Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 1031: What occurs when a temperate bacteriophage enters a state called lysogeny?

A. The bacterial cell is lysed.
B. Many new viruses are produced.
C. Most normal bacterial functions are turned off.
D. The virus may become integrated into the host genome. (Correct Answer)

Explanation: ### Explanation **1. Why the Correct Answer is Right:** Lysogeny is a hallmark of **temperate bacteriophages** (e.g., Lambda phage). Unlike the lytic cycle, where the virus immediately replicates and kills the host, in lysogeny, the viral DNA is incorporated into the bacterial chromosome. Once integrated, the viral genome is referred to as a **prophage**. The prophage replicates synchronously with the host DNA without causing immediate harm, allowing the virus to persist in a latent state for generations. **2. Why the Other Options are Incorrect:** * **Option A & B:** These describe the **Lytic Cycle**. In this phase, the phage hijacks the host machinery to produce new virions, eventually leading to the rupture (lysis) of the bacterial cell wall to release the progeny. * **Option C:** During lysogeny, the bacterium continues its normal physiological functions. In fact, the presence of a prophage can sometimes grant the bacterium new phenotypic traits (Lysogenic Conversion) rather than turning functions off. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Lysogenic Conversion:** This is a high-yield concept where a non-pathogenic bacterium becomes pathogenic due to the prophage. * *Classic Examples:* **Diphtheria toxin** (*C. diphtheriae*), **Cholera toxin** (*V. cholerae*), **Botulinum toxin** (*C. botulinum*), and **Shiga toxin** (*S. dysenteriae*). * **Induction:** The transition from the lysogenic to the lytic cycle is called induction, often triggered by environmental stress or UV light. * **Transduction:** Lysogenic phages are responsible for **Specialized Transduction**, where specific bacterial genes adjacent to the integration site are transferred to another bacterium.

Question 1032: Rabies can be confirmed in patients early in illness by?

A. Antigen detection by corneal smears immunofluorescence
B. Antigen detection by immunofluorescence of skin biopsy (Correct Answer)
C. Demonstration of neutralizing antibodies
D. Demonstration of Negri bodies in hippocampus

Explanation: **Explanation:** The diagnosis of Rabies in a living patient (antemortem) relies on detecting the virus before the onset of advanced neurological symptoms. **Why Option B is correct:** Rabies virus is highly neurotropic. After replicating in muscle cells, it travels via retrograde axonal transport to the CNS and subsequently spreads centrifugally to highly innervated peripheral sites. The **skin of the nape of the neck** is rich in hair follicles surrounded by sensory nerve networks. **Direct Fluorescent Antibody (DFA)** testing of a full-thickness skin biopsy from this region can detect viral antigens in these cutaneous nerves with high sensitivity and specificity early in the clinical course. **Why other options are incorrect:** * **Option A:** While corneal impression smears were historically used, they are no longer preferred due to low sensitivity and the risk of procedural injury to the patient. * **Option C:** Neutralizing antibodies usually appear late in the disease (often after the first week of symptoms) or may not appear at all in unvaccinated individuals until they are near death. They are not reliable for "early" confirmation. * **Option D:** Negri bodies (intracytoplasmic eosinophilic inclusions) are the pathological hallmark of Rabies but are typically identified during **post-mortem** examination of the brain (hippocampus/Purkinje cells). They are present in only about 70% of cases. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Post-mortem diagnosis:** DFA of brain tissue. * **Most sensitive antemortem tests:** RT-PCR of saliva or DFA of skin biopsy. * **Negri Bodies:** Found in the Hippocampus (Ammon’s horn) and Cerebellum. * **Incubation Period:** Usually 1–3 months; determined by the distance of the bite from the CNS.

Question 1033: A rise in the anti-HBc immunoglobulin in a patient indicates which of the following?

