A 20-year-old man presents to the emergency department with complaints of severe malaise, fevers, and sore throat for the past 7 days. He also has had episodes of nausea and vomiting during this period. He does not smoke or drink alcohol. There is no family history of liver disease. His blood pressure is 130/80 mm Hg, temperature is 38.3℃ (100.9℉), pulse is 102/min, and respiratory rate is 20/min. On physical examination, he appears ill with bilateral cervical lymphadenopathy. His tonsils are erythematous and enlarged. There is no jaundice and he is mildly dehydrated. Abdominal examination demonstrates splenomegaly. The laboratory findings are shown below:
Hemoglobin 15 g/dL
Platelet count 95,000/mm³
Leukocytes 13,500/mm³
Neutrophils 50%
Atypical lymphocytes 34%
AST 232 U/L
ALT 312 U/L
ALP 120 U/L
GGT 35 U/L
Total bilirubin 1.2 mg/dL
Direct bilirubin 0.2 mg/dL
PT 12 seconds
The serologic test for hepatitis A, B, and C, CMV, and leptospirosis are negative. Serology for both serum IgM and IgG antibodies for EBV capsid antigen are positive, but the heterophile antibody test is negative. What is the most likely reason for the negative heterophile test?
Q82
A 4-year-old Caucasian male patient presents with recurrent infections. During examination of his CD4 T-cells, it is noticed that his T-cells lack CD40 ligand. Which type of immunoglobulin is likely to be present in excess?
Q83
A young researcher is studying the structure of class I and class II major histocompatibility complex (MHC) molecules. He understands that these molecules are proteins, but the structures of class I MHC molecules are different from those of class II. Although all these molecules consist of α and β chains, some of their domains are polymorphic, meaning they are different in different individuals. He calls them ‘P’ domains. The other domains are nonpolymorphic, which remain invariant in all individuals. He calls these domains ‘N’ domains. Which of the following are examples of ‘N’ domains?
Q84
A previously healthy 6-week-old infant is brought to the emergency department because of fever, fatigue, and dry cough for one day. She has been feeding poorly and had difficulty latching on to breastfeed since yesterday. She has had nasal congestion. The mother reports that her daughter has not been going through as many diapers as usual. She was born by uncomplicated vaginal delivery at 42 weeks' gestation. Her mother is a cystic fibrosis carrier. The patient has been treated with acetaminophen for the last 24 hours, and vitamin D drops since birth. She appears irritable, pale, and lethargic. She is at the 25th percentile for both length and weight; she had the same percentiles at birth. Her temperature is 38.2°C (100.7°F) and respirations are 64/min. Pulse oximetry on room air shows an oxygen saturation of 92%. Examination shows an ill-appearing infant with a cough and nasal flaring. Mucous membranes are dry. Chest examination shows intercostal and supraclavicular retractions. Expiratory wheezes are heard on auscultation. Which of the following is the most likely causal organism?
Q85
A 7-year-old boy with a sore throat, fever, and generalized malaise is admitted to the pediatric floor. On physical examination, he has diffuse white exudate on both tonsils, and also a palpable spleen with mild hepatomegaly. His blood smear shows large and abundant lymphocytes with blue-gray cytoplasm, irregular nuclei, and dark chromatin with inconspicuous nucleoli. Which of the following is the most likely diagnosis?
Q86
A 4-year-old boy is brought to the emergency department for evaluation of a fever for 1 day. The mother reports that he has had severe pain in his lower extremities and difficulty eating since yesterday. He has not had a cough, nausea, or vomiting. He was born at term and has been healthy. His immunizations are up-to-date. He appears irritable. His temperature is 38.5°C (101.3°F). Examination shows several flesh-colored, tender papules over the trunk, knees, palms, and soles. There are multiple 2-mm, reddish macules on the hard palate. The remainder of the examination shows no abnormalities. Which of the following is the most likely causal organism of this patient's symptoms?
