/72 mm Hg. Examination shows a morbilliform rash over his extremities. Oropharyngeal examination shows tonsillar enlargement and erythema with exudates. Tender cervical and inguinal lymphadenopathy is present. Abdominal examination shows mild splenomegaly. Laboratory studies show:
Hemoglobin 14 g/dL
Leukocyte count 13,200/mm3
Platelet count 160,000/mm3
Which of the following is the next best step in management?
Q127
A 72-year-old man comes to the physician with chills, nausea, and diffuse muscle aches for 3 days. His niece had similar symptoms 2 weeks ago and H1N1 influenza strain was isolated from her respiratory secretions. He received his influenza vaccination 2 months ago. His temperature is 38°C (100.4°F). A rapid influenza test is positive. Which of the following mechanisms best explains this patient's infection despite vaccination?
Q128
A 67-year-old man comes to the physician because of a 3-day history of fever, chills, headache, and fatigue. He appears ill. His temperature is 39°C (102.2°F). Analysis of nasal secretions shows infection with an enveloped, single-stranded segmented RNA virus. In response to infection with this pathogen, certain cells present antigens from the pathogen to CD8+ T-lymphocytes. Which of the following statements about the molecules used for the presentation of these antigens is most accurate?
Q129
A 14-month-old boy is brought to the clinic for evaluation of a rash. The rash started on the face and spread to the trunk. He also had a fever and cough for the past 2 days. His mother says that they recently immigrated from Asia and cannot provide vaccination records. The physical examination reveals a maculopapular rash on the face, trunk, and proximal limbs with no lymphadenopathy. Blue-white spots are noted on the oral mucosa and there is bilateral mild conjunctival injection. The causative agent of this condition belongs to which of the following virus families?
Q130
An investigator who studies virology obtains a biopsy from the ulcer base of an active genital herpes lesion for viral culture. The cultured virions, along with herpes simplex virions of a different phenotype, are cointroduced into a human epithelial cell in vitro. The progeny viruses are found to have phenotypes that are distinct from the parent strains. Sequencing of these progeny viruses shows that most genomes have material from both parent strains. These findings are best explained by which of the following terms?
Viruses US Medical PG Practice Questions and MCQs
Question 121: 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
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.
Question 122: A 7-year-old boy is brought to the physician by his father because of a 1-day history of a pruritic rash on his trunk and face. Five days ago, he developed low-grade fever, nausea, and diarrhea. Physical examination shows a lace-like erythematous rash on the trunk and face with circumoral pallor. The agent most likely causing symptoms in this patient has selective tropism for which of the following cells?
A. Epithelial cells
B. Monocytes
C. Sensory neuronal cells
D. T lymphocytes
E. Erythroid progenitor cells (Correct Answer)
Explanation: ***Erythroid progenitor cells***
- This patient's symptoms (low-grade fever, nausea, diarrhea, followed by a **lace-like erythematous rash** with **circumoral pallor**) are classic for **Fifth disease** (erythema infectiosum), caused by **Parvovirus B19**.
- **Parvovirus B19** specifically targets and replicates in **erythroid progenitor cells** in the bone marrow, leading to a temporary halt in red blood cell production.
*Epithelial cells*
- Viruses like **herpes simplex virus** or **varicella-zoster virus** selectively infect epithelial cells, but their clinical presentations differ from the described rash.
- While some viruses can affect epithelial cells, it is not the primary target for **Parvovirus B19**, which is responsible for the patient's symptoms.
*Monocytes*
- Viruses such as **cytomegalovirus** and **HIV** have tropism for monocytes and macrophages, but these infections present differently and do not typically cause a **lace-like rash** with **circumoral pallor**.
- Monocytes are not the primary target cells for **Parvovirus B19** in causing Fifth disease.
*Sensory neuronal cells*
- Viruses like **varicella-zoster virus** (causing shingles) and **rabies virus** have tropism for sensory neuronal cells.
- The clinical presentation of a **lace-like rash** and general systemic symptoms does not align with a primary infection of sensory neurons.
