Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 821: Which flavivirus is associated with congenital microcephaly?

A. West Nile virus
B. Yellow fever virus
C. Zika virus (Correct Answer)
D. Dengue virus

Explanation: **Explanation:** **Zika virus (Option C)** is the correct answer. It is a member of the *Flaviviridae* family, primarily transmitted by the *Aedes aegypti* mosquito. Unlike most other flaviviruses, Zika virus exhibits significant **neurotropism** and can cross the placental barrier (vertical transmission). It infects neural progenitor cells, leading to cell death and disrupted cortical development, which manifests clinically as **Congenital Zika Syndrome**. The hallmark of this syndrome is **microcephaly**, often accompanied by intracranial calcifications, ocular damage, and clubfoot. **Analysis of Incorrect Options:** * **West Nile virus (Option A):** While it can cause neuroinvasive diseases like encephalitis or meningitis in adults, it is not a recognized cause of congenital microcephaly. * **Yellow fever virus (Option B):** Primarily causes a hemorrhagic fever characterized by jaundice (due to liver necrosis/Councilman bodies) and "black vomit." It is not associated with congenital malformations. * **Dengue virus (Option D):** Though transmitted by the same *Aedes* mosquito, Dengue typically presents as "breakbone fever" or Hemorrhagic Fever/Shock Syndrome. It does not cause microcephaly. **High-Yield NEET-PG Pearls:** * **Transmission:** Mosquito-borne (*Aedes*), sexual, blood transfusion, and vertical (transplacental). * **Diagnosis:** RT-PCR (blood/urine) during the acute phase; IgM ELISA later. * **Clinical Sign:** Look for a maculopapular rash, conjunctivitis, and arthralgia in the mother. * **Guillain-Barré Syndrome (GBS):** Zika is also strongly associated with an increased risk of GBS in adults.

Question 822: What is the most common cause of the common cold?

A. Coronavirus
B. Echovirus
C. Rhinovirus (Correct Answer)
D. Arbovirus

Explanation: **Explanation:** **1. Why Rhinovirus is Correct:** Rhinovirus (a genus within the *Picornaviridae* family) is the most frequent cause of the common cold, accounting for **30% to 50%** of all cases in adults and children. It is a non-enveloped, positive-sense ssRNA virus. It primarily replicates in the upper respiratory tract because its optimal growth temperature is **33°C**, which is lower than the core body temperature. There are over 100 serotypes, which explains why humans do not develop lasting immunity and suffer from repeated infections throughout life. **2. Analysis of Incorrect Options:** * **A. Coronavirus:** This is the **second most common** cause of the common cold (approx. 10–15%). While significant, it ranks below Rhinovirus in overall prevalence. * **B. Echovirus:** Part of the *Enterovirus* genus. While it can cause respiratory symptoms, it is more classically associated with aseptic meningitis, rashes, and infantile diarrhea. * **C. Arbovirus:** This is a functional group (Arthropod-borne viruses) including Dengue, Zika, and Japanese Encephalitis. These are transmitted via vectors (mosquitoes/ticks) and typically cause systemic febrile illnesses or encephalitis, not the common cold. **3. NEET-PG High-Yield Pearls:** * **Receptor:** Most Rhinoviruses (90%) use **ICAM-1** (CD54) to enter host cells. * **Acid Lability:** Unlike other Picornaviruses (like Poliovirus), Rhinoviruses are **acid-labile**, meaning they are destroyed by gastric acid and therefore do not cause GI infections. * **Seasonality:** Rhinovirus peaks in early fall and spring; Coronaviruses peak in winter. * **Transmission:** Primarily via direct contact (hand-to-hand) and large-particle aerosols. Handwashing is the most effective prevention.

Question 823: All of the following are markers of active replication of chronic hepatitis B, except:

A. HBV DNA
B. HBV DNA Polymerase
C. HBeAg
D. AST & ALT (Correct Answer)

Explanation: **Explanation:** The core concept in Hepatitis B (HBV) serology is distinguishing between **viral replication markers** and **biochemical markers of liver injury**. **Why AST & ALT is the correct answer:** AST (Aspartate Aminotransferase) and ALT (Alanine Aminotransferase) are intracellular enzymes released into the bloodstream when hepatocytes are damaged. While elevated levels indicate **active liver inflammation or necrosis** (hepatitis), they do not directly measure the virus's ability to replicate. In chronic HBV, a patient can have high viral replication with normal transaminases (Immune Tolerant phase) or low replication with high transaminases (due to other causes like superinfection or alcohol). **Why the other options are markers of active replication:** * **HBV DNA (Option A):** This is the most sensitive and direct quantitative marker of viral load and active replication. * **HBV DNA Polymerase (Option B):** This enzyme is required for the synthesis of viral DNA; its presence in the serum directly correlates with active viral assembly. * **HBeAg (Option C):** The 'e' antigen is a secretory product of the nucleocapsid gene. Its presence is a classic qualitative marker indicating high infectivity and active viral replication. **High-Yield Clinical Pearls for NEET-PG:** * **Window Period:** The time between the disappearance of HBsAg and the appearance of Anti-HBs. The only marker present is **Anti-HBc IgM**. * **Best marker of infectivity:** HBV DNA (Quantitative) > HBeAg (Qualitative). * **First marker to appear:** HBsAg. * **Pre-core Mutants:** These patients are HBeAg negative but still have high HBV DNA levels and active liver disease. * **Ground Glass Hepatocytes:** Histological hallmark of chronic HBV infection.

