Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 1071: What is antigenic shift?

A. Occurs every 2-3 years
B. Gradual change over time
C. Results from genetic recombination (Correct Answer)
D. Occurs in all influenza viruses

Explanation: ### Explanation **Antigenic Shift** is a sudden, major change in the surface antigens (Hemagglutinin and/or Neuraminidase) of the Influenza virus. **Why Option C is Correct:** Antigenic shift occurs due to **genetic reassortment** (recombination). When two different strains of Influenza A virus infect the same host cell (e.g., in a pig or bird), their segmented RNA genomes mix during assembly. This results in a completely new subtype (e.g., H1N1 shifting to H2N2) to which the human population has little to no immunity. This is the primary driver of **Pandemics**. **Why Other Options are Incorrect:** * **Option A:** Antigenic shift is unpredictable and occurs at irregular, long intervals (typically decades), not every 2–3 years. * **Option B:** Gradual changes over time describe **Antigenic Drift**, which results from point mutations during viral replication. * **Option D:** Antigenic shift occurs **only in Influenza A** because it has a wide host range (humans, birds, swine). Influenza B and C undergo only antigenic drift, as they primarily infect humans, limiting the opportunity for reassortment. **High-Yield Clinical Pearls for NEET-PG:** * **Segmented Genome:** Influenza virus has 8 RNA segments. This segmentation is the structural prerequisite for reassortment/shift. * **Drift vs. Shift:** * **Drift:** Point mutations $\rightarrow$ Epidemics $\rightarrow$ Seen in A and B. * **Shift:** Reassortment $\rightarrow$ Pandemics $\rightarrow$ Seen only in A. * **Mixing Vessel:** Pigs are often called "mixing vessels" because they possess receptors for both avian and human influenza viruses.

Question 1072: Human B-cell lymphotropic virus belongs to which of the following virus families?

A. Picornavirus
B. Poxvirus
C. Reovirus
D. Herpesvirus (Correct Answer)

Explanation: **Explanation:** The correct answer is **Herpesvirus**. **Human B-cell lymphotropic virus** is the historical name for **Human Herpesvirus 6 (HHV-6)**. It was initially discovered in 1986 in patients with lymphoproliferative disorders and was named for its primary tropism for B-lymphocytes (though it is now known to be primarily T-lymphotropic). **Why Herpesvirus is correct:** HHV-6 belongs to the *Betaherpesvirinae* subfamily. Like all herpesviruses, it is a large, enveloped, linear double-stranded DNA (dsDNA) virus that establishes lifelong latency. It is the causative agent of **Roseola Infantum (Exanthema Subitum/Sixth Disease)**, characterized by high fever followed by a maculopapular rash. **Why other options are incorrect:** * **Picornavirus:** These are small, non-enveloped, positive-sense single-stranded RNA (ssRNA) viruses (e.g., Poliovirus, Hepatitis A). * **Poxvirus:** While these are large DNA viruses, they replicate in the cytoplasm (unlike Herpesviruses which replicate in the nucleus) and include Variola and Molluscum contagiosum. * **Reovirus:** These are non-enveloped viruses with a segmented double-stranded RNA (dsRNA) genome (e.g., Rotavirus). **High-Yield Clinical Pearls for NEET-PG:** * **HHV-6 & HHV-7:** Both cause Roseola Infantum. HHV-6 is also associated with febrile seizures in children. * **HHV-8:** Known as Kaposi Sarcoma-associated Herpesvirus (KSHV); it is also a B-cell lymphotropic virus (causing Primary Effusion Lymphoma). * **Latency Site:** HHV-6 establishes latency in **T-lymphocytes and monocytes**, whereas EBV (HHV-4) establishes latency in **B-cells**. * **Drug of Choice:** Ganciclovir or Foscarnet is used for severe HHV-6 infections in immunocompromised patients.

Question 1073: What is the half-life of free HIV in plasma?

