Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 41: Which type of sample can be used to isolate poliovirus earliest?

A. Stool (Correct Answer)
B. Blood
C. Throat swab
D. Cerebrospinal fluid

Explanation: **Explanation:** Poliovirus is an enterovirus transmitted primarily via the fecal-oral route. Understanding its pathogenesis is key to identifying the correct diagnostic sample. **Why Stool is Correct:** After ingestion, the virus multiplies in the lymphoid tissues of the pharynx and the Peyer’s patches of the small intestine. It is excreted in the **stool** for several weeks, starting as early as **2–5 days after infection** (often before the onset of paralysis). Because the viral load is highest and persists longest in the gastrointestinal tract, stool is the most reliable and earliest sample for isolation. **Analysis of Incorrect Options:** * **Throat Swab:** While the virus can be isolated from the throat during the first week of illness, it disappears rapidly (usually within 7 days), making it less reliable than stool. * **Blood:** Viremia occurs early in the "minor illness" phase (pre-paralytic). However, it is transient and usually subsides by the time clinical symptoms of meningitis or paralysis appear, making it difficult to capture. * **Cerebrospinal Fluid (CSF):** Paradoxically, poliovirus is **rarely isolated from CSF**, even in patients with paralytic polio. Diagnosis of CNS involvement is usually clinical or via PCR, but not primary isolation from CSF. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** Virus isolation from stool (two samples collected 24 hours apart). * **Culture:** Poliovirus is typically grown on Monkey Kidney or human cell lines (e.g., HeLa, HEp-2), showing characteristic **cytopathic effects (CPE)** like cell rounding and pyknosis. * **Specimen Transport:** Stool samples should be sent at **4°C** (Reverse Cold Chain) to maintain viral viability. * **Key Fact:** Polio is a positive-sense, single-stranded RNA virus belonging to the *Picornaviridae* family.

Question 42: Which of the following cells does HIV infect?

A. NK cells
B. T-helper cells (Correct Answer)
C. T suppressor cells
D. Plasma cells

Explanation: ### Explanation **Correct Option: B. T-helper cells** The Human Immunodeficiency Virus (HIV) primarily targets cells that express the **CD4 receptor** on their surface. The hallmark of HIV pathogenesis is the infection and subsequent depletion of **CD4+ T-helper cells**. The viral envelope glycoprotein **gp120** binds specifically to the CD4 molecule. For successful entry, HIV also requires co-receptors: **CCR5** (found on macrophages/T-cells, dominant in early infection) or **CXCR4** (found on T-cells, dominant in late-stage infection). Once inside, the virus replicates, leading to cell death and a progressive decline in cell-mediated immunity. **Analysis of Incorrect Options:** * **A. NK cells:** Natural Killer cells are part of the innate immune system. While they play a role in fighting viral infections, they do not express the CD4 receptor and are not the primary target for HIV entry. * **C. T suppressor cells:** These are **CD8+ T-cells**. Because they lack the CD4 receptor, HIV cannot directly infect them. In fact, in early HIV infection, the CD8 count may temporarily rise as the body attempts to kill infected cells. * **D. Plasma cells:** These are terminally differentiated B-cells that produce antibodies. They do not express CD4 receptors and are not targets for HIV infection. **High-Yield Clinical Pearls for NEET-PG:** * **Primary Receptor:** CD4. * **Co-receptors:** CCR5 (M-tropic) and CXCR4 (T-tropic). * **Other target cells:** Macrophages, Monocytes, and Dendritic cells (all express low levels of CD4). * **Microglial cells:** These are the primary targets of HIV in the Central Nervous System (CNS), leading to HIV-associated dementia. * **Indicator of AIDS:** A CD4+ T-cell count **< 200 cells/mm³** or the presence of an AIDS-defining illness.

Question 43: Antigenic variation is seen in which of the following organisms?