A. Acute infection (Correct Answer)
B. Carrier state
C. Prodromal phase
D. Convalescence

Explanation: **Explanation:** The detection of **anti-HBc (Antibody to Hepatitis B core antigen)** is a critical marker in the serological diagnosis of Hepatitis B. 1. **Why Acute Infection is Correct:** Anti-HBc is the first antibody to appear after HBsAg. During an **acute infection**, the body produces **IgM anti-HBc**. This is the only reliable marker during the "Window Period"—the gap where HBsAg has disappeared but anti-HBs has not yet appeared. A rise in these titers signifies active viral replication and the host's primary immune response. 2. **Why Other Options are Incorrect:** * **Carrier State:** Defined by the persistence of HBsAg for >6 months. While anti-HBc (IgG type) is present, it does not "rise" acutely; rather, it remains stable. * **Prodromal Phase:** In this early stage, HBsAg and HBeAg are typically the only detectable markers. Anti-HBc usually appears only after the onset of clinical symptoms or biochemical evidence of liver injury (elevated ALT). * **Convalescence:** This phase is characterized by the disappearance of HBsAg and the appearance of **anti-HBs** (protective antibody). While IgG anti-HBc persists, it is the rise of anti-HBs that defines recovery. **High-Yield Clinical Pearls for NEET-PG:** * **Window Period Marker:** IgM anti-HBc is the "sole marker" of acute HBV infection during the window period. * **IgM vs. IgG:** IgM anti-HBc indicates **acute** infection; IgG anti-HBc indicates **past** exposure or chronic infection. * **Vaccination vs. Natural Infection:** Vaccinated individuals are **Anti-HBs positive** but **Anti-HBc negative** (since vaccines contain only the surface antigen). Natural infection results in positivity for both.

Question 1034: Which of the following viruses does not infect salivary glands?

A. Echovirus
B. Hepatitis C
C. HIV
D. Orthomyxovirus (Correct Answer)

Explanation: **Explanation:** The question tests the knowledge of viral tropism and the specific involvement of salivary glands in various viral infections. **Correct Option: D (Orthomyxovirus)** Orthomyxoviruses (e.g., Influenza virus) primarily infect the respiratory epithelium. They bind to sialic acid receptors on the surface of respiratory cells to initiate infection. They are **not** known to infect or replicate in the salivary glands. While "Mumps" was historically classified under Myxoviruses, it belongs to the **Paramyxovirus** family, which is the classic cause of parotitis (salivary gland inflammation). **Analysis of Incorrect Options:** * **Echovirus (Option A):** As a member of the Picornaviridae family (Enteroviruses), Echoviruses can cause viral sialadenitis, particularly in children, leading to swelling of the parotid glands. * **Hepatitis C (Option B):** HCV is associated with extrahepatic manifestations, most notably **Sjögren’s-like syndrome**. The virus can infect the salivary gland epithelium, leading to chronic inflammation and xerostomia. * **HIV (Option C):** HIV is frequently found in saliva and can cause **HIV-Associated Salivary Gland Disease (HIV-SGD)**, characterized by cystic lymphoid hyperplasia of the parotid glands and xerostomia. **NEET-PG High-Yield Pearls:** * **Mumps (Paramyxovirus):** The most common viral cause of bilateral parotitis. Complications include orchitis (most common in post-pubertal males) and aseptic meningitis. * **CMV (Cytomegalovirus):** Historically called "Salivary Gland Virus" because it remains latent in salivary gland tissues and is shed in saliva. * **Rabies (Rhabdovirus):** Replicates in the salivary glands of animals, ensuring transmission through bites. * **EBV (Epstein-Barr Virus):** Replicates in the oropharyngeal epithelium and is shed in high titers in saliva ("Kissing disease").

Question 1035: A 6-month-old baby presents with a 5-day history of fever, cough, and cold. Which of the following statements is false regarding the likely causative virus in this scenario?

A. The cytopathic effect is syncytium formation.
B. Ribavirin is the drug of choice.
C. It is the most important cause of bronchiolitis. (Correct Answer)
D. It causes upper respiratory tract infection.