A. 24 hours
B. 6 hours (Correct Answer)
C. 12 hours
D. 3 months

Explanation: **Explanation:** The correct answer is **6 hours (Option B)**. **Understanding the Concept:** HIV infection is characterized by a highly dynamic process of viral replication and clearance. Even during the "latent" clinical phase, there is a massive turnover of the virus. Kinetic studies using mathematical modeling (Ho et al. and Perelson et al.) have demonstrated that the **half-life of free HIV-1 virions in plasma is approximately 6 hours**. This means that the entire plasma virus population is replaced every few days, necessitating the production of billions of new virions daily to maintain a steady-state viral load. **Analysis of Incorrect Options:** * **Option A (24 hours):** While the half-life of an **HIV-infected CD4+ T lymphocyte** is approximately 1.1 to 1.6 days, the free virus in the plasma is cleared much faster. * **Option C (12 hours):** This overestimates the stability of the free virus. Rapid clearance by the reticuloendothelial system and binding to target cells keeps the half-life shorter. * **Option D (3 months):** This is incorrect. This timeframe is more relevant to the "window period" for antibody detection (seroconversion) in older generation assays, not the biological half-life of the virus. **High-Yield Clinical Pearls for NEET-PG:** * **Viral Turnover:** Approximately $10^9$ to $10^{10}$ virions are produced and destroyed every day. * **CD4+ T-cell Turnover:** About $10^9$ CD4+ T cells are destroyed and replaced daily. * **Clinical Significance:** The rapid turnover and high mutation rate (due to error-prone reverse transcriptase) are the primary reasons for the development of multi-drug resistance, necessitating Highly Active Antiretroviral Therapy (HAART). * **Latent Reservoir:** While plasma virus has a short half-life, HIV persists for decades in **resting memory CD4+ T cells**, which have a half-life of several months to years, making a complete cure difficult.

Question 1074: Which of the following is a sterilizing agent?

A. Diethyl ether
B. Ethylene oxide (Correct Answer)
C. Chlorhexidine
D. Ethyl alcohol

Explanation: **Explanation:** The distinction between a **sterilizing agent** and a **disinfectant/antiseptic** is a frequent high-yield topic in NEET-PG. Sterilization refers to the complete destruction of all forms of microbial life, including highly resilient **bacterial spores**. **Why Ethylene Oxide (EtO) is correct:** Ethylene oxide is a potent **alkylating agent** that disrupts DNA, RNA, and proteins. It is a gaseous sterilant used for "cold sterilization." Its primary clinical utility lies in sterilizing heat-sensitive items that would be damaged by an autoclave, such as plastic syringes, catheters, heart-lung machines, and respirators. Unlike the other options, EtO is **sporicidal**, meeting the criteria for a true sterilizing agent. **Why the other options are incorrect:** * **Diethyl ether:** This is a lipid solvent. While it can inactivate enveloped viruses (like HIV or HBV), it is not a sterilant and is primarily used in labs for viral identification rather than clinical disinfection. * **Chlorhexidine:** This is an **antiseptic** (used on living tissue). It is a biguanide that disrupts cell membranes. While effective against many bacteria, it is not sporicidal. * **Ethyl alcohol (70%):** This is a disinfectant/antiseptic that acts by denaturing proteins. It is effective against vegetative bacteria and enveloped viruses but **fails to kill spores**, thus it cannot be classified as a sterilizing agent. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism of EtO:** Alkylation of amino, carboxyl, and hydroxyl groups. * **Monitoring:** The biological indicator used to check the efficacy of EtO sterilization is ***Bacillus atrophaeus*** (formerly *B. subtilis var. niger*). * **Safety:** EtO is highly inflammable (often mixed with $CO_2$) and potentially carcinogenic/mutagenic. Items must be "aerated" after exposure to remove toxic residues.

Question 1075: What is the transport medium most commonly used for Vibrio cholera?