Question 824: Viruses are sharply differentiated from bacteria by which of the following characteristics?

A. Presence of either DNA or RNA as genetic material
B. Simple cellular structure
C. Complex multiplication process
D. All of the above (Correct Answer)

Explanation: **Explanation:** Viruses are fundamentally different from bacteria, representing a unique class of obligate intracellular parasites. The distinction lies in their structural, genetic, and reproductive characteristics: 1. **Genetic Material (Option A):** Unlike bacteria (and all cellular life), which contain both DNA and RNA, a virus typically possesses **only one type of nucleic acid**—either DNA or RNA—never both. This is a hallmark feature used to classify viruses (e.g., Retroviruses vs. Herpesviruses). 2. **Cellular Structure (Option B):** Bacteria are prokaryotic cells with a complex organization including a cell wall, cell membrane, cytoplasm, and ribosomes. In contrast, viruses have a **simple, non-cellular structure** consisting merely of a nucleic acid core surrounded by a protein coat (capsid) and, occasionally, a lipid envelope. They lack metabolic machinery and ribosomes. 3. **Multiplication Process (Option C):** Bacteria multiply by binary fission (an asexual cellular division). Viruses undergo a **complex replication cycle** involving attachment, penetration, uncoating, biosynthesis of components, and assembly. They do not "grow" in size; they are assembled from pre-formed components within a host cell. **Clinical Pearls for NEET-PG:** * **Smallest Virus:** Parvovirus (DNA) / Picornavirus (RNA). * **Largest Virus:** Poxvirus (resembles a brick shape). * **Exceptions to the Rule:** While most viruses have one nucleic acid, some large viruses (like Mimivirus) may contain traces of both, but for exam purposes, the "either/or" rule remains the standard differentiator. * **Antibiotic Sensitivity:** Bacteria are sensitive to antibiotics (targeting cell walls/ribosomes), whereas viruses are not, due to the absence of these structures.

Question 825: Which disease is caused by Parvovirus?

A. Aplastic anemia
B. Erythema infectiosum
C. Arthritis
D. All of the above (Correct Answer)

Explanation: **Explanation:** **Parvovirus B19** is a small, non-enveloped single-stranded DNA virus that specifically targets and replicates in **erythroid progenitor cells** (via the P-antigen receptor). This tropism explains its diverse clinical manifestations. 1. **Erythema Infectiosum (Fifth Disease):** This is the most common presentation in children, characterized by a "slapped-cheek" rash followed by a reticular (lace-like) body rash. The rash is immune-mediated. 2. **Aplastic Anemia:** Because the virus destroys red blood cell precursors, it causes a transient cessation of erythropoiesis. While usually subclinical in healthy individuals, it leads to a life-threatening **Transient Aplastic Crisis (TAC)** in patients with high RBC turnover (e.g., Sickle Cell Anemia, Thalassemia, Hereditary Spherocytosis). 3. **Arthritis:** In adults (more commonly females), the infection often presents as acute, symmetrical polyarthritis involving small joints of the hands, mimicking Rheumatoid Arthritis. Since Parvovirus B19 is responsible for all three conditions, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Hydrops Fetalis:** If a pregnant woman is infected, the virus can cross the placenta, cause severe fetal anemia, and lead to high-output cardiac failure and fetal death. * **Receptor:** It uses the **P-antigen** (globoside) on RBCs as its cellular receptor. Individuals lacking P-antigen are immune to infection. * **Pure Red Cell Aplasia (PRCA):** In immunocompromised patients, persistent infection can lead to chronic PRCA. * **Diagnosis:** Detection of **IgM antibodies** (acute) or **PCR** (especially in aplastic crisis where antibody response may be delayed).

Question 826: Which of the following statements about Hepatitis B virus (HBV) is false?