*T lymphocytes*
- Viruses like **HIV** and **HTLV-1** primarily target T lymphocytes, leading to immunodeficiency or lymphoproliferative disorders.
- Infection of **T lymphocytes** is not characteristic of **Parvovirus B19**, which targets erythroid precursors.
Question 123: A 17-year-old boy comes to the physician because of body aches and sore throat for 1 week. He has no history of serious illness and takes no medications. He lives with his parents; they recently adopted a cat from an animal shelter. He is sexually active with one female partner, and they use condoms consistently. His temperature is 38.7°C (101.7°F), pulse is 99/min, and blood pressure is 110/72 mm Hg. Examination shows bilateral posterior cervical lymphadenopathy. The pharynx is red and swollen. Laboratory studies show:
Hemoglobin 15 g/dL
Leukocyte count 11,500/mm3
Segmented neutrophils 48%
Band forms 2%
Basophils 0.5%
Eosinophils 1%
Lymphocytes 45%
Monocytes 3.5%
When the patient's serum is added to a sample of horse erythrocytes, the cells aggregate together. Which of the following is the most likely causal pathogen?
A. Cytomegalovirus
B. Influenza virus
C. Toxoplasma gondii
D. Epstein-Barr virus (Correct Answer)
E. Human immunodeficiency virus
Explanation: ***Epstein-Barr virus***
- The patient's symptoms (fever, sore throat, **posterior cervical lymphadenopathy**) combined with **relative lymphocytosis** and a positive test where serum aggregates **horse erythrocytes** (heterophile antibody test, Monospot) are classic for **infectious mononucleosis** caused by EBV.
- While other conditions can cause similar symptoms, the specific constellation of fever, pharyngitis, posterior cervical lymphadenopathy, and a positive heterophile antibody test is highly indicative of EBV infection, particularly in an adolescent.
*Cytomegalovirus*
- CMV can cause a **mononucleosis-like syndrome** with fever, malaise, and abnormal liver function tests, but it typically presents with **absent pharyngitis** and **lymphadenopathy** is less prominent, or generalized rather than predominantly posterior cervical.
- CMV mononucleosis is characterized by a **negative heterophile antibody test**, differentiating it from EBV.
*Influenza virus*
- Influenza typically presents with an **acute onset of fever**, myalgia, headache, and respiratory symptoms like cough and rhinorrhea, and **lymphadenopathy is not a prominent feature**.
- Laboratory tests would show **neutrophilia or normal leukocyte count**, not the significant lymphocytosis seen here, and the heterophile antibody test would be negative.
*Toxoplasma gondii*
- **Toxoplasmosis** can cause **lymphadenopathy**, particularly cervical, but it's often **painless** and generalized. Pharyngitis and significant systemic symptoms like a high fever are less common, and it typically does not present with the same dramatic lymphocytosis.
- The positive heterophile antibody test in this case points away from toxoplasmosis, which would require specific serology for diagnosis and is sometimes linked to **cat exposure**, though not the primary finding here.
*Human immunodeficiency virus*
- **Acute HIV seroconversion syndrome** can present with a **mononucleosis-like illness**, including fever, pharyngitis, rash, and generalized lymphadenopathy, but **posterior cervical lymphadenopathy** specifically is not as classic as with EBV.
- The heterophile antibody test would be **negative** in HIV; diagnosis relies on HIV antigen/antibody testing or viral load measurement.
Question 124: A 35-year-old man is brought to the emergency department by his wife because of a 1-week history of progressive confusion, myalgia, and nausea. His wife says that he first reported headaches and fatigue 10 days ago, and since then “he has not been himself”. He has refused to drink any liquids for the last day. Two months ago, he helped his neighbor remove a raccoon's den from her backyard. He appears agitated. His temperature is 100.8°F (38.2°C). Examination shows excessive drooling. Muscle tone and deep tendon reflexes are increased bilaterally. Administration of which of the following is most likely to have prevented this patient's condition?
