Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 641: What is the primary site of action of the poliovirus in the nervous system?

A. Medullary cone
B. Posterior nerve roots
C. Anterior horn of the spinal cord (Correct Answer)
D. Posterior horn of the spinal cord

Explanation: **Explanation:** The correct answer is **C. Anterior horn of the spinal cord.** Poliovirus is a neurotropic enterovirus. After initial replication in the oropharynx and gastrointestinal tract (Peyer's patches), the virus enters the bloodstream (viremia). In a small percentage of cases, it crosses the blood-brain barrier or spreads via retrograde axonal transport to the Central Nervous System (CNS). The virus has a specific predilection for **lower motor neurons (LMNs)** located in the **anterior horn cells** of the spinal cord and the motor nuclei of the brainstem. Destruction of these cells leads to the classic presentation of **asymmetrical flaccid paralysis** with loss of reflexes, while sensory functions remain intact. **Why other options are incorrect:** * **A. Medullary cone (Conus Medullaris):** This is the terminal end of the spinal cord. While polio can affect this region, it is a topographical location, not the specific cellular site of action. * **B & D. Posterior nerve roots/Posterior horn:** These structures are responsible for **sensory** conduction. Poliovirus characteristically spares the sensory pathways; therefore, there is no sensory loss in poliomyelitis. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Fecal-oral route (most common). * **Specimen of choice:** Stool is the most reliable sample for viral isolation (excreted for weeks). * **Most common presentation:** Asymptomatic/Inapparent infection (>90%). * **Post-Polio Syndrome:** Occurs decades after recovery due to the failure of compensated motor neurons. * **Vaccines:** **Salk (IPV)** uses killed virus (IgG response); **Sabin (OPV)** uses live-attenuated virus (IgA and IgG response) and can cause Vaccine-Associated Paralytic Polio (VAPP).

Question 642: Which of the following is an enveloped DNA virus?

A. Rhabdovirus
B. Hepatitis B virus (Correct Answer)
C. Adenovirus
D. Parvovirus

Explanation: **Explanation:** To answer this question, one must recall the structural classification of DNA and RNA viruses. Most DNA viruses are double-stranded and non-enveloped, with a few critical exceptions. **1. Why Hepatitis B Virus (HBV) is correct:** Hepatitis B belongs to the **Hepadnaviridae** family. It is a unique DNA virus characterized by a **circular, partially double-stranded DNA** genome and an **envelope** derived from the host cell membrane. The envelope contains the clinically significant Hepatitis B surface antigen (HBsAg). **2. Analysis of Incorrect Options:** * **Rhabdovirus (Option A):** While it is enveloped (bullet-shaped), it is an **RNA virus** (negative-sense, single-stranded), not a DNA virus. It causes Rabies. * **Adenovirus (Option C):** This is a DNA virus, but it is **non-enveloped** (naked). It typically causes respiratory infections, conjunctivitis, and gastroenteritis. * **Parvovirus (Option D):** This is a DNA virus, but it is **non-enveloped**. Notably, it is the only medically important **single-stranded** DNA virus (B19 causes Erythema Infectiosum). **NEET-PG High-Yield Pearls:** * **Mnemonic for Enveloped DNA Viruses:** Remember "**HHP**" — **H**epadna (HBV), **H**erpesvirus, and **P**oxvirus. (Note: Poxvirus is the only DNA virus that replicates in the cytoplasm). * **Mnemonic for Non-Enveloped DNA Viruses:** "**PAP**" — **P**apilloma, **A**deno, and **P**arvo. * **HBV Key Fact:** It uses **reverse transcriptase** during its replication cycle, despite being a DNA virus. * **Smallest DNA Virus:** Parvovirus. * **Largest DNA Virus:** Poxvirus.

Question 643: Which of the following sequential dengue serotypes infection are associated with the maximum risk of dengue hemorrhagic fever?

