Poxviruses — MCQs

10 questions

A 43-year-old male visits the emergency room around 4 weeks after getting bitten by a bat during a cave diving trip. After cleansing the wound with water, the patient reports that he felt well enough not to seek medical attention immediately following his trip. He does endorse feeling feverish in the past week but a new onset of photophobia and irritability led him to seek help today. What would the post-mortem pathology report show if the patient succumbs to this infection?

Researchers are investigating a new strain of a virus that has been infecting children over the past season and causing dermatitis. They have isolated the virus and have run a number of tests to determine its structure and characteristics. They have found that this new virus has an outer coating that is high in phospholipids. Protein targeting assays and immunofluorescence images have shown that the outer layer contains numerous surface proteins. On microscopy, these surface proteins are also expressed around the nucleus of cells derived from the infected tissue of the children. This virus’s structure most closely resembles which of the following?

A previously healthy 5-year-old boy is brought to the physician because of increasing weakness and a retroauricular rash that started 2 days ago. The rash spread rapidly and involves the trunk and extremities. Last week, he had a mild sore throat, pink eyes, and a headache. His family recently immigrated from Ethiopia. His immunization status is unknown. The patient appears severely ill. His temperature is 38.5°C (101.3°F). Examination shows tender postauricular and suboccipital lymphadenopathy. There is a nonconfluent, maculopapular rash over the torso and extremities. Infection with which of the following is the most likely cause of this patient's symptoms?

An investigator studying viral replication isolates the genetic material of an unidentified virus strain. After exposing a cell culture to the isolated, purified viral genetic material, the cells begin to produce viral polymerase and subsequently replicate the viral genome. Infection with the investigated strain is most likely to cause which of the following conditions?

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 previously healthy 2-year-old girl is brought to the physician by her mother after she noticed multiple painless, nonpruritic papules on her abdomen. The child attends daycare three times per week, and this past week one child was reported to have similar lesions. Her immunizations are up-to-date. Her brother had chickenpox one month ago. She is at the 50th percentile for height and the 60th percentile for weight. Vital signs are within normal limits. Examination shows several skin-colored, nontender, pearly papules with central umbilication on the abdomen and extremities. The remainder of the examination shows no abnormalities. Which of the following is the most likely diagnosis?

A 48-year-old man with HIV comes to the physician because of skin lesions over his face and neck for 2 weeks. They are not itchy or painful. He does not have fever or a sore throat. He was treated for candidal esophagitis 3 months ago. He is sexually active with his wife, who knows of his condition, and uses condoms consistently. He is currently receiving triple antiretroviral therapy with lamivudine, abacavir, and efavirenz. He is 175 cm (5 ft 9 in) tall and weighs 58 kg (128 lb); BMI is 18.8 kg/m2. Examination shows multiple skin colored papules over his face and neck with a dimpled center. Cervical lymphadenopathy is present. The remainder of the examination is unremarkable. His hemoglobin concentration is 12.1 g/dL, leukocyte count is 4,900/mm3, and platelet count is 143,000/mm3; serum studies and urinalysis show no abnormalities. CD4+ T-lymphocyte count is 312/mm3 (normal ≥ 500). Which of the following is the most likely cause of this patient's findings?

A 30-year-old woman who is 24-weeks pregnant presents to the emergency department with fever, painful urination, and headache. The patient's blood pressure is 111/67 mm Hg, the pulse is 95/min, the respiratory rate is 16/min, and the temperature is 38.3°C (101.1°F). Physical examination reveals bilateral tender inguinal lymphadenopathy and painful genital lesions. On closer inspection, the patient’s genital lesions contain clear fluid and measure 5–6 mm in diameter. What is the appropriate description of these lesions?

A 27-year-old male who works on an organic farm is diagnosed with infection by N. americanus, a helminthic parasite. Eosinophils require which antibody isotype to destroy these parasites via antibody-dependent cellular cytotoxicity?