A. Influenza virus (Correct Answer)
B. Hepatitis virus
C. Yellow fever virus
D. Leptospira

Explanation: **Explanation:** **Antigenic variation** is a mechanism by which an infectious agent alters its surface proteins to evade the host's immune response. This phenomenon is most classically demonstrated by the **Influenza virus**. **Why Influenza is Correct:** Influenza viruses (primarily Type A) possess a **segmented RNA genome** (8 segments). This structure allows for two types of antigenic changes: 1. **Antigenic Drift:** Point mutations in the Hemagglutinin (HA) and Neuraminidase (NA) genes, leading to seasonal epidemics. 2. **Antigenic Shift:** A major change resulting from the **reassortment** of genome segments between different strains (e.g., human and avian) infecting the same cell. This leads to new subtypes and causes global **pandemics**. **Analysis of Incorrect Options:** * **Hepatitis Virus:** While Hepatitis C (HCV) shows high genetic diversity due to an error-prone RNA polymerase (quasi-species), it does not undergo the classic "shift and drift" reassortment seen in Influenza. Hepatitis B is a DNA virus and is relatively stable. * **Yellow Fever Virus:** This is an Arbovirus (Flavivirus) with a single serotype. Infection or vaccination provides lifelong immunity because the virus does not undergo significant antigenic variation. * **Leptospira:** While there are many serovars of *Leptospira interrogans*, an individual strain does not rapidly change its surface antigens within a host to evade immunity in the same manner as Influenza or *Borrelia*. **High-Yield Clinical Pearls for NEET-PG:** * **Segmented Viruses:** Remember the mnemonic **BOAR** (Bunyavirus, Orthomyxovirus, Arenavirus, Reovirus). These are the viruses capable of reassortment (Antigenic Shift). * **Antigenic Drift** occurs in both Influenza A and B; **Antigenic Shift** occurs **only in Influenza A**. * Other organisms famous for antigenic variation: *Borrelia recurrentis* (relapsing fever), *Trypanosoma brucei* (VSG genes), and *Neisseria gonorrhoeae* (pili).

Question 44: What are the typical features of arboviral infection regarding viral titre and duration of viremia?

A. Low titre and prolonged viremia
B. Low titre and transient viremia
C. High titre and prolonged viremia (Correct Answer)
D. High titre and transient viremia

Explanation: ### Explanation **1. Why "High titre and prolonged viremia" is correct:** Arboviruses (Arthropod-borne viruses) must complete a complex biological cycle involving an invertebrate vector (e.g., mosquito, tick) and a vertebrate host. For a vector to successfully ingest the virus during a blood meal and subsequently transmit it, the vertebrate host must maintain a **high viral load (high titre)** in the blood. Furthermore, because the vector's feeding is intermittent and opportunistic, the virus must persist in the host's bloodstream for an extended period (**prolonged viremia**) to increase the statistical probability of being picked up by a biting insect. **2. Analysis of Incorrect Options:** * **Options A & B (Low titre):** If the viral titre were low, the small volume of blood ingested by a mosquito (often microliters) would likely contain no viral particles, leading to a "dead-end" for the transmission cycle. * **Options B & D (Transient viremia):** Transient or short-lived viremia would provide a very narrow window for transmission. Most successful arboviruses (like Dengue or West Nile) ensure a sustained viremic phase to maximize the chances of vector infection. **3. NEET-PG High-Yield Pearls:** * **Dead-end Hosts:** Humans are often "dead-end" hosts for certain arboviruses (e.g., Japanese Encephalitis) because they do not develop a high enough titre to reinfect the vector. * **Extrinsic Incubation Period:** This is the time taken by the virus to replicate within the *vector* before it becomes infective. * **Intrinsic Incubation Period:** The time between the vector bite and the onset of symptoms in the *human* host. * **Common Vectors:** *Aedes aegypti* (Dengue, Zika, Chikungunya), *Culex* (Japanese Encephalitis, West Nile), and *Ixodes* ticks (KFD).

Question 45: Suckling mice are used in the isolation of which virus?