A. DENV-1/DENV-2 (Correct Answer)
B. DENV-3/DENV-2
C. DENV-4/DENV-2
D. Similar risk with all

Explanation: ### Explanation The correct answer is **A. DENV-1/DENV-2**. #### 1. The Underlying Concept: Antibody-Dependent Enhancement (ADE) The risk of severe dengue (Dengue Hemorrhagic Fever/Dengue Shock Syndrome) is significantly higher during a **secondary infection** with a heterologous serotype. This occurs due to **Antibody-Dependent Enhancement (ADE)**. When a person previously infected with DENV-1 is later infected with DENV-2, the pre-existing non-neutralizing antibodies against DENV-1 bind to the DENV-2 virus. Instead of neutralizing it, these antibodies facilitate the entry of the virus into macrophages via **Fc receptors**. This leads to increased viral replication, a massive release of cytokines ("cytokine storm"), and subsequent vascular permeability. #### 2. Why DENV-1 followed by DENV-2? Epidemiological studies and clinical data have consistently shown that the sequence of **DENV-1 followed by DENV-2** carries the highest statistical risk for developing DHF. While any secondary infection can cause severe disease, the DENV-2 Southeast Asian genotype is particularly notorious for high virulence when following a primary DENV-1 infection. #### 3. Analysis of Other Options * **Options B & C:** While secondary infections with DENV-3 or DENV-4 following other serotypes can cause DHF, they are statistically less frequent or less severe compared to the DENV-1/DENV-2 sequence. * **Option D:** The risk is not uniform. The specific sequence of serotypes, the interval between infections (usually 6 months to several years), and the virulence of the specific strain all influence the clinical outcome. #### Clinical Pearls for NEET-PG: * **Serotypes:** There are 4 serotypes (DENV 1-4). Type 2 is most commonly associated with DHF. * **Vector:** *Aedes aegypti* (Day biter, breeds in artificial collections of clean water). * **Tourniquet Test:** A positive test (≥10 petechiae/square inch) is a clinical indicator of capillary fragility in DHF. * **Gold Standard Diagnosis:** Viral isolation or PCR (early phase); IgM ELISA (after 5 days). NS1 antigen is a reliable early marker.

Question 644: Which influenza strain, not of human origin, can cause a pandemic?

A. H1N1
B. H2N2
C. H5N1 (Correct Answer)
D. H9N1

Explanation: ### Explanation **Correct Answer: C. H5N1** **Concept: Antigenic Shift and Pandemic Potential** A pandemic occurs when a new influenza virus emerges to which the general population has little to no immunity, and it demonstrates the ability for sustained human-to-human transmission. While **H5N1 (Avian Influenza)** is primarily a pathogen of birds (not of human origin), it is considered a significant pandemic threat. This is due to its high virulence, its ability to cross the species barrier to infect humans directly, and the potential for **Antigenic Shift**—a process where the virus acquires new gene segments (reassortment), potentially gaining the ability for efficient human-to-human spread. **Analysis of Options:** * **A. H1N1:** This strain is already established in the human population. It caused the 1918 Spanish Flu and the 2009 Swine Flu pandemic. Since it currently circulates as a seasonal flu strain, it is considered of human/swine origin in the clinical context. * **B. H2N2:** This strain caused the 1957 "Asian Flu" pandemic. Like H1N1, it is a known human pathogen that has previously circulated in the population. * **D. H9N1:** While H9 subtypes (like H9N2) are found in birds and can occasionally infect humans, they have not shown the same high-pathogenicity or pandemic potential historically associated with the H5 or H7 clusters. **High-Yield Clinical Pearls for NEET-PG:** * **Antigenic Shift:** Major genetic changes (reassortment) leading to **Pandemics**. Occurs only in Influenza A. * **Antigenic Drift:** Minor point mutations leading to **Epidemics** and the need for annual vaccine updates. Occurs in both Influenza A and B. * **Drug of Choice:** Oseltamivir (Neuraminidase inhibitor) is the preferred treatment for both seasonal and avian influenza. * **Gold Standard Diagnosis:** RT-PCR is the most sensitive and specific test for identifying influenza strains.