A. It is a DNA virus.
B. It spreads by blood transfusions.
C. HBsAg is a marker of infection.
D. It has the least chance of chronicity. (Correct Answer)

Explanation: **Explanation:** **Why Option D is the correct (false) statement:** Hepatitis B Virus (HBV) does **not** have the least chance of chronicity; in fact, it has a significant risk of becoming chronic, especially when acquired early in life. The risk of chronicity is inversely proportional to age: approximately **90% in neonates** (vertical transmission), 30% in children, and **5–10% in immunocompetent adults**. Among viral hepatitides, Hepatitis C (HCV) has the highest overall rate of chronicity (~75–85%), while Hepatitis A and E do not cause chronic infection at all (except HEV in transplant recipients). **Analysis of incorrect options:** * **Option A (True):** HBV is a member of the *Hepadnaviridae* family. It is a partially double-stranded **DNA virus** that replicates via reverse transcription. * **Option B (True):** HBV is primarily transmitted through parenteral routes (**blood transfusions**, contaminated needles), sexual contact, and vertical transmission (mother-to-child). * **Option C (True):** **HBsAg** (Surface Antigen) is the first serological marker to appear in the blood and is the hallmark of active infection (both acute and chronic). **High-Yield Clinical Pearls for NEET-PG:** * **Window Period:** The time when HBsAg and Anti-HBs are both negative; **Anti-HBc IgM** is the only marker of acute infection during this phase. * **Infectivity Marker:** **HBeAg** indicates high viral replication and high infectivity. * **Ground Glass Hepatocytes:** Characteristic histopathological finding in chronic HBV infection due to HBsAg accumulation in the endoplasmic reticulum. * **Dane Particle:** The complete, infectious 42nm virion of HBV.

Question 827: Which of the following is a common clinical manifestation of human parvovirus B19 infection?

A. Aplastic crisis in hemolytic anemia patients
B. Anemia in the neonatal period
C. Erythema infectiosum (Correct Answer)
D. Hydrops fetalis

Explanation: **Explanation:** **Parvovirus B19** is a small, non-enveloped DNA virus that specifically targets and replicates in **erythroid progenitor cells** (via the P antigen). **1. Why Erythema Infectiosum is the correct answer:** Also known as **Fifth Disease**, this is the **most common** clinical manifestation of Parvovirus B19, primarily seen in children. It is characterized by a classic "slapped-cheek" rash on the face, followed by a reticular, lace-like maculopapular rash on the trunk and extremities. The rash is immune-mediated and typically appears after the viremic phase has resolved. **2. Analysis of Incorrect Options:** * **A. Aplastic crisis:** While Parvovirus B19 causes this, it occurs specifically in patients with high red cell turnover (e.g., Sickle Cell Anemia, Hereditary Spherocytosis). It is a severe complication, not the most "common" manifestation in the general population. * **B. Anemia in the neonatal period:** Parvovirus infection during pregnancy can lead to fetal loss or hydrops, but it is not a standard cause of neonatal anemia. * **D. Hydrops fetalis:** This occurs if a non-immune pregnant woman is infected, leading to severe fetal anemia and high-output cardiac failure. While high-yield, it occurs in less than 10% of maternal infections. **Clinical Pearls for NEET-PG:** * **Receptor:** P antigen (Globoside) on RBCs. * **Adults:** Often present with **symmetrical arthralgia/arthritis** (mimicking Rheumatoid Arthritis). * **Immunocompromised:** Can cause **Pure Red Cell Aplasia (PRCA)** and chronic anemia. * **Diagnosis:** Detection of IgM antibodies or PCR (especially in aplastic crisis where antibody response may be delayed).

Question 828: During the first week of infection, Dengue virus can be best detected by which method?

A. NS1 antigen detection (Correct Answer)
B. MACELISA
C. ICT test to detect IgM
D. None of the above

Explanation: **Explanation:** The laboratory diagnosis of Dengue virus infection depends on the timing of the clinical presentation. During the **first week (Days 1–5)** of illness, the patient is in the viremic phase. **1. Why NS1 Antigen Detection is correct:** The **NS1 (Non-Structural Protein 1)** is a highly conserved glycoprotein secreted by Dengue-infected cells. It reaches high concentrations in the serum during the early febrile phase (Day 1 to Day 9). Because it appears before antibodies are detectable, it is the **gold standard for early diagnosis** (alongside RT-PCR). It is highly specific and can be detected even before the fever subsides. **2. Why the other options are incorrect:** * **MAC-ELISA (IgM Antibody Capture ELISA):** This detects IgM antibodies. IgM typically appears only after **Day 5–7** of the illness. Testing for IgM during the first week often yields a false negative because the immune system has not yet produced a measurable antibody response. * **ICT (Immunochromatographic Test) for IgM:** Similar to ELISA, this rapid test detects antibodies. While faster, it lacks the sensitivity of ELISA and is still dependent on the patient being in the late acute or convalescent phase (after the first week). **3. High-Yield Clinical Pearls for NEET-PG:** * **Best Diagnostic Sequence:** * **Days 1–5:** NS1 Antigen or RT-PCR (RT-PCR is the most sensitive but NS1 is more widely used). * **After Day 5:** IgM MAC-ELISA. * **Secondary Infection:** In secondary dengue, IgG levels rise rapidly (anamnesis), while IgM may be low or undetectable. * **Vector:** *Aedes aegypti* (Day biter). * **Hallmark Lab Finding:** Thrombocytopenia (low platelet count) and hemoconcentration (rising hematocrit).