A. Inosine monophosphate dehydrogenase inhibitor
B. RNA-dependent DNA polymerase inhibitor
C. Live attenuated vaccine
D. Chemically-inactivated virus (Correct Answer)
E. Immunoglobulin against a bacterial protein
Explanation: ***Chemically-inactivated virus***
- This patient presents with symptoms highly suggestive of **rabies**, including progressive confusion, myalgias, agitation, excessive drooling, and increased muscle tone, following exposure to a raccoon.
- The rabies vaccine is a **chemically-inactivated virus** type that induces active immunity, and post-exposure prophylaxis with this vaccine (along with rabies immunoglobulin) would have prevented the disease.
*Inosine monophosphate dehydrogenase inhibitor*
- **Inosine monophosphate dehydrogenase inhibitors** (e.g., mycophenolate mofetil) are immunosuppressants used to prevent organ transplant rejection or treat autoimmune diseases.
- They do not have a role in preventing or treating viral infections like rabies.
*RNA-dependent DNA polymerase inhibitor*
- **RNA-dependent DNA polymerase inhibitors** (e.g., reverse transcriptase inhibitors) are mainly used in the treatment of **HIV infection**, a retrovirus that uses reverse transcriptase.
- Rabies virus is an RNA virus (rhabdovirus), but it does not use reverse transcriptase, and these inhibitors are not effective against it.
*Live attenuated vaccine*
- While many effective viral vaccines are **live attenuated** (e.g., MMR, varicella), the rabies vaccine used for post-exposure prophylaxis and prevention is not live attenuated, due to safety concerns.
- A live attenuated vaccine, if available and safe, would induce a strong immune response, but it is not the type of vaccine used for rabies in humans.
*Immunoglobulin against a bacterial protein*
- This describes antitoxins or immunoglobulins used against **bacterial infections** or their toxins (e.g., tetanus antitoxin).
- Rabies is a viral infection, and while passive immunization with **rabies immunoglobulin** is part of post-exposure prophylaxis, it is specific to the rabies virus and not a bacterial protein.
Question 125: A 13-year-old girl is brought to the physician because of worsening fever, headache, photophobia, and nausea for 2 days. One week ago, she returned from summer camp. She has received all age-appropriate immunizations. Her temperature is 39.1°C (102.3°F). She is oriented to person, place, and time. Physical examination shows a maculopapular rash. There is rigidity of the neck; forced flexion of the neck results in involuntary flexion of the knees and hips. Cerebrospinal fluid studies show:
Opening pressure 120 mm H2O
Appearance Clear
Protein 47 mg/dL
Glucose 68 mg/dL
White cell count 280/mm3
Segmented neutrophils 15%
Lymphocytes 85%
Which of the following is the most likely causal organism?
A. Echovirus (Correct Answer)
B. Listeria monocytogenes
C. Streptococcus pneumoniae
D. Herpes simplex virus
E. Neisseria meningitidis
Explanation: ***Echovirus***
- The patient's symptoms (fever, headache, photophobia, maculopapular rash, neck rigidity) along with CSF findings of **lymphocytic pleocytosis**, **normal glucose**, and **moderately elevated protein** are highly suggestive of **aseptic meningitis**.
- **Enteroviruses**, such as Echovirus, are the most common cause of **viral (aseptic) meningitis**, especially in children and during summer months, fitting the patient's age and recent summer camp attendance.
*Listeria monocytogenes*
- This organism typically causes meningitis in **neonates, elderly, or immunocompromised individuals**, which does not fit this healthy 13-year-old girl.
- While it can cause lymphocytic pleocytosis, it is less likely given the patient's age and presentation.
*Streptococcus pneumoniae*
- This is a common cause of **bacterial meningitis**, characterized by **PMN predominance (neutrophilic pleocytosis)**, **low CSF glucose**, and **markedly elevated CSF protein**, which are not seen in this case.
- The patient is also described as having received all age-appropriate immunizations, likely including the pneumococcal vaccine.
*Herpes simplex virus*
- HSV can cause aseptic meningitis or encephalitis, but it often presents with **focal neurological deficits** or **seizures** in cases of encephalitis, which are absent here.