Explanation: **Explanation** The clinical presentation of a 6-month-old with fever, cough, and cold is highly suggestive of **Respiratory Syncytial Virus (RSV)** infection, which is the most common cause of lower respiratory tract infections (bronchiolitis and pneumonia) in infants. **Why Option C is the "False" Statement (Correct Answer):** In the context of NEET-PG questions, this is a "trick" question regarding the phrasing. While RSV is indeed the most important cause of bronchiolitis, the question asks for the **false** statement. However, in many standardized exams, if all options seem factually true, the "false" statement often refers to outdated clinical guidelines. Currently, **Ribavirin (Option B)** is no longer considered the routine "drug of choice" due to lack of proven benefit and toxicity; it is reserved only for severe, life-threatening cases. *Note: If the question identifies Option C as the key, it implies a technical error in the question stem or a specific focus on the fact that RSV primarily causes URTI in older children/adults, but for an infant, Option B is the more classically "false" statement in modern practice.* **Analysis of Other Options:** * **Option A:** RSV belongs to the Paramyxoviridae family. Its surface **F (fusion) protein** causes infected cells to fuse with neighboring cells, leading to the characteristic **syncytium formation** (multinucleated giant cells). * **Option D:** RSV is a ubiquitous virus. While it is famous for bronchiolitis, it most commonly presents as a simple **upper respiratory tract infection** (common cold) in adults and older children. **High-Yield Clinical Pearls for NEET-PG:** * **Family:** Paramyxoviridae (ssRNA, enveloped). * **Diagnosis:** Rapid antigen detection (DFA) or PCR. * **Prophylaxis:** **Palivizumab** (monoclonal antibody against F protein) is used for high-risk premature infants. * **Seasonality:** Peak incidence occurs in winter months. * **Radiology:** Hyperinflation and patchy infiltrates are common in RSV bronchiolitis.

Question 1036: Which of the following statements about p24 antigen is FALSE?

A. It is seen 3 weeks after infection. (Correct Answer)
B. It cannot be seen in the first week of infection.
C. It cannot be detected after seroconversion.
D. It is detected by ELISA.

Explanation: ### Explanation The **p24 antigen** is a major core protein of HIV-1. Understanding its kinetics is crucial for diagnosing early infection. **1. Why Option A is the Correct (False) Statement:** The p24 antigen typically becomes detectable in the serum **1 to 2 weeks** (approximately 10–14 days) after infection. Stating that it is first seen at 3 weeks is incorrect because it appears earlier, during the "window period" before antibodies develop. **2. Analysis of Other Options:** * **Option B:** It cannot be seen in the first week. This is **true**. There is an initial "eclipse phase" (7–10 days) where neither viral RNA nor p24 antigen is detectable in the blood. * **Option C:** It cannot be detected after seroconversion. This is **true**. As the body produces anti-p24 antibodies (seroconversion), these antibodies bind to the p24 antigen to form immune complexes. This "masks" the antigen, making it undetectable by standard assays until the late stages of AIDS. * **Option D:** It is detected by ELISA. This is **true**. 4th Generation ELISA (Combo assays) simultaneously detects both the p24 antigen and HIV antibodies, significantly shortening the window period. **Clinical Pearls for NEET-PG:** * **Window Period:** The time between infection and the appearance of detectable antibodies. p24 antigen is the earliest **protein** marker to bridge this gap. * **Earliest Marker:** The very first marker to appear is **HIV-RNA** (detected by NAT) at ~7–10 days, followed by **p24 antigen** at ~10–14 days. * **Biphasic Appearance:** p24 levels peak during acute infection, disappear after seroconversion, and reappear in the terminal stage (AIDS) as the immune system collapses and viral replication surges.

Question 1037: Which test is used for the diagnosis of HIV infection during the window period?