Q87
A previously healthy 15-year-old girl is brought to the emergency department 24 hours after the onset of a severe headache. She returned from a 1-week camping trip 3 days ago; she went spelunking and swimming in a freshwater lake during the trip. She is agitated, uncooperative, and oriented only to person. Her temperature is 38.9°C (102°F), pulse is 112/min, respirations are 20/min, and blood pressure is 100/68 mm Hg. There are several crusted insect bites on her extremities. Neurologic examination shows diffuse hyperreflexia and an extensor plantar response bilaterally. Her neck is supple without lymphadenopathy. An MRI of the brain shows asymmetrical, bitemporal hyperintensities. A lumbar puncture is performed. Cerebrospinal fluid analysis shows:
Opening pressure 150 mm H2O
Glucose 58 mg/dL
Protein 108 mg/dL
Leukocyte count 150/mm3
Segmented neutrophils 15%
Lymphocytes 85%
Erythrocyte count 25/mm3
Which of the following is the most likely causal pathogen?
Q88
A previously healthy 53-year-old man is brought to the emergency department 45 minutes after the onset of a severe headache. He returned from a vacation in the mountains 4 days ago, during which he went swimming in a freshwater lake. On arrival, he is confused. His temperature is 39°C (102.2°F) and blood pressure is 105/68 mm Hg. Neurologic examination shows diffuse hyperreflexia. An MRI of the brain shows asymmetrical, bitemporal hyperintensities. A lumbar puncture is performed. Cerebrospinal fluid analysis shows:
Leukocyte count 120/mm3
Segmented neutrophils 10%
Lymphocytes 90%
Erythrocyte count 15/mm3
Glucose 45 mg/dL
Opening pressure 130 mm Hg
Protein 75 mg/dL
Which of the following is the most likely causal pathogen?
Q89
Two patients are vaccinated for poliomyelitis. Patient A receives the Sabin oral vaccine, and Patient B receives the Salk intramuscular vaccine. Six weeks after their initial vaccinations, which of the following would be the greatest difference regarding these two patients?
Q90
A mother brings her 2-year-old son to your office after she noticed a “sore on the back of his throat.” She states that her son had a fever and was complaining of throat pain 2 days ago. The child has also been fussy and eating poorly. On examination, the child has met all appropriate developmental milestones and appears well-nourished. He has submandibular and anterior cervical lymphadenopathy. On oral examination, less than 10 lesions are visible on bilateral tonsillar pillars and soft palate with surrounding erythema. After 4 days, the lesions disappear without treatment. Which of the following is the most likely causative agent?
Viruses US Medical PG Practice Questions and MCQs
Question 81: A 20-year-old man presents to the emergency department with complaints of severe malaise, fevers, and sore throat for the past 7 days. He also has had episodes of nausea and vomiting during this period. He does not smoke or drink alcohol. There is no family history of liver disease. His blood pressure is 130/80 mm Hg, temperature is 38.3℃ (100.9℉), pulse is 102/min, and respiratory rate is 20/min. On physical examination, he appears ill with bilateral cervical lymphadenopathy. His tonsils are erythematous and enlarged. There is no jaundice and he is mildly dehydrated. Abdominal examination demonstrates splenomegaly. The laboratory findings are shown below:
Hemoglobin 15 g/dL
Platelet count 95,000/mm³
Leukocytes 13,500/mm³
Neutrophils 50%
Atypical lymphocytes 34%
AST 232 U/L
ALT 312 U/L
ALP 120 U/L
GGT 35 U/L
Total bilirubin 1.2 mg/dL
Direct bilirubin 0.2 mg/dL
PT 12 seconds
The serologic test for hepatitis A, B, and C, CMV, and leptospirosis are negative. Serology for both serum IgM and IgG antibodies for EBV capsid antigen are positive, but the heterophile antibody test is negative. What is the most likely reason for the negative heterophile test?
A. Concurrent viral hepatitis A infection
B. CMV infection
C. Low specificity
D. Age of the patient
E. False negative (Correct Answer)
Explanation: ***False negative***
- The **heterophile antibody test (Monospot test)** has a sensitivity of only **70-92%** for infectious mononucleosis, meaning false negatives occur in **10-25% of cases**.