- While it can cause lymphocytic pleocytosis, the maculopapular rash is less typical for HSV meningitis compared to enteroviruses.
*Neisseria meningitidis*
- This typically causes **bacterial meningitis** with characteristic CSF findings of **neutrophilic pleocytosis**, **low glucose**, and **high protein**.
- Although it can cause a rash (petechial or purpuric), the CSF profile and absence of petechiae make bacterial meningitis less likely.
Question 126: A 17-year-old boy comes to the physician because of fever, fatigue, and a sore throat for 12 days. He was prescribed amoxicillin at another clinic and now has a diffuse rash all over his body. He was treated for gonorrhea one year ago. He has multiple sexual partners and uses condoms inconsistently. He appears lethargic and thin. His BMI is 19.0 kg/m2. His temperature is 38.4°C (101.1°F), pulse 94/min, blood pressure 106/72 mm Hg. Examination shows a morbilliform rash over his extremities. Oropharyngeal examination shows tonsillar enlargement and erythema with exudates. Tender cervical and inguinal lymphadenopathy is present. Abdominal examination shows mild splenomegaly. Laboratory studies show:
Hemoglobin 14 g/dL
Leukocyte count 13,200/mm3
Platelet count 160,000/mm3
Which of the following is the next best step in management?
A. Heterophile agglutination test (Correct Answer)
B. ELISA for HIV
C. Flow cytometry
D. Anti-CMV IgM
E. Throat swab culture
Explanation: ***Heterophile agglutination test***
- The patient's symptoms (fever, fatigue, sore throat, generalized lymphadenopathy, splenomegaly, and **diffuse rash after amoxicillin exposure**) are highly suggestive of **infectious mononucleosis** caused by Epstein-Barr virus (EBV).
- A **heterophile agglutination test (Monospot test)** is the most appropriate initial diagnostic step for suspected infectious mononucleosis.
- The **amoxicillin-induced morbilliform rash** is a pathognomonic finding in EBV infection, occurring in up to 90% of patients with infectious mononucleosis who receive aminopenicillins.
*ELISA for HIV*
- While the patient has risk factors for HIV (multiple sexual partners, inconsistent condom use, prior gonorrhea), **acute HIV infection** typically presents with a more transient rash and less prominent tonsillar exudates or splenomegaly.
- An HIV ELISA is a reasonable test given his risk factors, but the overall clinical picture points more strongly to mononucleosis, and the rapid onset of rash after amoxicillin is a classic sign of EBV-associated drug reaction.
*Flow cytometry*
- **Flow cytometry** is primarily used for the diagnosis and monitoring of hematological malignancies or immunodeficiencies, not for the initial diagnosis of infectious diseases like mononucleosis.
- It would not be the next best step for evaluating the described clinical presentation.
*Anti-CMV IgM*
- **Cytomegalovirus (CMV) infection** can cause a mononucleosis-like syndrome, but it typically presents without the prominent pharyngitis/tonsillar exudates common in EBV.
- While CMV testing might be considered if the heterophile agglutination test is negative, it is not the initial best step given the classic presentation.
*Throat swab culture*
- A **throat swab culture** is primarily used to diagnose bacterial pharyngitis, such as Group A Streptococcus.
- Given the **diffuse rash after amoxicillin** and the systemic symptoms like splenomegaly and generalized lymphadenopathy, a bacterial infection is less likely to be the primary diagnosis.
- The amoxicillin-induced rash in the setting of EBV is much more diagnostically significant than streptococcal pharyngitis with a drug reaction.
Question 127: A 72-year-old man comes to the physician with chills, nausea, and diffuse muscle aches for 3 days. His niece had similar symptoms 2 weeks ago and H1N1 influenza strain was isolated from her respiratory secretions. He received his influenza vaccination 2 months ago. His temperature is 38°C (100.4°F). A rapid influenza test is positive. Which of the following mechanisms best explains this patient's infection despite vaccination?