Question 645: Epstein-Barr virus (EBV) causes which of the following conditions?

A. Burkitt's lymphoma
B. Hodgkin's lymphoma
C. Nasopharyngeal carcinoma
D. All of the above (Correct Answer)

Explanation: **Explanation:** Epstein-Barr Virus (EBV), also known as Human Herpesvirus 4 (HHV-4), is a potent oncogenic virus that infects B-lymphocytes and epithelial cells. It establishes latency and drives cellular proliferation through viral proteins like EBNA (Epstein-Barr Nuclear Antigen) and LMP (Latent Membrane Protein), leading to various malignancies. * **Burkitt’s Lymphoma:** EBV is strongly associated with the African (endemic) form of Burkitt’s lymphoma. It involves a characteristic **t(8;14)** chromosomal translocation involving the *c-myc* oncogene. * **Hodgkin’s Lymphoma:** EBV is found in approximately 40-50% of Hodgkin’s cases, particularly the **Mixed Cellularity** subtype. The virus is often detected within the pathognomonic Reed-Sternberg cells. * **Nasopharyngeal Carcinoma:** This is an epithelial tumor highly prevalent in Southern China. EBV DNA is found in 100% of undifferentiated nasopharyngeal carcinomas. **Why "All of the above" is correct:** Since EBV is a proven etiological agent for all three conditions listed, Option D is the most comprehensive answer. **High-Yield Clinical Pearls for NEET-PG:** * **Infectious Mononucleosis (Glandular Fever):** The primary infection caused by EBV, characterized by fever, sore throat, lymphadenopathy, and **atypical lymphocytes (Downey cells)** on peripheral smear. * **Paul-Bunnell Test:** Detects heterophile antibodies, a classic diagnostic marker for EBV. * **Other EBV associations:** Oral Hairy Leukoplakia (in HIV patients), Gastric Carcinoma, and Post-transplant lymphoproliferative disorder (PTLD). * **Receptor:** EBV enters B-cells via the **CD21** receptor (also known as CR2).

Question 646: SARS is a type of?

A. Rhinovirus
B. Lentivirus
C. Calicivirus
D. Coronavir us (Correct Answer)

Explanation: **Explanation:** **SARS (Severe Acute Respiratory Syndrome)** is caused by the **SARS-associated coronavirus (SARS-CoV)**. Coronaviruses are a family of large, enveloped, positive-sense, single-stranded RNA viruses characterized by club-shaped surface projections (peplomers) that create a "crown-like" appearance under electron microscopy. They primarily cause respiratory and enteric diseases in mammals and birds. **Analysis of Options:** * **Coronaviridae (Correct):** SARS-CoV (identified in 2003) and SARS-CoV-2 (COVID-19) belong to the genus *Betacoronavirus*. They bind to the ACE2 receptor in the lower respiratory tract, leading to severe pneumonia and respiratory failure. * **Rhinovirus:** These belong to the *Picornaviridae* family. They are the most common cause of the "common cold" and typically affect the upper respiratory tract, unlike the severe lower respiratory involvement seen in SARS. * **Lentivirus:** This is a genus under the *Retroviridae* family (e.g., HIV). These are characterized by long incubation periods and the ability to integrate their genome into the host cell's DNA. * **Calicivirus:** This family includes the Norovirus, which is a leading cause of epidemic viral gastroenteritis (vomiting and diarrhea), not primary respiratory syndromes. **High-Yield Clinical Pearls for NEET-PG:** * **Genome:** Coronaviruses have the largest genome among all RNA viruses (~30 kb). * **Receptor:** SARS-CoV and SARS-CoV-2 use the **ACE2 receptor**, whereas MERS-CoV uses the **DPP4 receptor**. * **Morphology:** Helical symmetry (unique for positive-sense RNA viruses) and "solar corona" appearance. * **Zoonosis:** SARS originated in bats and was transmitted to humans via **palm civets**.