- Heterophile antibodies typically appear **1-2 weeks after symptom onset**, and this patient has been symptomatic for only **7 days**, making it likely the heterophile antibodies have not yet developed to detectable levels.
- The positive **EBV IgM and IgG for capsid antigen** confirm acute EBV infection, so the negative heterophile test is a **false negative** result.
- False negatives are especially common **early in the course of illness**.
*Age of the patient*
- Age 20 years is actually within the **peak sensitivity range** for heterophile antibody testing (adolescents and young adults 15-25 years have 85-90% sensitivity).
- The heterophile test is **less sensitive in young children (<12 years)**, with sensitivity as low as 30-50% in children under 4 years.
- This patient's age would not explain the negative result.
*Concurrent viral hepatitis A infection*
- Serologic testing for **hepatitis A is negative**, ruling out co-infection.
- Hepatitis A co-infection would not cause a false negative heterophile test.
*CMV infection*
- Serologic testing for **CMV is negative**, and the patient has **positive EBV-specific serology**.
- While CMV can cause heterophile-negative mononucleosis syndrome, the confirmed EBV infection makes this irrelevant.
*Low specificity*
- The heterophile antibody test has **high specificity (95-100%)** for infectious mononucleosis, meaning false positives are rare.
- The limitation of the test is its **low sensitivity**, not low specificity, which explains false negatives but doesn't directly answer why this specific test is negative.
Question 82: A 4-year-old Caucasian male patient presents with recurrent infections. During examination of his CD4 T-cells, it is noticed that his T-cells lack CD40 ligand. Which type of immunoglobulin is likely to be present in excess?
A. IgA
B. IgE
C. IgM (Correct Answer)
D. IgG
E. IgD
Explanation: ***IgM***
- A lack of **CD40 ligand** on T-cells prevents proper **T-cell dependent B-cell activation** and subsequent **class switching**.
- This leads to an inability to produce other immunoglobulin isotopes, resulting in an accumulation of **IgM** and a deficiency of IgG, IgA, and IgE.
*IgA*
- **IgA** production requires **T-cell help** and functioning CD40-CD40L interaction for class switching.
- In the absence of CD40 ligand, **IgA production** will be significantly impaired, not in excess.
*IgE*
- **IgE** class switching is also a **T-cell dependent process** that relies on the CD40-CD40L pathway.
- A deficiency in CD40 ligand would therefore lead to **reduced IgE levels**, not elevated ones.
*IgG*
- **IgG** is the most abundant immunoglobulin and its production is highly dependent on **T-cell signaling** via CD40-CD40L interaction for class switching.
- A lack of CD40 ligand would result in a severe **deficiency of IgG**, predisposing the patient to recurrent infections.
*IgD*
- **IgD** is primarily found on the surface of naïve B cells and its production is **not typically regulated** by class switching mechanisms involving the CD40-CD40L pathway in the same way as other isotypes.
- While its role is less understood, its levels are generally **not significantly elevated** in conditions affecting class switching.
Question 83: A young researcher is studying the structure of class I and class II major histocompatibility complex (MHC) molecules. He understands that these molecules are proteins, but the structures of class I MHC molecules are different from those of class II. Although all these molecules consist of α and β chains, some of their domains are polymorphic, meaning they are different in different individuals. He calls them ‘P’ domains. The other domains are nonpolymorphic, which remain invariant in all individuals. He calls these domains ‘N’ domains. Which of the following are examples of ‘N’ domains?
A. β2-microglobulin in class I molecules and β1 domain in class II molecules
B. α1 domain in class I molecules and α1 domain in class II molecules
C. α3 domain in class I molecules and β2 domain in class II molecules (Correct Answer)
D. α2 domain in class I molecules and β2 domain in class II molecules
E. α1-α2 domains in class I molecules and α1-β1 domains in class II molecules
Explanation: ***α3 domain in class I molecules and β2 domain in class II molecules***
- The **α3 domain of MHC Class I** molecules and the **β2 domain of MHC Class II** molecules are invariant (non-polymorphic) and interact with the co-receptors **CD8** and **CD4**, respectively.
- These domains are crucial for the stability and proper function of the MHC molecules, as well as for T cell recognition, making their conserved nature essential.