Q10

A 24-year-old female comes to the physician because of flu-like symptoms and a new rash for 2 days. She denies contacts with sick individuals or recent travel abroad, but recently went camping in Vermont. Vital signs are within normal limits. Examination of the lateral right thigh shows a circular red ring with central clearing. Which of the following is the natural reservoir of the pathogen responsible for this patient's symptoms?

Poxviruses US Medical PG Practice Questions and MCQs

Question 1: A 43-year-old male visits the emergency room around 4 weeks after getting bitten by a bat during a cave diving trip. After cleansing the wound with water, the patient reports that he felt well enough not to seek medical attention immediately following his trip. He does endorse feeling feverish in the past week but a new onset of photophobia and irritability led him to seek help today. What would the post-mortem pathology report show if the patient succumbs to this infection?

A. Howell-Jolly bodies
B. Heinz bodies
C. Psammoma bodies
D. Pick bodies
E. Negri bodies (Correct Answer)

Explanation: ***Negri bodies*** - This patient's symptoms (fever, photophobia, irritability) and history of a bat bite point to rabies. **Negri bodies** are eosinophilic inclusions found in the cytoplasm of hippocampal and Purkinje cells in cases of rabies. - They are **pathognomonic** for rabies infection and represent viral nucleocapsid proteins. *Howell-Jolly bodies* - These are **nuclear remnants** found in red blood cells that indicate impaired splenic function or asplenia. - They are not associated with viral infections like rabies and are observed in conditions like sickle cell disease or after splenectomy. *Heinz bodies* - **Heinz bodies** are inclusions within red blood cells composed of denatured hemoglobin. - They are typically seen in conditions involving **oxidative stress** to red blood cells, such as G6PD deficiency or alpha-thalassemia, not rabies. *Psammoma bodies* - These are **calcified, laminated, concentric spherules** found in some tumors (e.g., papillary thyroid carcinoma, meningioma, serous ovarian cystadenocarcinoma). - They are a marker of specific neoplastic conditions and have no relevance to viral infections. *Pick bodies* - **Pick bodies** are aggregates of tau protein found in neurons, characteristic of **Pick's disease**, a type of frontotemporal dementia. - They are neurodegenerative markers and are unrelated to infectious diseases.

Question 2: Researchers are investigating a new strain of a virus that has been infecting children over the past season and causing dermatitis. They have isolated the virus and have run a number of tests to determine its structure and characteristics. They have found that this new virus has an outer coating that is high in phospholipids. Protein targeting assays and immunofluorescence images have shown that the outer layer contains numerous surface proteins. On microscopy, these surface proteins are also expressed around the nucleus of cells derived from the infected tissue of the children. This virus’s structure most closely resembles which of the following?

A. Adenovirus
B. Papillomavirus
C. Herpesvirus (Correct Answer)
D. Poxvirus
E. Hepadnavirus

Explanation: ***Herpesvirus*** - The presence of an **outer coating high in phospholipids** indicates an **enveloped virus**. Herpesviruses are large, enveloped DNA viruses. - The detection of surface proteins expressed around the **nucleus** is a key finding that suggests the virus **buds from the nuclear membrane**, a characteristic feature of **herpesviruses**. - Many herpesviruses cause dermatitis, including **HSV-1, HSV-2, VZV**, and **HHV-6**. *Adenovirus* - Adenoviruses are **non-enveloped viruses**, meaning they lack a lipid outer coating, which contradicts the phospholipid-rich coating described. - Their replication and assembly occur exclusively in the **nucleus**, but they do not acquire an envelope by budding from the nuclear membrane. *Papillomavirus* - Papillomaviruses are also **non-enveloped viruses** with an **icosahedral capsid**, lacking the phospholipid envelope described. - They replicate and assemble in the nucleus but do not possess the phospholipid-rich outer coating. *Poxvirus* - While poxviruses are **enveloped**, they acquire their envelope from the **Golgi apparatus** or **plasma membrane**, not the nuclear membrane. - They are also distinct from other DNA viruses in that they replicate entirely in the **cytoplasm**, not the nucleus, which does not explain the perinuclear protein expression. *Hepadnavirus* - Hepadnaviruses (e.g., **Hepatitis B virus**) are enveloped DNA viruses, but their envelope is acquired by budding through the **endoplasmic reticulum** and **Golgi apparatus**, not the nuclear membrane. - The perinuclear localization of surface proteins in immunofluorescence is not a characteristic feature of hepadnaviruses.