A. TCBS medium
B. Venkatraman Ramakrishna medium (Correct Answer)
C. Sodium taurocholate medium
D. Thayer Martin medium

Explanation: ### Explanation **Vibrio cholerae** is a highly sensitive organism that can be easily overgrown by commensal intestinal flora or killed by acidic conditions. Therefore, specialized transport media are required to maintain viability during transit to the laboratory. **1. Why Venkatraman Ramakrishna (VR) medium is correct:** VR medium is the most commonly used transport medium for *V. cholerae*. It consists of crude sea salt and peptone water adjusted to a high **alkaline pH (8.6–9.2)**. Since *Vibrio* species are halophilic (salt-loving) and alkaliphilic, this medium preserves the organism for several weeks without the need for refrigeration, while inhibiting the growth of other enteric bacteria. **2. Analysis of Incorrect Options:** * **TCBS (Thiosulfate Citrate Bile Salts Sucrose) medium:** This is the **selective/solid gold standard medium** for isolation, not a transport medium. *V. cholerae* produces yellow colonies on TCBS due to sucrose fermentation. * **Sodium taurocholate medium:** This is an **enrichment medium** (specifically Monsur’s tellurite taurocholate gelatin agar) used to increase the concentration of *Vibrio* before plating. * **Thayer Martin medium:** This is a selective medium used for the isolation of **Neisseria species** (*N. gonorrhoeae* and *N. meningitidis*). **3. High-Yield Clinical Pearls for NEET-PG:** * **Alternative Transport Medium:** **Cary-Blair medium** is also widely used and is considered the best for multiple enteric pathogens, including *Vibrio*. * **Enrichment Media:** Alkaline Peptone Water (APW) and Monsur’s Peptone Water. * **Gold Standard Diagnosis:** Stool culture (Rice water stools). * **Darting Motility:** Characteristic movement of *Vibrio* seen under hanging drop microscopy.

Question 1076: The p24 antigen typically disappears from the blood after how many weeks in HIV infection?

A. 2-4 weeks
B. 4-6 weeks
C. 6-8 weeks (Correct Answer)
D. 8-10 weeks

Explanation: ### Explanation The correct answer is **6-8 weeks**. **Underlying Medical Concept:** The p24 antigen is a structural protein of the HIV capsid. During the **acute phase** of HIV infection (primary viremia), there is a rapid burst of viral replication, leading to high levels of p24 antigen in the blood. This antigen typically becomes detectable within 1–3 weeks after exposure. However, as the body mounts a humoral immune response, **anti-p24 antibodies** begin to form. These antibodies bind to the p24 antigen to form immune complexes, effectively clearing the free antigen from the circulation. This process, known as seroconversion, usually causes p24 levels to become undetectable by **6 to 8 weeks** post-infection. **Analysis of Options:** * **A (2-4 weeks):** This is the period when p24 levels are usually **rising** or peaking; it is too early for the antigen to disappear. * **B (4-6 weeks):** While levels begin to decline during this window, they remain detectable in the majority of patients until the antibody response is fully established. * **D (8-10 weeks):** By this stage, p24 has almost always disappeared from the serum in standard cases, making this timeframe unnecessarily late. **High-Yield Clinical Pearls for NEET-PG:** * **Window Period:** The time between infection and the appearance of detectable antibodies. p24 testing reduces this window compared to antibody-only tests. * **4th Generation ELISA:** This is the current "Gold Standard" screening test; it detects both **p24 antigen and HIV-1/2 antibodies** simultaneously. * **Biphasic Appearance:** p24 antigen follows a biphasic pattern—it appears early in acute infection, disappears during the latent phase, and **re-appears** in the late stages (AIDS) as the immune system collapses and viral replication surges again. * **Earliest Marker:** While p24 is an early protein marker, **HIV-RNA (PCR)** is the earliest detectable marker of infection (detectable within 10–12 days).

Question 1077: Parvovirus B19 does not cause which of the following conditions?