*β2-microglobulin in class I molecules and β1 domain in class II molecules*
- **β2-microglobulin** is a non-polymorphic component of MHC Class I molecules, but the **β1 domain in Class II MHC** is **highly polymorphic** and forms part of the peptide-binding groove.
- Thus, this option incorrectly states that β1 domain of Class II is non-polymorphic.
*α1 domain in class I molecules and α1 domain in class II molecules*
- Both the **α1 domain of Class I MHC** and the **α1 domain of Class II MHC** are **polymorphic**, contributing to the diversity of the peptide-binding groove.
- This option incorrectly identifies these polymorphic domains as non-polymorphic.
*α2 domain in class I molecules and β2 domain in class II molecules*
- While the **β2 domain of MHC Class II** is non-polymorphic, the **α2 domain of MHC Class I** is **highly polymorphic** and forms part of the peptide-binding groove.
- This option incorrectly states that α2 domain of Class I is non-polymorphic.
*α1-α2 domains in class I molecules and α1-β1 domains in class II molecules*
- The **α1 and α2 domains of Class I MHC** molecules and the **α1 and β1 domains of Class II MHC** molecules all contain the **polymorphic residues** that form the peptide-binding groove.
- Therefore, these are examples of 'P' (polymorphic) domains, not 'N' (non-polymorphic) domains.
Question 84: A previously healthy 6-week-old infant is brought to the emergency department because of fever, fatigue, and dry cough for one day. She has been feeding poorly and had difficulty latching on to breastfeed since yesterday. She has had nasal congestion. The mother reports that her daughter has not been going through as many diapers as usual. She was born by uncomplicated vaginal delivery at 42 weeks' gestation. Her mother is a cystic fibrosis carrier. The patient has been treated with acetaminophen for the last 24 hours, and vitamin D drops since birth. She appears irritable, pale, and lethargic. She is at the 25th percentile for both length and weight; she had the same percentiles at birth. Her temperature is 38.2°C (100.7°F) and respirations are 64/min. Pulse oximetry on room air shows an oxygen saturation of 92%. Examination shows an ill-appearing infant with a cough and nasal flaring. Mucous membranes are dry. Chest examination shows intercostal and supraclavicular retractions. Expiratory wheezes are heard on auscultation. Which of the following is the most likely causal organism?
A. Respiratory syncytial virus (Correct Answer)
B. Coronavirus
C. Listeria monocytogenes
D. Streptococcus pneumoniae
E. Rhinovirus
Explanation: ***Respiratory syncytial virus***
- The patient's age (6 weeks), symptoms of **bronchiolitis** (wheezing, retractions, tachypnea), and **hypoxia** are highly characteristic of a **Respiratory Syncytial Virus (RSV)** infection.
- RSV is the most common cause of **bronchiolitis** in infants and young children, especially during colder months.
*Coronavirus*
- While coronaviruses can cause respiratory infections, they are a less common cause of classic **bronchiolitis** in this age group compared to RSV.
- The constellation of severe bronchiolitis symptoms makes RSV a more likely primary pathogen in this specific case.
*Listeria monocytogenes*
- *Listeria* typically causes **meningitis** or **sepsis** in neonates and young infants, often presenting with fever, poor feeding, and lethargy, but not typically with severe respiratory symptoms like wheezing and retractions.
- This pathogen is usually acquired perinatally and would present earlier than 6 weeks with more systemic signs of infection.
*Streptococcus pneumoniae*
- *S. pneumoniae* is a common cause of **pneumonia** and **otitis media** in infants, but it rarely causes acute **bronchiolitis** with prominent wheezing and retractions as the primary presentation.
- While pneumonia can cause respiratory distress, the expiratory wheezes point more towards airway obstruction characteristic of viral bronchiolitis.
*Rhinovirus*
- Rhinovirus can cause upper and lower respiratory tract infections in infants, including some cases of **bronchiolitis**.
- However, RSV is statistically the most common and severe cause of bronchiolitis with significant hypoxia and respiratory distress in this age group.