Question 647: Varicella zoster virus presumably infects which of the following structures?

A. Dorsal root ganglion (Correct Answer)
B. Anterior horn cells
C. Hippocampus
D. Ventral nerve root

Explanation: **Explanation** **Correct Option: A. Dorsal root ganglion** Varicella-Zoster Virus (VZV), a member of the *Alphaherpesvirinae* subfamily, exhibits a unique life cycle characterized by primary infection (Chickenpox) followed by lifelong latency. During the primary infection, the virus spreads from skin lesions via retrograde axonal transport through sensory nerve fibers to the **Dorsal Root Ganglia (DRG)** or cranial nerve ganglia. Here, the virus remains dormant in a non-replicative state. Reactivation of the virus (due to waning immunity or stress) leads to **Herpes Zoster (Shingles)**, where the virus travels anterograde down the sensory nerve to cause a painful, vesicular rash in a specific dermatomal distribution. **Incorrect Options:** * **B. Anterior horn cells:** These are motor neurons. VZV primarily targets sensory neurons. Damage to anterior horn cells is characteristic of **Poliovirus**, leading to lower motor neuron paralysis. * **C. Hippocampus:** This area of the brain is specifically targeted by the **Rabies virus** (forming Negri bodies) and is often involved in **Herpes Simplex Virus (HSV-1) encephalitis**, which typically affects the temporal lobes. * **D. Ventral nerve root:** These roots carry motor efferent fibers. VZV latency is established in the sensory (afferent) cell bodies located in the dorsal ganglion, not the nerve roots themselves. **High-Yield Clinical Pearls for NEET-PG:** * **Tzanck Smear:** Shows Multinucleated Giant Cells with Cowdry Type A intranuclear inclusions (common to HSV and VZV). * **Ramsay Hunt Syndrome:** Reactivation of VZV in the Geniculate ganglion involving CN VII. * **Post-herpetic Neuralgia:** The most common complication of Shingles. * **Vaccine:** Live attenuated strains (Oka strain) are used for both Varicella and Zoster prevention.

Question 648: What is the function of the gp120 protein of the HIV envelope?

A. Cell fusion
B. Cell penetration
C. Attachment to the CD4 receptor (Correct Answer)
D. Integration of nucleic acid

Explanation: ### Explanation The HIV envelope contains two essential glycoproteins derived from the precursor **gp160**: the surface subunit **gp120** and the transmembrane subunit **gp41**. **1. Why the correct answer is right:** * **gp120 (Surface protein):** Its primary role is **attachment**. It binds specifically to the **CD4 receptor** found on T-helper cells, macrophages, and dendritic cells. Following this initial binding, gp120 undergoes a conformational change that allows it to bind to co-receptors (**CCR5** or **CXCR4**), which is a prerequisite for viral entry. **2. Why the incorrect options are wrong:** * **A & B (Cell fusion and penetration):** These are the functions of **gp41**. Once gp120 binds to the receptors, gp41 is exposed and mediates the fusion of the viral envelope with the host cell membrane, allowing the viral capsid to penetrate the cytoplasm. * **D (Integration of nucleic acid):** This is mediated by the enzyme **Integrase (p32)**, which is a product of the *pol* gene, not an envelope protein. **3. High-Yield Clinical Pearls for NEET-PG:** * **Maraviroc:** A drug that acts as a CCR5 antagonist, preventing the gp120-co-receptor interaction. * **Enfuvirtide:** A fusion inhibitor that targets gp41. * **Mnemonic:** **gp120** is for **1**nitial attachment; **gp41** is for **4**usion. * **Antigenic Variation:** The *env* gene (encoding gp120) is the most variable gene of HIV, which is the primary reason why developing an effective vaccine is challenging.