Question 3: A previously healthy 5-year-old boy is brought to the physician because of increasing weakness and a retroauricular rash that started 2 days ago. The rash spread rapidly and involves the trunk and extremities. Last week, he had a mild sore throat, pink eyes, and a headache. His family recently immigrated from Ethiopia. His immunization status is unknown. The patient appears severely ill. His temperature is 38.5°C (101.3°F). Examination shows tender postauricular and suboccipital lymphadenopathy. There is a nonconfluent, maculopapular rash over the torso and extremities. Infection with which of the following is the most likely cause of this patient's symptoms?

A. Togavirus (Correct Answer)
B. Human herpesvirus 6
C. Parvovirus
D. Varicella zoster virus
E. Paramyxovirus

Explanation: ***Togavirus*** - This patient's presentation is classic for **rubella** (German measles), caused by the **rubella virus**, a **togavirus**. - The hallmark clinical finding is **tender postauricular and suboccipital lymphadenopathy**, which appears before the rash and is pathognomonic for rubella. - The **maculopapular rash** begins on the face (retroauricular region) and spreads cephalocaudally to the trunk and extremities over 2-3 days. - The prodrome includes **mild symptoms** (low-grade fever, sore throat, mild conjunctivitis, headache), which is characteristic of rubella. - The patient's **unknown immunization status** and immigration from a region with lower vaccination coverage increases the likelihood of rubella infection. *Paramyxovirus* - **Measles virus** is a paramyxovirus that causes rubeola, but the clinical presentation differs significantly from this case. - Measles typically presents with the **"3 Cs"**: severe **cough**, **coryza** (profuse nasal discharge), and **conjunctivitis** (more prominent than rubella). - **Koplik spots** (white spots on buccal mucosa) are pathognomonic for measles and appear before the rash. - Measles causes **higher fever** (often >40°C) and more severe systemic illness than described here. - While measles can have lymphadenopathy, the **prominent postauricular and suboccipital nodes are characteristic of rubella, not measles**. *Human herpesvirus 6* - **HHV-6** causes **roseola infantum** (exanthem subitum), typically in infants 6-24 months old. - The classic presentation is **high fever for 3-5 days** that suddenly resolves, followed immediately by a rash (**"fever then rash"**). - This patient had prodromal symptoms followed by rash while still febrile, which does not fit roseola. - Roseola does not cause significant lymphadenopathy or conjunctivitis. *Parvovirus* - **Parvovirus B19** causes **erythema infectiosum** (fifth disease), characterized by a **"slapped cheek"** facial erythema followed by a reticular (lacy) rash on the trunk and extremities. - The rash pattern and prominent lymphadenopathy in this case are not consistent with fifth disease. - Fifth disease typically causes mild or no fever and lacks the retroauricular distribution seen here. *Varicella zoster virus* - **VZV** causes **chickenpox**, which presents with a **pruritic, vesicular rash** that appears in successive crops and progresses through stages (macule → papule → vesicle → crust). - This patient has a **maculopapular, nonconfluent rash** without vesicles, which is inconsistent with chickenpox. - Chickenpox does not typically cause prominent postauricular lymphadenopathy.

Question 4: An investigator studying viral replication isolates the genetic material of an unidentified virus strain. After exposing a cell culture to the isolated, purified viral genetic material, the cells begin to produce viral polymerase and subsequently replicate the viral genome. Infection with the investigated strain is most likely to cause which of the following conditions?