Question 85: A 7-year-old boy with a sore throat, fever, and generalized malaise is admitted to the pediatric floor. On physical examination, he has diffuse white exudate on both tonsils, and also a palpable spleen with mild hepatomegaly. His blood smear shows large and abundant lymphocytes with blue-gray cytoplasm, irregular nuclei, and dark chromatin with inconspicuous nucleoli. Which of the following is the most likely diagnosis?
A. Toxoplasmosis
B. Infectious mononucleosis (Correct Answer)
C. Cytomegalovirus infection
D. Viral hepatitis
E. Graves' disease
Explanation: ***Infectious mononucleosis***
- The classic triad of **fever**, **pharyngitis (sore throat with exudates)**, and **lymphadenopathy** is highly characteristic of infectious mononucleosis, often caused by the **Epstein-Barr virus (EBV)**.
- The presence of **splenomegaly**, sometimes hepatomegaly, and **atypical lymphocytes** on a blood smear (large lymphocytes with blue-gray cytoplasm and irregular nuclei) further supports this diagnosis.
*Toxoplasmosis*
- While it can cause **lymphadenopathy** and sometimes **hepatosplenomegaly**, toxoplasmosis typically does not present with prominent **exudative pharyngitis** as described.
- The specific morphology of atypical lymphocytes on the blood smear is more indicative of a viral infection like EBV.
*Cytomegalovirus infection*
- **CMV can cause a mononucleosis-like syndrome** with fever, malaise, and atypical lymphocytes, but **exudative tonsillitis is less common** and often less severe than in EBV mononucleosis.
- While hepatosplenomegaly can occur, the clinical picture strongly favors EBV given the pharyngeal findings.
*Viral hepatitis*
- Viral hepatitis primarily manifests with **liver inflammation**, leading to symptoms like **jaundice**, dark urine, and elevated liver enzymes, which are not mentioned here.
- While some forms can cause generalized malaise and mild hepatomegaly, exudative tonsillitis and distinctive atypical lymphocytes are not typical features.
*Graves' disease*
- Graves' disease is an **autoimmune disorder** causing **hyperthyroidism**, characterized by symptoms like **tachycardia**, weight loss, tremor, and exophthalmos.
- It does not present with fever, sore throat, splenomegaly, or atypical lymphocytes on a blood smear.
Question 86: A 4-year-old boy is brought to the emergency department for evaluation of a fever for 1 day. The mother reports that he has had severe pain in his lower extremities and difficulty eating since yesterday. He has not had a cough, nausea, or vomiting. He was born at term and has been healthy. His immunizations are up-to-date. He appears irritable. His temperature is 38.5°C (101.3°F). Examination shows several flesh-colored, tender papules over the trunk, knees, palms, and soles. There are multiple 2-mm, reddish macules on the hard palate. The remainder of the examination shows no abnormalities. Which of the following is the most likely causal organism of this patient's symptoms?
A. Coxsackie A virus (Correct Answer)
B. Measles virus
C. Rubella virus
D. Human herpesvirus 6
E. Herpes simplex virus 1
Explanation: ***Coxsackie A virus***
- The presentation of **fever**, **painful papules** on the trunk, knees, palms, and soles, and **reddish macules on the hard palate** (enanthem) is classic for **hand-foot-and-mouth disease (HFMD)**, which is primarily caused by **Coxsackie A virus**.
- The child's reports of **severe pain in lower extremities** and **difficulty eating** are consistent with the painful lesions (enanthem and exanthem) associated with HFMD.
*Measles virus*
- Measles typically presents with a **prodrome of cough, coryza, and conjunctivitis**, followed by a **maculopapular rash that starts on the face** and spreads downwards.
- While measles can cause an enanthem (**Koplik spots**), these are usually white spots on the buccal mucosa, not reddish macules on the hard palate. The rash distribution described is also atypical for measles.
*Rubella virus*
- Rubella (German measles) causes a **milder maculopapular rash** that originates on the face and spreads rapidly, often accompanied by **postauricular and occipital lymphadenopathy**.
- The described **tender papules** on palms and soles and **mucosal lesions** are not characteristic features of rubella.