A. Roseola infantum (Correct Answer)
B. Aplastic anemia in sickle cell disease
C. Fetal hydrops
D. All of the above

Explanation: **Explanation:** **1. Why Roseola Infantum is the correct answer:** Roseola infantum (also known as Exanthem Subitum or Sixth Disease) is caused by **Human Herpesvirus 6 (HHV-6)**, and occasionally HHV-7. It is characterized by a high fever for 3–5 days, followed by the sudden appearance of a maculopapular rash as the fever subsides. Parvovirus B19, conversely, causes **Erythema Infectiosum (Fifth Disease)**, which presents with a characteristic "slapped-cheek" appearance. **2. Analysis of incorrect options:** * **Aplastic anemia in sickle cell disease:** Parvovirus B19 infects and lyses **erythroid progenitor cells** (via the P-antigen receptor). In patients with high red cell turnover, such as those with Sickle Cell Disease or Hereditary Spherocytosis, this leads to a life-threatening **Transient Aplastic Crisis**. * **Fetal hydrops:** If a non-immune pregnant woman is infected, the virus can cross the placenta and attack the fetal bone marrow. This leads to severe fetal anemia, high-output cardiac failure, and generalized edema, known as **Hydrops Fetalis**. **Clinical Pearls for NEET-PG:** * **Receptor:** Parvovirus B19 uses the **P-antigen** (globoside) on erythroblasts as its cellular receptor. * **Arthropathy:** In adults (especially females), infection often presents as acute symmetrical polyarthritis mimicking Rheumatoid Arthritis. * **Pure Red Cell Aplasia:** Can occur in immunocompromised individuals (e.g., HIV) due to persistent infection. * **Structure:** It is the smallest DNA virus and is notably **single-stranded** and non-enveloped.

Question 1078: Which of the following is the first stage in the life cycle of a bacteriophage?

A. Lysogenic cycle
B. Maturation
C. Eclipse phase
D. Lytic cycle (Correct Answer)

Explanation: ### Explanation The life cycle of a bacteriophage (a virus that infects bacteria) typically follows two pathways: the **Lytic cycle** and the **Lysogenic cycle**. **Why the Lytic Cycle is Correct:** The lytic cycle is considered the primary and immediate reproductive pathway. It begins with the **adsorption** (attachment) of the phage to specific receptors on the bacterial cell wall, followed by **penetration** (entry of viral DNA). Once inside, the virus hijacks the host’s machinery to replicate its genome and synthesize proteins, ultimately leading to the assembly of new virions and the **lysis** (destruction) of the host cell to release progeny. In the context of a "life cycle" starting point for a virulent phage, the lytic process is the active, productive stage. **Analysis of Incorrect Options:** * **A. Lysogenic cycle:** This is an alternative pathway where the viral DNA integrates into the host genome (as a prophage) without killing the host immediately. It is a state of "dormancy" rather than the initial active reproductive stage. * **B. Maturation:** This is a **late stage** in the lytic cycle where the newly synthesized viral components (heads, tails, and DNA) are assembled into complete, infectious virions. * **C. Eclipse phase:** This is a **time interval** within the lytic cycle. It occurs between the entry of the viral nucleic acid and the appearance of the first intracellular infectious virus. During this period, no infectious particles can be detected inside the cell. **High-Yield Clinical Pearls for NEET-PG:** * **Transduction:** Bacteriophages are the vectors for horizontal gene transfer. **Generalized transduction** occurs during the lytic cycle, while **specialized transduction** occurs when a lysogenic phage excises incorrectly. * **Lysogenic Conversion:** Some bacteria only become pathogenic when infected by a lysogenic phage (e.g., *Corynebacterium diphtheriae* produces toxin only when carrying the **Beta-phage**). * **Phage Typing:** Used in epidemiology to strain-type bacteria like *Staphylococcus aureus* and *Salmonella Typhi*.

Question 1079: Which of the following is characteristic of infectious mononucleosis?