A. Rotavirus infection
B. Poliomyelitis (Correct Answer)
C. Hepatitis B
D. Rabies
E. Influenza

Explanation: ***Poliomyelitis*** - The isolation of **purified viral genetic material** directly leading to viral protein production (polymerase) and genome replication indicates the virus has an **RNA genome that can directly serve as mRNA**. - **Poliovirus** is a **positive-sense single-stranded RNA (+ssRNA) virus**, meaning its genome can immediately be translated by host ribosomes upon entry, acting like mRNA. *Rotavirus infection* - Rotavirus is a **double-stranded RNA (dsRNA) virus** and requires its own **RNA-dependent RNA polymerase** to synthesize mRNA before protein production and genome replication can occur. - Its purified genetic material alone would not directly lead to viral protein synthesis in the absence of viral enzymes. *Hepatitis B* - Hepatitis B virus (HBV) is a **DNA virus** and replicates through an **RNA intermediate** via **reverse transcriptase**. - Its genetic material cannot directly initiate the production of viral polymerase or genome replication without complex cellular machinery and viral enzymes. *Rabies* - Rabies virus is a **negative-sense single-stranded RNA (-ssRNA) virus**, which means its genome cannot be directly translated into protein. - It requires its own **RNA-dependent RNA polymerase** to first synthesize complementary positive-sense mRNA strands. *Influenza* - Influenza virus is also a **negative-sense single-stranded RNA (-ssRNA) virus**. - Like rabies, it carries its own **RNA-dependent RNA polymerase** to transcribe its genome into mRNA before protein synthesis can begin.

Question 5: 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 6: A previously healthy 2-year-old girl is brought to the physician by her mother after she noticed multiple painless, nonpruritic papules on her abdomen. The child attends daycare three times per week, and this past week one child was reported to have similar lesions. Her immunizations are up-to-date. Her brother had chickenpox one month ago. She is at the 50th percentile for height and the 60th percentile for weight. Vital signs are within normal limits. Examination shows several skin-colored, nontender, pearly papules with central umbilication on the abdomen and extremities. The remainder of the examination shows no abnormalities. Which of the following is the most likely diagnosis?

A. Cutaneous lichen planus
B. Verruca vulgaris
C. Chickenpox
D. Molluscum contagiosum (Correct Answer)
E. Insect bites

Explanation: ***Molluscum contagiosum*** - The presence of **painless, nonpruritic, skin-colored, pearly papules with central umbilication** is pathognomonic for molluscum contagiosum. - The history of exposure in a daycare setting and a child with similar lesions further supports this diagnosis, as it is a **highly contagious viral infection**. *Cutaneous lichen planus* - Characterized by **pruritic, purple, polygonal papules and plaques** (the 6 P's), which are distinct from the described lesions. - While it can affect children, the classic presentation does not match the **umbilicated, pearly appearance**. *Verruca vulgaris* - Commonly known as **warts**, these are typically **rough, hyperkeratotic papules** with an irregular surface, unlike the smooth, pearly lesions of molluscum. - Warts are not typically described as having central umbilication. *Chickenpox* - Presents as a **pruritic rash** that progresses from macules to papules, then to **vesicles ("dewdrop on a rose petal")**, and finally crusts. - The lesions in the question are described as painless, nonpruritic papules with central umbilication, which is inconsistent with active chickenpox. The child is also immunized and her brother had chickenpox one month ago, making a very recent infection less likely, and the lesions do not fit the description of chickenpox scars. *Insect bites* - Typically present as **pruritic, erythematous papules or wheals** that may have a central punctum. - The lesions described are painless, nonpruritic, and pearly with central umbilication, which is not characteristic of typical insect bites.

Question 7: A 48-year-old man with HIV comes to the physician because of skin lesions over his face and neck for 2 weeks. They are not itchy or painful. He does not have fever or a sore throat. He was treated for candidal esophagitis 3 months ago. He is sexually active with his wife, who knows of his condition, and uses condoms consistently. He is currently receiving triple antiretroviral therapy with lamivudine, abacavir, and efavirenz. He is 175 cm (5 ft 9 in) tall and weighs 58 kg (128 lb); BMI is 18.8 kg/m2. Examination shows multiple skin colored papules over his face and neck with a dimpled center. Cervical lymphadenopathy is present. The remainder of the examination is unremarkable. His hemoglobin concentration is 12.1 g/dL, leukocyte count is 4,900/mm3, and platelet count is 143,000/mm3; serum studies and urinalysis show no abnormalities. CD4+ T-lymphocyte count is 312/mm3 (normal ≥ 500). Which of the following is the most likely cause of this patient's findings?