*Human herpesvirus 6*
- Human herpesvirus 6 (HHV-6) is the primary cause of **roseola infantum**, characterized by a **high fever followed by a blanching macular or maculopapular rash** that appears as the fever subsides.
- The rash of roseola typically does not involve distinct papules on the palms and soles or painful oral lesions.
*Herpes simplex virus 1*
- HSV-1 primarily causes **oral herpes (cold sores)**, **gingivostomatitis**, or **herpetic whitlow**.
- While it can cause oral lesions, the widespread **flesh-colored papules** on the trunk and extremities, particularly the palms and soles, are not typical for a primary HSV-1 infection in this age group.
Question 87: A previously healthy 15-year-old girl is brought to the emergency department 24 hours after the onset of a severe headache. She returned from a 1-week camping trip 3 days ago; she went spelunking and swimming in a freshwater lake during the trip. She is agitated, uncooperative, and oriented only to person. Her temperature is 38.9°C (102°F), pulse is 112/min, respirations are 20/min, and blood pressure is 100/68 mm Hg. There are several crusted insect bites on her extremities. Neurologic examination shows diffuse hyperreflexia and an extensor plantar response bilaterally. Her neck is supple without lymphadenopathy. An MRI of the brain shows asymmetrical, bitemporal hyperintensities. A lumbar puncture is performed. Cerebrospinal fluid analysis shows:
Opening pressure 150 mm H2O
Glucose 58 mg/dL
Protein 108 mg/dL
Leukocyte count 150/mm3
Segmented neutrophils 15%
Lymphocytes 85%
Erythrocyte count 25/mm3
Which of the following is the most likely causal pathogen?
A. Naegleria fowleri
B. Herpes simplex virus (HSV) (Correct Answer)
C. West Nile virus (WNV)
D. Tick-borne encephalitis virus (TBEV)
E. La Crosse virus (LACV)
Explanation: ***Herpes simplex virus (HSV)***
- The combination of **bitemporal hyperintensities** on MRI, **agitation**, **uncooperativeness**, **fever**, and **lymphocytic pleocytosis** with elevated protein in the CSF is highly suggestive of **HSV encephalitis**. The history of a camping trip and insect bites is a distractor, but the MRI and CSF findings are classic for HSV.
- HSV encephalitis often presents with **focal neurologic deficits**, such as diffuse hyperreflexia and an extensor plantar response, and behavioral changes, reflecting the **temporal lobe involvement**.
*Naegleria fowleri*
- Causes **primary amebic meningoencephalitis**, which is rapidly fatal and typically associated with recent exposure to **warm freshwater**. While she went swimming in a freshwater lake, the MRI findings and CSF profile (lymphocytic predominance, moderate protein elevation) are not typical for Naegleria, which usually causes a **hemorrhagic necrosis** and often a **neutrophilic pleocytosis**.
- The disease progression with Naegleria fowleri is much more aggressive and rapid, usually leading to death within 3-7 days, and the MRI findings would often be more widespread.
*West Nile virus (WNV)*
- Transmitted by **mosquitoes** and can cause West Nile encephalitis, characterized by fever, headache, body aches, and sometimes a rash. However, WNV encephalitis typically does not show the characteristic **bitemporal hyperintensities** on MRI and often presents with more generalized encephalopathy or flaccid paralysis rather than focal temporal lobe findings.
- While CSF can show lymphocytic pleocytosis, the specific MRI findings are less consistent with WNV compared to HSV.
*Tick-borne encephalitis virus (TBEV)*
- Transmitted by **ticks** and is common in parts of Europe and Asia. It can cause encephalitis with fever, headache, and neurologic symptoms. However, it typically does not present with the specific **bitemporal hyperintensities** seen in this patient's MRI.
- The clinical picture, particularly the MRI findings and behavioral changes, is not a classic presentation for TBEV, which often has a more diffuse cerebral involvement.
*La Crosse virus (LACV)*
- Transmitted by **mosquitoes** and primarily affects children, causing encephalitis with fever, headache, nausea, and seizures. While it can cause an acute encephalitis, it typically does not feature the distinct **bitemporal hyperintensities** on MRI that are characteristic of HSV encephalitis.