Question 649: Which of the following is not an RNA virus?

A. Ebola
B. Simian 40 (Correct Answer)
C. Rabies
D. Vesicular stomatitis virus

Explanation: **Explanation:** The core concept tested here is the classification of viruses based on their genetic material. Most animal viruses contain either DNA or RNA. **Why Simian 40 (SV40) is the correct answer:** Simian Virus 40 is a **DNA virus**. Specifically, it belongs to the *Polyomaviridae* family. It is a small, non-enveloped virus with a **circular, double-stranded DNA (dsDNA)** genome. It is historically significant in microbiology as a model organism for studying DNA replication and oncogenesis. **Why the other options are incorrect:** * **Ebola Virus:** A member of the *Filoviridae* family, it contains a linear, negative-sense, single-stranded RNA (**ssRNA**) genome. * **Rabies Virus:** A member of the *Rhabdoviridae* family (genus *Lyssavirus*), it is a classic example of a bullet-shaped, negative-sense **ssRNA** virus. * **Vesicular Stomatitis Virus (VSV):** Also a member of the *Rhabdoviridae* family, it contains a negative-sense **ssRNA** genome. It is often studied alongside Rabies due to their structural similarities. **High-Yield Clinical Pearls for NEET-PG:** * **DNA Virus Mnemonic:** Remember "**HHAPPPPy**" viruses: **H**erpes, **H**epadna (HBV), **A**deno, **P**apilloma, **P**olyoma (includes SV40, JC, BK), **P**ox (the only DNA virus that replicates in the cytoplasm), and **P**arvo (the only ssDNA virus). * **SV40 Significance:** It was a contaminant in early Salk polio vaccines (derived from monkey kidney cells), though it has not been proven to cause cancer in humans. * **Rhabdoviruses:** Both Rabies and VSV are characterized by **Negri bodies** (intracytoplasmic inclusions) and a bullet-shaped morphology.

Question 650: Which virus is used as a vector in vaccine preparation?

A. Rhinovirus
B. Vaccinia (Correct Answer)
C. Adenovirus
D. Ebola

Explanation: **Explanation:** **Vaccinia virus** is the classic example of a viral vector used in vaccine preparation. It is a large, complex, enveloped dsDNA virus belonging to the Poxviridae family. Its primary utility as a vector stems from its large genome, which can accommodate significant amounts of foreign DNA without losing the ability to replicate. Historically, it was used as the live vaccine to eradicate Smallpox. In modern medicine, recombinant Vaccinia strains (like Modified Vaccinia Ankara - MVA) are engineered to express antigens from other pathogens (e.g., Rabies, HIV), triggering a robust T-cell and B-cell immune response. **Analysis of Incorrect Options:** * **Rhinovirus (A):** Primarily causes the common cold. It is not used as a vector due to its high diversity (over 100 serotypes) and limited genomic capacity. * **Adenovirus (C):** While Adenoviruses *are* widely used as vectors (e.g., AstraZeneca/Covishield COVID-19 vaccine), in the context of standard microbiology textbooks and historical "gold standard" vector questions for NEET-PG, **Vaccinia** is the preferred answer unless the question specifies modern mRNA/viral vector platforms. * **Ebola (D):** This is a highly pathogenic Filovirus. It is the *target* of vaccines (often using Vesicular Stomatitis Virus as a vector), not a vector itself. **High-Yield Clinical Pearls for NEET-PG:** * **Smallpox Eradication:** Smallpox was declared eradicated in 1980; the vaccine used was live Vaccinia, not Variola. * **Vector Properties:** An ideal vector should be safe, trigger a strong immune response, and have a stable genome. * **Complication:** A rare but serious side effect of the Vaccinia vaccine is *Eczema vaccinatum* and *Post-vaccinial encephalitis*.

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Virology — MCQs

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