A. Arbovirus (Correct Answer)
B. Herpes virus
C. Pox virus
D. HIV

Explanation: **Explanation:** The use of **suckling mice** (mice less than 48 hours old) is a classic method for the isolation of **Arboviruses** (e.g., Dengue, Japanese Encephalitis) and certain **Coxsackieviruses**. These viruses are often difficult to grow in standard cell lines or embryonated eggs. Suckling mice are preferred because they lack a mature immune system and possess specific undifferentiated tissues that are highly susceptible to viral replication. For Arboviruses, the specimen is typically inoculated **intracerebrally**, leading to encephalitis, paralysis, or death, which confirms viral presence. **Analysis of Incorrect Options:** * **Herpes virus (B):** These are typically isolated using **cell cultures** (e.g., Human embryonic lung fibroblasts) where they produce characteristic Cowdry Type A intranuclear inclusion bodies. * **Pox virus (C):** The gold standard for isolation is the **Chorioallantoic Membrane (CAM)** of embryonated hen’s eggs, where they produce visible "pocks." * **HIV (D):** As a retrovirus, HIV is isolated using **co-culture of patient’s PBMCs** (Peripheral Blood Mononuclear Cells) with stimulated healthy T-lymphocytes. Diagnosis is primarily via ELISA, Western Blot, or p24 antigen detection. **High-Yield Clinical Pearls for NEET-PG:** * **Suckling mice** are also the specific medium to differentiate **Coxsackie A** (diffuse myositis/flaccid paralysis) from **Coxsackie B** (focal myositis/spastic paralysis). * **Intracerebral inoculation** in mice is the most sensitive method for isolating the **Rabies virus** (though Negri bodies in brain tissue is the classic histological finding). * Modern virology has largely replaced animal inoculation with **Cell Culture**, but suckling mice remain the "gold standard" for primary isolation of unknown Arboviruses in many reference labs.

Question 46: Cytomegalovirus (CMV) retinitis in HIV patients typically occurs when the CD4 count falls below which threshold?

A. 50 cells/mm³
B. 100 cells/mm³ (Correct Answer)
C. 200 cells/mm³
D. 150 cells/mm³

Explanation: **Explanation:** **Correct Answer: B. 100 cells/mm³** Cytomegalovirus (CMV) is a double-stranded DNA virus (Beta-herpesvirus) that remains latent in myeloid progenitor cells. In HIV-infected individuals, CMV reactivation is a classic opportunistic infection that occurs during advanced stages of immunosuppression. While the risk significantly increases as the CD4 count drops, **100 cells/mm³** is the clinically established threshold below which CMV retinitis typically manifests. It is the most common cause of blindness in AIDS patients, characterized by "pizza-pie" or "cottage cheese and ketchup" fundoscopic appearances (hemorrhage with yellowish-white exudates). **Analysis of Incorrect Options:** * **A. 50 cells/mm³:** While CMV risk is highest and most severe below 50 cells/mm³ (often the threshold for disseminated CMV or CMV colitis), the initial clinical threshold for monitoring and onset of retinitis is 100 cells/mm³. * **C. 200 cells/mm³:** This is the threshold for defining AIDS and the primary cutoff for *Pneumocystis jirovecii* pneumonia (PCP) prophylaxis. CMV rarely occurs at this level. * **D. 150 cells/mm³:** This threshold is associated with an increased risk of fungal infections like *Histoplasma capsulatum* in endemic areas, but it is not the standard cutoff for CMV. **High-Yield Clinical Pearls for NEET-PG:** * **Drug of Choice:** Ganciclovir (or Valganciclovir). Foscarnet is used for ganciclovir-resistant strains. * **Histology:** "Owl’s eye" intranuclear inclusion bodies. * **Screening:** HIV patients with CD4 <100 cells/mm³ should undergo regular ophthalmologic screening every 3–6 months. * **Immune Reconstitution Inflammatory Syndrome (IRIS):** Starting HAART in a patient with CMV can lead to "Immune Recovery Uveitis."