- The geographic distribution of LACV is typically in the Midwestern and eastern United States, and while possible, the specific MRI pattern strongly points away from LACV.
Question 88: A previously healthy 53-year-old man is brought to the emergency department 45 minutes after the onset of a severe headache. He returned from a vacation in the mountains 4 days ago, during which he went swimming in a freshwater lake. On arrival, he is confused. His temperature is 39°C (102.2°F) and blood pressure is 105/68 mm Hg. Neurologic examination shows diffuse hyperreflexia. An MRI of the brain shows asymmetrical, bitemporal hyperintensities. A lumbar puncture is performed. Cerebrospinal fluid analysis shows:
Leukocyte count 120/mm3
Segmented neutrophils 10%
Lymphocytes 90%
Erythrocyte count 15/mm3
Glucose 45 mg/dL
Opening pressure 130 mm Hg
Protein 75 mg/dL
Which of the following is the most likely causal pathogen?
A. Herpes simplex virus (Correct Answer)
B. La Crosse virus
C. Rabies virus
D. Enterovirus
E. Naegleria fowleri
Explanation: ***Herpes simplex virus***
- The combination of **acute severe headache**, **fever**, **confusion**, **bitemporal hyperintensities** on MRI, and CSF showing a **lymphocytic pleocytosis** with some red blood cells (erythrocytes) is highly suggestive of **herpes simplex encephalitis (HSV encephalitis)**.
- HSV encephalitis characteristically causes **temporal lobe involvement**, leading to focal neurological deficits and neuropsychiatric symptoms, and CSF often has **lymphocytic predominance** with a slightly elevated protein.
*La Crosse virus*
- This virus causes **encephalitis** primarily in children and adolescents, often preceded by fever, headache, and nausea; seizures are common.
- While it can cause encephalitis, the **bitemporal hyperintensities** and specific CSF findings (lymphocytic pleocytosis, erythrocytes) are more classic for HSV.
*Rabies virus*
- Rabies presents as **encephalitis** with symptoms escalating from non-specific flu-like illness to neurological symptoms like hydrophobia, hallucinations, and paralysis.
- The patient's presentation with **acute severe headache** and rapid onset of confusion, without a history of animal bite, makes rabies less likely.
*Enterovirus*
- Enteroviruses are a common cause of **aseptic meningitis** and can cause **encephalitis**, especially in children.
- While they can cause fever, headache, and CSF lymphocytic pleocytosis, **bitemporal hyperintensities** and significant confusion pointing to focal neurological involvement are less characteristic.
*Naegleria fowleri*
- *Naegleria fowleri* causes **primary amoebic meningoencephalitis (PAM)**, typically in individuals with recent freshwater exposure, leading to rapid onset of severe headache, fever, and altered mental status.
- CSF analysis in PAM usually shows a **neutrophilic pleocytosis** rather than lymphocytic predominance, differentiating it from HSV encephalitis.
Question 89: Two patients are vaccinated for poliomyelitis. Patient A receives the Sabin oral vaccine, and Patient B receives the Salk intramuscular vaccine. Six weeks after their initial vaccinations, which of the following would be the greatest difference regarding these two patients?
A. Patient A has a lower level of serum IgA antibodies
B. Patient A has a higher level of serum IgG antibodies
C. Patient B has a higher level of duodenal IgA antibodies
D. Patient A has a higher level of duodenal IgA antibodies (Correct Answer)
E. Patient B has a lower level of serum IgM antibodies
Explanation: ***Patient A has a higher level of duodenal IgA antibodies***
- The **Sabin oral vaccine** (live attenuated virus) replicates in the gut, stimulating a strong **mucosal IgA response** in the duodenum and elsewhere within the gastrointestinal tract. This provides excellent local immunity against future re-infection by wild-type polio.
- This **mucosal immunity** is crucial for preventing the shedding of poliovirus and thus contributes to **herd immunity** by reducing transmission.