Question 829: Which virus is also known as Burkitt's virus?

A. Epstein-Barr virus (EBV) (Correct Answer)
B. Human Papillomavirus (HPV)
C. Human Immunodeficiency Virus (HIV)
D. Hepatitis A virus (HAV)

Explanation: **Explanation:** **Epstein-Barr Virus (EBV)**, also known as Human Gammaherpesvirus 4, is famously referred to as **Burkitt's virus** because of its definitive causal association with Burkitt lymphoma. This association was first discovered by Denis Burkitt and later confirmed by Epstein, Achong, and Barr, who isolated the virus from tumor cells. EBV infects B-lymphocytes via the **CD21 receptor** (CR2) and leads to immortalization of these cells. In the endemic (African) form of Burkitt lymphoma, EBV is found in nearly 100% of cases, characterized by the classic **t(8;14) translocation** involving the *c-myc* oncogene. **Analysis of Incorrect Options:** * **Human Papillomavirus (HPV):** Primarily associated with cervical cancer (Types 16, 18) and warts. It is not linked to Burkitt lymphoma. * **Human Immunodeficiency Virus (HIV):** While HIV increases the risk of developing EBV-related lymphomas (due to immunosuppression), the virus itself does not transform B-cells and is not called Burkitt's virus. * **Hepatitis A virus (HAV):** A picornavirus that causes acute hepatitis; it has no oncogenic potential. **NEET-PG High-Yield Pearls:** * **Other EBV Associations:** Nasopharyngeal carcinoma, Infectious Mononucleosis (Glandular fever), Oral Hairy Leukoplakia (in HIV), and Hodgkin Lymphoma (Mixed cellularity type). * **Diagnosis:** Monospot test (detects heterophile antibodies) and Paul-Bunnell test. * **Microscopy:** Burkitt lymphoma shows a characteristic **"Starry sky appearance"** due to tingible body macrophages against a background of malignant B-cells.

Question 830: Electron microscopy is helpful in the diagnosis of which of the following?

A. Rotavirus (Correct Answer)
B. Respiratory Syncytial Virus (RSV)
C. Herpesvirus
D. Prion

Explanation: **Explanation:** **1. Why Rotavirus is the Correct Answer:** Electron Microscopy (EM) is a classic diagnostic tool for **Rotavirus**, primarily because the virus is shed in extremely high concentrations in the stool (up to $10^{11}$ particles per gram) during the acute phase of gastroenteritis. Under EM, Rotavirus has a highly characteristic morphology: it appears as a **"wheel-like"** structure with a distinct rim and radiating spokes (Latin: *Rota* = wheel). While ELISA and PCR are now more common in clinical practice, EM remains the gold standard for visualizing its unique structure. **2. Why the Other Options are Incorrect:** * **Respiratory Syncytial Virus (RSV):** RSV is highly pleomorphic and fragile. Diagnosis is typically made via Rapid Antigen Detection Tests (RADT) or RT-PCR from nasopharyngeal swabs. * **Herpesvirus:** While EM can identify the family (showing a typical enveloped icosahedral symmetry), it cannot differentiate between members like HSV-1, HSV-2, or VZV. Tzanck smear or PCR is preferred. * **Prions:** Prions are misfolded proteins, not viruses. They lack a nucleic acid core and do not have a distinct "viral" morphology visible by standard negative-staining EM used for clinical diagnosis. **3. NEET-PG High-Yield Pearls:** * **Rotavirus:** Most common cause of severe diarrhea in children worldwide. It is a **double-stranded RNA** virus with a **segmented genome (11 segments)**. * **Immune Electron Microscopy (IEM):** Used to increase sensitivity by adding specific antibodies to clump viral particles together. * **Other EM-identifiable viruses:** Norwalk virus (Calicivirus), Astrovirus (star-shaped), and Adenovirus (space-vehicle appearance). * **Vaccines:** Rotavirus vaccines (Rotarix, RotaTeq) are live-attenuated and part of the Universal Immunization Programme (UIP) in India.

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