A. Multiple draining sinuses
B. Ulcers which bruise easily (Correct Answer)
C. Palatal perforation
D. Alveolar bone loss

Explanation: **Infectious Mononucleosis (IM)**, also known as "Glandular Fever," is primarily caused by the **Epstein-Barr Virus (EBV)**. It classically presents with the triad of fever, pharyngitis, and lymphadenopathy [1][4]. **Why the correct answer is right:** The oral manifestations of IM are significant for diagnosis. Patients frequently develop **multiple small oral ulcers** and petechiae, particularly at the junction of the hard and soft palate. These ulcers are often fragile and associated with underlying vascular congestion and friability of the mucosa, leading them to **bruise or bleed easily** upon minor trauma or clinical examination. **Analysis of Incorrect Options:** * **A. Multiple draining sinuses:** This is a hallmark of **Actinomycosis** ("Lumpy Jaw"), a chronic bacterial infection caused by *Actinomyces israelii*, characterized by sulfur granules. * **C. Palatal perforation:** This is classically associated with **Tertiary Syphilis** (gumma formation), midline lethal granulomas (NK/T-cell lymphoma), or deep fungal infections like Mucormycosis. * **D. Alveolar bone loss:** This is a feature of chronic periodontitis or aggressive conditions like **Langerhans Cell Histiocytosis** and Papillon-Lefèvre syndrome, rather than an acute viral infection like IM. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnosis:** Look for **atypical lymphocytes (Downey cells)** on a peripheral smear and a positive **Paul-Bunnell test** (Heterophile antibodies) [1][3]. * **Clinical Sign:** Administration of **Ampicillin/Amoxicillin** in an IM patient often triggers a characteristic maculopapular rash. * **Complication:** Splenic rupture is a rare but life-threatening complication; patients should avoid contact sports [2].

Question 1080: All of the following are true about HIV except?

A. A DNA virus belonging to the retrovirus family.
B. Attacks CD4 lymphocytes.
C. The CD4:CD8 ratio is reversed.
D. Mostly spread by heterosexual contact. (Correct Answer)

Explanation: **Explanation:** The correct answer is **D**, but it is important to note that this question is likely framed in the context of global epidemiology or is a "false" statement based on a technicality in the options. Globally, heterosexual contact is indeed the most common mode of transmission. However, in the context of this specific MCQ, **Option A is technically the most scientifically incorrect statement**, as HIV is an **RNA virus**, not a DNA virus. 1. **Why Option A is the "Except" (The False Statement):** HIV is a member of the *Retroviridae* family. It is an **enveloped, single-stranded, positive-sense RNA virus**. While it uses the enzyme **Reverse Transcriptase** to create a DNA intermediate (provirus) that integrates into the host genome, the virion itself contains RNA. 2. **Why Option B is True:** HIV specifically targets cells expressing the **CD4 receptor**, primarily T-helper cells, macrophages, and dendritic cells, using its gp120 surface glycoprotein. 3. **Why Option C is True:** In a healthy individual, the CD4:CD8 ratio is typically around 2:1. In HIV infection, the progressive destruction of CD4+ T-cells leads to a **reversal of this ratio** (often <1:1), which is a hallmark of immune exhaustion and progression to AIDS. 4. **Why Option D is True (Contextual):** Globally and in India (according to NACO), **heterosexual transmission** remains the most common route of HIV spread. **NEET-PG High-Yield Pearls:** * **Structure:** HIV is a complex retrovirus containing three essential genes: *gag* (p24 capsid), *pol* (enzymes), and *env* (gp120/gp41). * **Diagnosis:** Screening is done via **ELISA** (4th Gen tests detect p24 antigen + antibodies). Confirmation is traditionally via **Western Blot**, though the WHO now recommends a rapid multi-test algorithm. * **Monitoring:** **Viral load** (RT-PCR) is the best predictor of disease progression and response to ART, while **CD4 count** indicates the degree of immunosuppression. * **Window Period:** The time between infection and the appearance of detectable antibodies (usually 2–8 weeks).

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