A. p24 antigen (Correct Answer)
B. p18 antigen
C. gp120 antigen
D. gp41 antigen

Explanation: **Explanation:** The **window period** in HIV infection is the interval between the initial infection and the production of detectable antibodies (usually 3–4 weeks). During this phase, standard antibody tests (ELISA) will yield a false negative result. **Why p24 antigen is the correct answer:** The **p24 antigen** is a structural protein of the HIV capsid. It is the first marker to appear in the blood, becoming detectable as early as **10–14 days** after infection, well before seroconversion (antibody production). Modern 4th-generation ELISA tests combine p24 antigen detection with antibody testing to significantly shorten the window period. **Analysis of Incorrect Options:** * **p18 antigen:** This is a matrix protein of HIV-1. While it is a structural component, it is not used as a primary diagnostic marker for early infection because it does not reach detectable levels as reliably or as early as p24. * **gp120 and gp41:** These are envelope glycoproteins. While they are highly immunogenic and used in **Western Blot** to confirm HIV infection by detecting the *antibodies* formed against them, the antigens themselves are not typically measured for early diagnosis during the window period. **High-Yield Clinical Pearls for NEET-PG:** * **Earliest Marker:** HIV-RNA (detected by NAT/PCR) is the very first marker (detectable at ~10 days), followed by p24 antigen. * **Best Screening Test:** 4th Generation ELISA (p24 Ag + Ab combo). * **Confirmatory Test:** Western Blot (detects antibodies to gp41, gp120, and p24). * **Monitoring Progress:** CD4+ T-cell count is used to monitor immune status. * **Monitoring Treatment:** Viral load (HIV-RNA) is the best predictor of treatment efficacy and disease progression.

Question 1038: The Paul Bunnell test is used for the diagnosis of which condition?

A. Chickenpox
B. Yellow fever
C. Genital herpes
D. Infectious mononucleosis (Correct Answer)

Explanation: The **Paul-Bunnell test** is a classic diagnostic tool for **Infectious Mononucleosis (IM)**, caused by the **Epstein-Barr Virus (EBV)**. ### Explanation of the Correct Answer The test detects **heterophile antibodies** in the patient's serum. These are IgM antibodies produced during an EBV infection that, while not specific to the virus itself, have the unique property of agglutinating erythrocytes from other species (specifically **sheep or horse RBCs**). A positive result is indicated by the clumping (agglutination) of these foreign red cells when mixed with the patient's serum. ### Why the Other Options are Incorrect * **A. Chickenpox:** Caused by the Varicella-Zoster Virus (VZV). Diagnosis is primarily clinical or via Tzanck smear (showing multinucleated giant cells) and PCR. * **B. Yellow Fever:** A flavivirus infection diagnosed via IgM ELISA or PCR. It does not induce the production of heterophile antibodies. * **C. Genital Herpes:** Caused by HSV-2. Diagnosis involves viral culture, PCR, or Tzanck smear. ### High-Yield Clinical Pearls for NEET-PG * **Specific Test:** While the Paul-Bunnell test is screening, the **Monospot test** (using horse RBCs) is the rapid version used in clinics. * **Differential Diagnosis:** If a patient has IM-like symptoms (fever, sore throat, lymphadenopathy) but the Paul-Bunnell test is **negative**, consider **Cytomegalovirus (CMV)**, which causes "Heterophile-negative mononucleosis." * **Hematology:** Look for **Downey cells** (atypical T-lymphocytes) on a peripheral blood smear. * **Clinical Warning:** Avoid prescribing Ampicillin/Amoxicillin in suspected IM, as it often triggers a characteristic maculopapular rash.

Question 1039: Which virus reactivation is known to involve the eye?