A. Poxvirus (Correct Answer)
B. A herpesvirus
C. Papillomavirus
D. Coccidioides
E. Bartonella

Explanation: ***Poxvirus*** - The description of **skin-colored papules with a dimpled (umbilicated) center** is highly characteristic of **molluscum contagiosum**, which is caused by a poxvirus. This condition is common in immunocompromised individuals, such as those with HIV. - The patient's **HIV-positive status** and **CD4+ count of 312/mm³** indicate immunocompromise, making him susceptible to severe or widespread molluscum contagiosum, often seen on the face and neck. *A herpesvirus* - Herpes simplex virus typically causes **painful, clustered vesicles** on an erythematous base, often with recurrent outbreaks; this presentation does not match the described painless, umbilicated papules. - Varicella-zoster virus (another herpesvirus) causes chickenpox or shingles, presenting as **vesicles and crusts in a dermatomal pattern** (shingles) or diffuse rash (chickenpox), which is inconsistent with this patient's lesions. *Papillomavirus* - Human papillomavirus (HPV) causes **warts**, which are typically rough, hyperkeratotic papules or nodules, lacking the characteristic central umbilication seen in this patient. - While common in immunocompromised individuals, HPV lesions usually present differently and are not described as skin-colored with a dimpled center. *Coccidioides* - **Coccidioidomycosis** is a fungal infection that can cause various skin manifestations, including **erythema nodosum**, **erythema multiforme**, or subcutaneous nodules, but not the distinct umbilicated papules characteristic of molluscum contagiosum. - Systemic symptoms like fever, cough, and fatigue are common in disseminated coccidioidomycosis, and while skin lesions can occur, they do not typically present as solitary or multiple umbilicated papules. *Bartonella* - *Bartonella* infections, particularly *Bartonella henselae* (cat scratch disease) or *Bartonella quintana* (bacillary angiomatosis), typically present as **reddish-purple vascular lesions** (angiomatous papules or nodules) or localized lymphadenopathy. - The lesions described are skin-colored and umbilicated, not vascular, making *Bartonella* an unlikely cause.

Question 8: A 30-year-old woman who is 24-weeks pregnant presents to the emergency department with fever, painful urination, and headache. The patient's blood pressure is 111/67 mm Hg, the pulse is 95/min, the respiratory rate is 16/min, and the temperature is 38.3°C (101.1°F). Physical examination reveals bilateral tender inguinal lymphadenopathy and painful genital lesions. On closer inspection, the patient’s genital lesions contain clear fluid and measure 5–6 mm in diameter. What is the appropriate description of these lesions?

A. Pustule
B. Ulcer
C. Papule
D. Bulla
E. Vesicle (Correct Answer)

Explanation: ***Vesicle*** - A **vesicle** is defined as a **circumscribed, elevated lesion** (macule/papule) containing **clear fluid** and measuring less than 1 cm in diameter. - The patient's lesions, which are 5-6 mm in diameter and contain clear fluid, perfectly fit the description of vesicles, characteristic of **herpes simplex virus (HSV)** infection. *Pustule* - A **pustule** is a small, elevated lesion similar to a vesicle but filled with **pus**, not clear fluid. - Examples include acne or folliculitis, which are typically opaque and yellowish, unlike the described lesions. *Ulcer* - An **ulcer** is a defect or excavation of the skin past the **epidermis**, resulting in the loss of tissue. - The patient's lesions are described as fluid-filled and elevated, not as an open wound with tissue loss. *Papule* - A **papule** is a **solid, elevated lesion** measuring less than 1 cm in diameter. - While elevated and small, a papule does **not contain fluid**, which is a key characteristic of the described lesions. *Bulla* - A **bulla** is a **fluid-filled lesion** that is **larger than 1 cm** in diameter. - The lesions described are 5-6 mm, making them smaller than the definition of a bulla.