Question 47: Inclusion bodies are found in all of the following viral infections except:

A. Herpes simplex virus infection
B. Epstein-Barr virus infection (Correct Answer)
C. Poliovirus infection
D. Cytomegalovirus infection

Explanation: **Explanation:** Inclusion bodies are distinct structures (aggregates of viral proteins or nucleic acids) formed in the nucleus or cytoplasm of host cells during viral replication. They serve as "viral factories" and are often visible under a light microscope. **Why Epstein-Barr Virus (EBV) is the correct answer:** While EBV is a member of the *Herpesviridae* family, it is a notable exception because it **does not typically produce visible inclusion bodies** in infected cells. Instead, EBV infection is characterized by the presence of **atypical lymphocytes (Downey cells)** in the peripheral blood—which are actually activated T-cells reacting to the infected B-cells, rather than intracellular viral aggregates. **Analysis of Incorrect Options:** * **Herpes Simplex Virus (HSV):** Produces characteristic **Cowdry Type A** inclusions (eosinophilic intranuclear inclusions surrounded by a clear halo). * **Cytomegalovirus (CMV):** Produces large, prominent intranuclear inclusions with a clear halo, famously described as **"Owl’s eye" appearance**. It can also produce basophilic cytoplasmic inclusions. * **Poliovirus:** As an enterovirus, it replicates in the cytoplasm and produces eosinophilic **cytoplasmic inclusion bodies**. **High-Yield Clinical Pearls for NEET-PG:** * **Intranuclear Inclusions:** Cowdry Type A (HSV, VZV, Yellow Fever), Cowdry Type B (Adenovirus, Polio), and Owl’s eye (CMV). * **Intracitoplasmic Inclusions:** Negri bodies (Rabies - pathognomonic), Guarnieri bodies (Smallpox), Molluscum bodies (Molluscum contagiosum), and Henderson-Patterson bodies. * **Both Intra-nuclear & Intra-cytoplasmic:** Measles (Warthin-Finkeldey cells). * **EBV Key Association:** Infectious Mononucleosis, Burkitt Lymphoma, and Nasopharyngeal Carcinoma. Remember: **EBV = Atypical Lymphocytes, NOT Inclusion Bodies.**

Question 48: All of the following are examples of enveloped viruses EXCEPT:

A. Adenovirus (Correct Answer)
B. Coronavirus
C. Molluscum contagiosum virus
D. Herpes virus

Explanation: **Explanation:** The classification of viruses into **enveloped** and **non-enveloped (naked)** is a high-yield concept in NEET-PG Microbiology. Enveloped viruses derive their outer lipid bilayer from host cell membranes, making them more susceptible to heat, detergents, and lipid solvents (like alcohol). **1. Why Adenovirus is the Correct Answer:** Adenovirus is a **non-enveloped (naked) DNA virus**. It possesses a characteristic icosahedral capsid with "penton fibers" (spikes) projecting from the vertices. Because it lacks a lipid envelope, it is relatively stable in the environment and can survive the acidic conditions of the GI tract, which is why it can cause both respiratory infections and gastroenteritis. **2. Analysis of Incorrect Options:** * **Coronavirus:** An enveloped, positive-sense single-stranded RNA virus. Its envelope is derived from the endoplasmic reticulum-Golgi intermediate compartment (ERGIC). * **Molluscum Contagiosum Virus:** A member of the **Poxviridae** family. Poxviruses are unique, complex-shaped DNA viruses that possess an envelope, though they do not acquire it by budding from the plasma membrane. * **Herpes Virus:** A large, enveloped DNA virus. It acquires its envelope by budding through the **inner nuclear membrane** of the host cell. **Clinical Pearls for NEET-PG:** * **Mnemonic for Naked DNA Viruses:** "**P**apova, **A**deno, **P**arvo" (**PAP**). (Note: Parvovirus is the only one that is single-stranded). * **Mnemonic for Naked RNA Viruses:** "**P**icorna, **R**eo, **C**alici, **H**epe" (**P**e**RCH**). * **High-Yield Fact:** All DNA viruses are double-stranded except Parvovirus; all are icosahedral except Poxvirus; and all replicate in the nucleus except Poxvirus.

Question 49: Oral hairy leukoplakia is associated with which virus?