*Patient A has a lower level of serum IgA antibodies*
- While the **oral vaccine** primarily stimulates mucosal immunity (IgA), it does not necessarily lead to a *lower* level of IgA in the serum compared to the Salk vaccine, as both can induce systemic responses to varying degrees.
- The most significant difference is in the *mucosal* IgA, not serum IgA, and the oral vaccine is designed to *increase* mucosal IgA.
*Patient A has a higher level of serum IgG antibodies*
- Both Sabin and Salk vaccines induce systemic IgG antibodies, but the **Salk vaccine** (inactivated virus given intramuscularly) is generally better at stimulating **high levels of serum IgG antibodies**, as its primary site of action is systemic rather than mucosal.
- Sabin does induce serum IgG, but the Salk vaccine often results in a more robust and sustained systemic IgG response providing protection against viremia.
*Patient B has a higher level of duodenal IgA antibodies*
- The **Salk vaccine** is an **inactivated vaccine** administered intramuscularly, meaning it primarily stimulates a **systemic immune response** (IgG and IgM) and does not replicate in the gut.
- Therefore, it provokes a **minimal, if any, duodenal IgA response**, which is a key difference from the Sabin vaccine.
*Patient B has a lower level of serum IgM antibodies*
- Both vaccine types would induce a **primary immune response** including IgM antibodies in the serum after initial vaccination.
- There is no specific reason why the Salk vaccine would lead to a *lower* level of serum IgM compared to Sabin; typically, both would show an IgM rise followed by an IgG class switch.
Question 90: A mother brings her 2-year-old son to your office after she noticed a “sore on the back of his throat.” She states that her son had a fever and was complaining of throat pain 2 days ago. The child has also been fussy and eating poorly. On examination, the child has met all appropriate developmental milestones and appears well-nourished. He has submandibular and anterior cervical lymphadenopathy. On oral examination, less than 10 lesions are visible on bilateral tonsillar pillars and soft palate with surrounding erythema. After 4 days, the lesions disappear without treatment. Which of the following is the most likely causative agent?
A. Varicella-zoster
B. Staphylococcus aureus
C. Herpes simplex virus type 1
D. Coxsackievirus A (Correct Answer)
E. Epstein-Barr virus
Explanation: ***Coxsackievirus A***
- The presentation of **fever**, **throat pain**, **oral lesions** (fewer than 10, on tonsillar pillars and soft palate), and **spontaneous resolution within 4 days** is classic for **herpangina**, which is most commonly caused by **Coxsackievirus A**.
- **Herpangina** typically affects young children during summer/fall and is characterized by discrete, small lesions in the posterior oropharynx that resolve without specific treatment.
*Varicella-zoster*
- **Varicella-zoster virus** causes **chickenpox**, which presents with a widespread **vesicular rash** primarily on the trunk and face, rather than isolated oral lesions in the posterior oropharynx.
- While oral lesions can occur in chickenpox, they would generally be accompanied by the characteristic diffuse skin rash and would not be limited to the posterior pharynx as described.
*Staphylococcus aureus*
- **Staphylococcus aureus** can cause various infections, but it is not a typical cause of **viral-like oral vesicular lesions** such as those described.
- Bacterial pharyngitis caused by *S. aureus* is uncommon and would more likely present with bacterial tonsillitis, **abscess formation**, or **impetigo**-like skin lesions, not discrete oral vesicles.
*Herpes simplex virus type 1*
- **Herpes simplex virus type 1 (HSV-1)** typically causes **gingivostomatitis** in young children, characterized by numerous, painful **vesicles and ulcers on the gingiva, tongue, buccal mucosa, lips, and hard palate**.
- This presentation differs from the described lesions, which are fewer in number and localized to the **tonsillar pillars and soft palate**, as seen in herpangina.
*Epstein-Barr virus*
- **Epstein-Barr virus (EBV)** causes **infectious mononucleosis**, which presents with **fever**, **fatigue**, **pharyngitis**, and **lymphadenopathy**, often with **splenomegaly**.
- While pharyngitis can be severe, EBV does not typically cause the **discrete vesicular or ulcerative lesions limited to the tonsillar pillars and soft palate** as described in this case.