Question 9: A 27-year-old male who works on an organic farm is diagnosed with infection by N. americanus, a helminthic parasite. Eosinophils require which antibody isotype to destroy these parasites via antibody-dependent cellular cytotoxicity?

A. IgE (Correct Answer)
B. IgA
C. IgG
D. IgM
E. IgD

Explanation: ***IgE*** - **IgE** antibodies are crucial in the immune response against helminthic parasites, including *N. americanus*, by sensitizing **mast cells** and **eosinophils**. - When **IgE** binds to the surface of parasites, the **Fc receptor** on eosinophils recognizes the Fc portion of IgE, leading to the release of cytotoxic granules that destroy the parasite (antibody-dependent cellular cytotoxicity). *IgA* - **IgA** is primarily found in **mucosal secretions** and plays a role in defending against pathogens at mucosal surfaces, but it is not the primary isotype involved in eosinophil-mediated **ADCC** against helminths. - While IgA can bind to some immune cells, its main function is to **neutralize toxins** and prevent microbial adhesion at mucosal sites. *IgG* - **IgG** is the most abundant antibody in serum and is involved in various immune functions, including **opsonization**, **neutralization**, and **complement activation**. - Although IgG can mediate ADCC by **NK cells** and **macrophages**, it is not the primary isotype for eosinophil-mediated killing of helminths, which is dominated by IgE. *IgM* - **IgM** is typically the first antibody produced during a primary immune response and is very effective at **activating the complement system**. - Its large pentameric structure also limits its diffusion into tissues, and it does not play a direct role in eosinophil-mediated ADCC against helminthic parasites. *IgD* - **IgD** primarily functions as a **B cell receptor** on naive B lymphocytes, signaling for their activation and differentiation. - It is present in very low concentrations in serum and its role in host defense against parasites or in ADCC is negligible.

Question 10: A 24-year-old female comes to the physician because of flu-like symptoms and a new rash for 2 days. She denies contacts with sick individuals or recent travel abroad, but recently went camping in Vermont. Vital signs are within normal limits. Examination of the lateral right thigh shows a circular red ring with central clearing. Which of the following is the natural reservoir of the pathogen responsible for this patient's symptoms?

A. Rat
B. Rabbit
C. Tick
D. Mouse (Correct Answer)
E. Flea

Explanation: ***Mouse*** - The patient's symptoms, including **flu-like illness** and a **circular red rash with central clearing** (erythema migrans) after camping in Vermont, are classic for **Lyme disease**. - The causative agent, *Borrelia burgdorferi*, is primarily maintained in **white-footed mice** (genus *Peromyscus*) in its natural reservoir during its larval and nymphal stages. *Rat* - While **rats** can carry and transmit various diseases, they are not the primary natural reservoir for *Borrelia burgdorferi*, the pathogen responsible for Lyme disease. - Diseases associated with rats often include **leptospirosis** and **plague**, which present with different clinical pictures. *Rabbit* - **Rabbits** are known reservoirs for diseases like **tularemia** (*Francisella tularensis*), which can cause fever, skin lesions, and lymphadenopathy, but typically not the characteristic **erythema migrans** rash. - They are not a significant natural reservoir for *Borrelia burgdorferi*. *Tick* - The **tick** (specifically *Ixodes scapularis* or **deer tick**) is the **vector** that transmits *Borrelia burgdorferi* to humans, not the natural reservoir. - The tick acquires the bacteria from infected animal hosts such as mice and deer. *Flea* - **Fleas** are vectors for diseases such as **bubonic plague** (*Yersinia pestis*) and **endemic typhus** (*Rickettsia typhi*), which do not manifest with erythema migrans. - They are not involved in the transmission or natural history of **Lyme disease**.

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