A. Cytomegalovirus
B. Human immunodeficiency virus
C. Epstein-Barr virus (Correct Answer)
D. Human papillomavirus

Explanation: **Explanation:** **Oral Hairy Leukoplakia (OHL)** is a clinical condition characterized by white, corrugated (hairy), non-scrapable patches typically found on the lateral margins of the tongue. **1. Why Epstein-Barr Virus (EBV) is correct:** OHL is caused by the **opportunistic replication of EBV (Human Herpesvirus 4)** in the squamous epithelium of the tongue. It occurs almost exclusively in immunocompromised individuals, particularly those with HIV/AIDS. Unlike oral candidiasis, these lesions **cannot be scraped off**. The diagnosis is confirmed by detecting EBV DNA or proteins within the epithelial cells. **2. Why the other options are incorrect:** * **Cytomegalovirus (CMV):** While CMV causes opportunistic infections in HIV patients (like retinitis or esophagitis), it does not cause leukoplakia. * **Human Immunodeficiency Virus (HIV):** While OHL is a classic clinical marker for HIV progression and low CD4 counts, HIV itself is the underlying cause of immunosuppression, not the direct viral cause of the leukoplakia. * **Human Papillomavirus (HPV):** HPV is associated with oral warts (verruca vulgaris) and oropharyngeal cancers, but not with the "hairy" white patches of OHL. **Clinical Pearls for NEET-PG:** * **Diagnostic Marker:** OHL is often the first clinical sign of HIV infection or a signal that a patient’s CD4 count has dropped below **200 cells/mm³**. * **Histology:** Look for **Koilocytes** (ballooning cells) in the upper layers of the epithelium and "Cowdry type A" inclusions. * **Treatment:** Usually not required unless for cosmetic reasons; however, it often resolves with **Highly Active Antiretroviral Therapy (HAART)** as the immune system recovers. * **Differential Diagnosis:** Must be distinguished from **Oral Candidiasis** (which scrapes off) and **Leukoplakia** (a premalignant lesion associated with tobacco). OHL is **not** premalignant.

Question 50: HTLV-1 is known to cause which of the following conditions?

A. Tropical spastic paraparesis (Correct Answer)
B. Familial Mediterranean fever
C. Cutaneous T-cell lymphoma
D. Burkitt's lymphoma

Explanation: **Explanation:** **Human T-cell Lymphotropic Virus type 1 (HTLV-1)** is a complex retrovirus that primarily infects CD4+ T-cells. It is unique because, unlike HIV, it is oncogenic rather than cytopathic. **1. Why Option A is Correct:** HTLV-1 is the definitive causative agent of **Tropical Spastic Paraparesis (TSP)**, also known as **HTLV-1 Associated Myelopathy (HAM)**. This is a chronic, progressive neurological disorder characterized by demyelination of the spinal cord (lateral columns), leading to lower limb weakness, spasticity, and bladder dysfunction. It is most commonly seen in endemic areas like the Caribbean, Japan, and parts of Africa. **2. Analysis of Incorrect Options:** * **Option B (Familial Mediterranean Fever):** This is an autosomal recessive autoinflammatory genetic disorder caused by mutations in the *MEFV* gene; it is not viral. * **Option C (Cutaneous T-cell Lymphoma):** While HTLV-1 causes **Adult T-cell Leukemia/Lymphoma (ATLL)**, Cutaneous T-cell lymphomas like Mycosis Fungoides and Sézary Syndrome are generally not associated with HTLV-1. * **Option D (Burkitt's Lymphoma):** This is strongly associated with the **Epstein-Barr Virus (EBV)** and the c-myc gene translocation t(8;14). **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Similar to HIV (Blood, Sexual, Vertical/Breast milk). * **Oncogenesis:** The **Tax protein** of HTLV-1 is the key oncogenic driver; it activates host cell genes involved in proliferation and inhibits tumor suppressors. * **ATLL Presentation:** Look for "Flower cells" (clover-leaf nuclei) on peripheral smear and lytic bone lesions with hypercalcemia. * **Diagnosis:** Screening is done via ELISA; confirmation is via Western Blot or PCR.

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