A. Herpes zoster (Correct Answer)
B. Cytomegalovirus (CMV)
C. Epstein-Barr Virus (EBV)
D. Enterovirus 70

Explanation: **Explanation:** The correct answer is **Herpes zoster (Option A)**. **1. Why Herpes Zoster is Correct:** Herpes zoster is caused by the reactivation of the **Varicella-Zoster Virus (VZV)**, which remains latent in the sensory ganglia (like the trigeminal ganglion) after a primary chickenpox infection. When it reactivates along the **Ophthalmic division (V1) of the Trigeminal nerve**, it causes **Herpes Zoster Ophthalmicus (HZO)**. This typically presents with a painful vesicular rash in a dermatomal distribution and can lead to keratitis, uveitis, and secondary glaucoma. **2. Why other options are incorrect:** * **Cytomegalovirus (CMV):** While CMV causes retinitis (especially in HIV/AIDS patients with CD4 <50), it is generally considered an **opportunistic infection** or a manifestation of primary/persistent infection in the immunocompromised, rather than a classic "reactivation" syndrome defined by dermatomal or neural involvement like VZV. * **Epstein-Barr Virus (EBV):** EBV is primarily associated with infectious mononucleosis, Burkitt lymphoma, and nasopharyngeal carcinoma. It rarely involves the eye. * **Enterovirus 70:** This virus is a major cause of **Acute Hemorrhagic Conjunctivitis (AHC)**. However, this is an **acute primary infection** (often occurring in epidemics), not a reactivation of a latent virus. **3. NEET-PG High-Yield Pearls:** * **Hutchinson’s Sign:** Vesicles on the tip or side of the nose indicate involvement of the nasociliary branch of the trigeminal nerve and are highly predictive of ocular involvement in Herpes Zoster. * **Dendritic Ulcers:** While both HSV and VZV cause them, VZV ulcers are "pseudodendrites" (smaller, lack terminal bulbs, and are poorly stained with fluorescein) compared to the true dendrites of HSV. * **Treatment:** Oral Acyclovir, Valacyclovir, or Famciclovir started within 72 hours of rash onset.

Question 1040: A microbiologist working on influenza virus in a lab observes reversal of hemagglutination. What is this phenomenon called?

A. Hemolysis
B. Elution (Correct Answer)
C. Complement fixation
D. Precipitation

Explanation: ### Explanation **1. Why Elution is the Correct Answer:** The influenza virus possesses two major surface glycoproteins: **Hemagglutinin (HA)** and **Neuraminidase (NA)**. * **Hemagglutination:** Initially, the HA spikes bind to sialic acid receptors on Red Blood Cells (RBCs), forming a lattice (clumping). * **Elution:** Upon incubation at 37°C, the **Neuraminidase (NA)** enzyme acts as a "receptor-destroying enzyme." It cleaves the sialic acid bonds, releasing the virus from the RBC surface. This reversal of clumping, where the virus detaches and the RBCs settle to the bottom of the tube, is known as **Elution**. **2. Why Other Options are Incorrect:** * **A. Hemolysis:** This refers to the rupture of RBCs with the release of hemoglobin. While some viruses (like Mumps) can cause hemolysis, the reversal of clumping specifically describes elution. * **C. Complement Fixation:** This is an immunological test used to detect specific antibodies or antigens using the complement system. It is not a property of viral surface enzymes. * **D. Precipitation:** This occurs when a soluble antigen reacts with a soluble antibody to form an insoluble complex. Hemagglutination involves particulate antigens (cells), not soluble ones. **3. NEET-PG Clinical Pearls & High-Yield Facts:** * **Neuraminidase Inhibitors:** Drugs like **Oseltamivir** and **Zanamivir** work by inhibiting the NA enzyme, thereby preventing the elution/release of new virions from host cells. * **Antigenic Drift vs. Shift:** Drift involves point mutations in HA/NA (epidemics); Shift involves genetic reassortment (pandemics). * **H1N1:** The "swine flu" strain is a classic example of antigenic shift. * **Diagnosis:** The Hemagglutination Inhibition (HI) test is the gold standard for detecting antibodies against influenza.

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Virology — MCQs

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