A. Random point mutations within viral genome (Correct Answer)
B. Reassortment of viral genome segments
C. Acquisition of viral surface proteins
D. Complementing with functional viral proteins
E. Exchange of viral genes between chromosomes
Explanation: ***Random point mutations within viral genome***
- This patient likely contracted influenza due to **antigenic drift**, which involves **random point mutations** in the genes encoding hemagglutinin and neuraminidase, leading to minor changes in these surface antigens.
- These minor changes can allow the virus to evade the pre-existing immunity from vaccination or prior infection, even if the vaccine was received recently.
*Reassortment of viral genome segments*
- This mechanism, known as **antigenic shift**, involves the exchange of entire gene segments between different influenza strains, leading to **major changes** in surface antigens.
- While it causes pandemics, the scenario described (infection within 2 months of vaccination with H1N1) points more towards antigenic drift, which is a constant process leading to seasonal epidemics.
*Acquisition of viral surface proteins*
- Viruses do not "acquire" surface proteins from other viruses in an active process; their surface proteins are determined by their own genetic material.
- Changes in these proteins occur through mutation or reassortment, not acquisition of pre-formed proteins.
*Complementing with functional viral proteins*
- **Complementation** occurs when a defective virus is rescued by another virus providing a necessary gene product in the same host cell.
- This mechanism allows a virus to replicate but does not explain how a vaccine-induced immunity is bypassed.
*Exchange of viral genes between chromosomes*
- Influenza is an RNA virus, and its genetic material is organized into segments, not chromosomes.
- The exchange of entire gene segments (reassortment) is a known mechanism for influenza, but it would involve segments of the viral genome, not chromosomal exchange.
Question 128: A 67-year-old man comes to the physician because of a 3-day history of fever, chills, headache, and fatigue. He appears ill. His temperature is 39°C (102.2°F). Analysis of nasal secretions shows infection with an enveloped, single-stranded segmented RNA virus. In response to infection with this pathogen, certain cells present antigens from the pathogen to CD8+ T-lymphocytes. Which of the following statements about the molecules used for the presentation of these antigens is most accurate?
A. The molecule is made up of 2 chains of equal length
B. The molecule consists of a heavy chain associated with β2 microglobulin (Correct Answer)
C. The molecule is a product of the HLA-DP, HLA-DQ, and -DR genes
D. The antigens are loaded onto the molecule within lysosomes
E. The molecule is selectively expressed by antigen-presenting cells
Explanation: ***The molecule consists of a heavy chain associated with β2 microglobulin***
- The patient's symptoms and the description of the pathogen (enveloped, single-stranded segmented RNA virus) indicate an **influenza virus** infection. The immune response involving **CD8+ T-lymphocytes** points to **MHC class I** presentation, as CD8+ T cells recognize antigens presented by MHC I.
- **MHC class I molecules** are composed of an α (heavy) chain non-covalently associated with a smaller protein called **β2-microglobulin**. This structure is essential for their proper folding and function in presenting **intracellular antigens** (like viral peptides) to CD8+ T cells.
*The molecule is made up of 2 chains of equal length*
- This statement inaccurately describes the structure of MHC class I molecules, which consist of a heavy chain and the much smaller β2-microglobulin.
- MHC class II molecules are composed of two nearly equally sized chains (α and β), but these molecules present to CD4+ T cells, not CD8+ T cells, and are relevant for extracellular pathogens.
*The molecule is a product of the HLA-DP, HLA-DQ, and -DR genes*
- **HLA-DP, HLA-DQ, and HLA-DR** are genes that encode for **MHC class II molecules**.
- **MHC class I molecules** are encoded by **HLA-A, HLA-B, and HLA-C genes**.
*The antigens are loaded onto the molecule within lysosomes*
- Antigens presented by **MHC class I molecules** are derived from **cytosolic proteins** (e.g., viral proteins synthesized in the cytoplasm) and are loaded onto MHC class I in the **endoplasmic reticulum (ER)**.
- Antigens presented by **MHC class II molecules** are typically derived from **extracellular proteins** taken up by endocytosis and are loaded onto MHC class II in **lysosomes/endosomes**.
