Virology Practice Questions

Q: What is antigenic shift?

Results from genetic recombination. ### 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.

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

Herpesvirus. **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.

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

6 hours. **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.

Q: Which of the following is a sterilizing agent?

Ethylene oxide. **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.

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

Venkatraman Ramakrishna medium. ### 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.

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

Roseola infantum. **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.

Q: Which of the following is characteristic of infectious mononucleosis?

Ulcers which bruise easily. **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 1

What is antigenic shift?

Accepted Answer

Results from genetic recombination

Answer

Occurs every 2-3 years

Answer

Gradual change over time

Answer

Occurs in all influenza viruses

Question 2

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

Accepted Answer

Herpesvirus

Answer

Picornavirus

Answer

Poxvirus

Answer

Reovirus

Question 3

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

Accepted Answer

6 hours

Answer

24 hours

Answer

12 hours

Answer

3 months

Question 4

Which of the following is a sterilizing agent?

Accepted Answer

Ethylene oxide

Answer

Diethyl ether

Answer

Chlorhexidine

Answer

Ethyl alcohol

Question 5

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

Accepted Answer

Venkatraman Ramakrishna medium

Answer

TCBS medium

Answer

Sodium taurocholate medium

Answer

Thayer Martin medium

Question 6

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

Accepted Answer

6-8 weeks

Answer

2-4 weeks

Answer

4-6 weeks

Answer

8-10 weeks

Question 7

Parvovirus B19 does not cause which of the following conditions?

Accepted Answer

Roseola infantum

Answer

Aplastic anemia in sickle cell disease

Answer

Fetal hydrops

Answer

All of the above

Question 8

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

Accepted Answer

Lytic cycle

Answer

Lysogenic cycle

Answer

Maturation

Answer

Eclipse phase

Question 9

Which of the following is characteristic of infectious mononucleosis?

Accepted Answer

Ulcers which bruise easily

Answer

Multiple draining sinuses

Answer

Palatal perforation

Answer

Alveolar bone loss

Question 10

All of the following are true about HIV except?

Accepted Answer

Mostly spread by heterosexual contact.

Answer

A DNA virus belonging to the retrovirus family.

Answer

Attacks CD4 lymphocytes.

Answer

The CD4:CD8 ratio is reversed.

Virology — MCQs

Virology — MCQs

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