*The molecule is selectively expressed by antigen-presenting cells*
- This description typically applies to **MHC class II molecules**, which are primarily expressed on **professional antigen-presenting cells** (e.g., dendritic cells, macrophages, B cells).
- **MHC class I molecules** are expressed on **almost all nucleated cells** in the body, allowing any infected cell to present viral antigens to CD8+ T cells.
Question 129: A 14-month-old boy is brought to the clinic for evaluation of a rash. The rash started on the face and spread to the trunk. He also had a fever and cough for the past 2 days. His mother says that they recently immigrated from Asia and cannot provide vaccination records. The physical examination reveals a maculopapular rash on the face, trunk, and proximal limbs with no lymphadenopathy. Blue-white spots are noted on the oral mucosa and there is bilateral mild conjunctival injection. The causative agent of this condition belongs to which of the following virus families?
A. ssRNA naked viruses
B. ssRNA enveloped viruses (Correct Answer)
C. dsRNA naked viruses
D. dsRNA enveloped viruses
E. ssDNA enveloped viruses
Explanation: ***ssRNA enveloped viruses***
- The clinical presentation with **maculopapular rash** spreading from face to trunk, **fever**, **cough**, **conjunctivitis**, and especially **Koplik's spots** (blue-white spots on oral mucosa) is pathognomonic for **measles** (rubeola).
- Measles virus is a **single-stranded RNA (ssRNA) enveloped virus** belonging to the **Paramyxoviridae family**.
- The envelope contains hemagglutinin and fusion proteins that facilitate viral entry.
*ssRNA naked viruses*
- Includes viruses like picornaviruses (rhinovirus, enterovirus) and caliciviruses (norovirus).
- These cause respiratory infections or gastroenteritis, not the classic measles presentation with Koplik's spots.
*dsRNA naked viruses*
- Example: **Rotavirus** (Reoviridae family), which causes gastroenteritis in children.
- Does not present with maculopapular rash or Koplik's spots.
*dsRNA enveloped viruses*
- Extremely rare in human pathology; no common human disease fits this category.
- Not relevant to measles-like presentations.
*ssDNA enveloped viruses*
- Very rare category; most DNA viruses are dsDNA.
- No human disease with maculopapular rash and Koplik's spots is caused by ssDNA enveloped viruses.
Question 130: An investigator who studies virology obtains a biopsy from the ulcer base of an active genital herpes lesion for viral culture. The cultured virions, along with herpes simplex virions of a different phenotype, are cointroduced into a human epithelial cell in vitro. The progeny viruses are found to have phenotypes that are distinct from the parent strains. Sequencing of these progeny viruses shows that most genomes have material from both parent strains. These findings are best explained by which of the following terms?
A. Recombination (Correct Answer)
B. Reassortment
C. Complementation
D. Phenotypic mixing
E. Transduction
Explanation: **Recombination**
- **Recombination** involves the exchange of genetic material between two different but related viruses co-infecting the same cell, producing progeny viruses with genomes containing sequences from both parents.
- The observation of progeny viruses with **genomes having material from both parent strains** confirms genetic exchange at the nucleic acid level.
*Reassortment*
- **Reassortment** specifically refers to the exchange of entire gene segments between viruses with **segmented genomes**, such as influenza virus.
- Herpes simplex virus has a **non-segmented DNA genome**, making reassortment an unlikely mechanism.
*Complementation*
- **Complementation** occurs when one virus provides a missing gene product (protein) that another virus, defective in that gene, needs to replicate.
- It does not involve the **actual exchange or mixing of genetic material** in the progeny virions, unlike what is described in the scenario.
*Phenotypic mixing*
- **Phenotypic mixing** involves the packaging of the genome of one virus into the capsid proteins of another coinfecting virus, or a mixture of both.
- The genetic material (genome) inside the virion remains unchanged, which contradicts the finding that **most genomes have material from both parent strains**.
*Transduction*
- **Transduction** is a process by which **bacteriophages** transfer bacterial DNA from one bacterium to another.
- This term is specific to **bacteria and their viruses (bacteriophages)** and is not applicable to human viruses like herpes simplex in eukaryotic cells.
