Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 1141: Which of the following statements is true about bacteriophages?

A. It is a bacterium.
B. It imparts toxigenicity to bacteria. (Correct Answer)
C. It helps in transformation.
D. It transfers only chromosomal genes.

Explanation: ### Explanation **Correct Option: B. It imparts toxigenicity to bacteria.** Bacteriophages are viruses that infect bacteria. In a process known as **Lysogenic Conversion**, a temperate phage integrates its genome (as a prophage) into the bacterial chromosome. This genetic addition can provide the bacterium with new phenotypic traits, most notably the ability to produce potent exotoxins. Without the presence of these specific phages, the bacteria remain non-toxigenic and non-pathogenic. **Analysis of Incorrect Options:** * **A. It is a bacterium:** Incorrect. Bacteriophages are viruses, not bacteria. They consist of a nucleic acid core (DNA or RNA) surrounded by a protein coat (capsid). * **C. It helps in transformation:** Incorrect. Bacteriophages are the mediators of **Transduction**. Transformation involves the uptake of naked DNA from the environment, whereas transduction is the transfer of DNA via a viral vector. * **D. It transfers only chromosomal genes:** Incorrect. Phages can transfer both chromosomal DNA and episomal DNA (like plasmids). Furthermore, in **Specialized Transduction**, only specific genes adjacent to the phage integration site are transferred, while in **Generalized Transduction**, any part of the bacterial genome can be packaged. **High-Yield Clinical Pearls for NEET-PG:** * **COBEDS Mnemonic:** The most important toxins acquired via lysogenic conversion are: * **C**holera toxin (*Vibrio cholerae*) * **O** antigen of *Salmonella* * **B**otulinum toxin (*Clostridium botulinum*) * **E**rythrogenic toxin (*Streptococcus pyogenes*) * **D**iphtheria toxin (*Corynebacterium diphtheriae*) * **S**higa toxin (*Shigella* and EHEC) * **Diphtheria Toxin:** Produced only by strains of *C. diphtheriae* infected by the **Beta-phage**. * **Bacteriophage Typing:** Used in epidemiological studies to track the source of outbreaks (e.g., *Staphylococcus aureus* or *Salmonella Typhi*).

Question 1142: Which of the following is NOT caused by Adenovirus?

A. Hemorrhagic cystitis
B. Diarrhea
C. Respiratory tract infection
D. Infectious mononucleosis (Correct Answer)

Explanation: **Explanation:** **Infectious mononucleosis** is the correct answer because it is caused by the **Epstein-Barr Virus (EBV)**, a member of the Herpesviridae family, not Adenovirus. It is clinically characterized by the triad of fever, pharyngitis, and lymphadenopathy, often accompanied by atypical lymphocytosis (Downey cells). **Why the other options are incorrect:** Adenoviruses are non-enveloped, double-stranded DNA viruses known for their diverse tissue tropism, affecting the respiratory, ocular, urinary, and gastrointestinal tracts: * **Hemorrhagic cystitis:** Adenovirus types **11 and 21** are classic causes of acute hemorrhagic cystitis, particularly in children and immunocompromised patients. * **Diarrhea:** Adenovirus types **40 and 41** (subgenus F) are significant causes of infantile gastroenteritis. These are "fastidious" strains that do not grow in routine cell cultures. * **Respiratory tract infection:** This is the most common presentation. Types 1-7 are frequently implicated in pharyngitis, pneumonia, and **Pharyngoconjunctival fever** (often associated with swimming pools). **High-Yield Clinical Pearls for NEET-PG:** * **Structure:** Icosahedral symmetry with characteristic **fibers (pentons)** projecting from the vertices, which act as hemagglutinins and mediate attachment. * **Intranuclear Inclusions:** Produces prominent "smudge cells" (basophilic intranuclear inclusions). * **Military Importance:** Types 4 and 7 are associated with outbreaks of Acute Respiratory Disease (ARD) in military recruits; a live oral vaccine is used in this specific population. * **Keratoconjunctivitis:** Types 8, 19, and 37 cause "Shipyard eye" (Epidemic keratoconjunctivitis).

Question 1143: HHV-6B causes which of the following?

A. Carcinoma cervix
B. Carcinoma endometrium
C. Clear cell carcinoma
D. Focal encephalitis (Correct Answer)

Explanation: **Explanation:** Human Herpesvirus 6 (HHV-6), specifically the **HHV-6B** variant, is a ubiquitous beta-herpesvirus. While it is primarily known as the causative agent of **Exanthema Subitum (Roseola Infantum)** in children, it is highly neurotropic. In immunocompromised individuals—particularly hematopoietic stem cell transplant (HSCT) recipients—HHV-6B reactivation frequently targets the central nervous system, leading to **limbic encephalitis** or **focal encephalitis**. This typically presents with memory loss, seizures, and focal temporal lobe involvement on MRI, making it a critical differential for neurological decline in transplant patients. **Analysis of Incorrect Options:** * **Options A & B (Carcinoma Cervix & Endometrium):** These are primarily associated with High-Risk **Human Papillomavirus (HPV)** types 16 and 18. HHV-6 has no established causal link with gynecological malignancies. * **Option C (Clear Cell Carcinoma):** This malignancy (often of the kidney or ovary) is associated with genetic mutations (e.g., VHL gene) or endometriosis, not viral infections like HHV-6. **High-Yield Clinical Pearls for NEET-PG:** * **HHV-6 Variants:** HHV-6B causes Roseola; HHV-6A is more common in neuroinflammatory diseases like Multiple Sclerosis (though the link is still being studied). * **Roseola Infantum:** Characterized by high fever for 3–5 days, followed by the sudden appearance of a maculopapular rash as the fever subsides ("Fever falls, rash appears"). * **Drug Association:** HHV-6 reactivation is strongly linked to **DRESS syndrome** (Drug Reaction with Eosinophilia and Systemic Symptoms). * **Treatment:** Ganciclovir or Foscarnet are the preferred agents for severe HHV-6 infections.

Question 1144: Which of the following is the causative agent of a variety of cutaneous warts (plantar, common, and flat) and is associated with cervical neoplasia?

A. Human papillomavirus (Correct Answer)
B. West Nile virus
C. Tick-borne encephalitis virus
D. Polyomavirus

Explanation: **Explanation:** **Human Papillomavirus (HPV)** is the correct answer as it is a small, non-enveloped DNA virus with a specific tropism for squamous epithelium. It is the primary causative agent for various types of cutaneous warts: * **Plantar warts:** HPV-1 * **Common warts (Verruca vulgaris):** HPV-2 and HPV-4 * **Flat warts:** HPV-3 and HPV-10 Furthermore, high-risk genotypes (specifically **HPV-16 and 18**) are the leading cause of **cervical neoplasia** and carcinoma. They produce E6 and E7 oncoproteins, which inhibit tumor suppressor proteins p53 and pRb, respectively, leading to malignant transformation. **Incorrect Options:** * **West Nile Virus:** A member of the *Flaviviridae* family, transmitted by *Culex* mosquitoes. It typically causes West Nile fever or neuroinvasive diseases like encephalitis, not cutaneous warts or cervical cancer. * **Tick-borne Encephalitis Virus:** Also a Flavivirus, transmitted by *Ixodes* ticks. It primarily affects the central nervous system. * **Polyomavirus:** While related to Papillomaviruses, the most clinically significant human polyomaviruses are BK virus (hemorrhagic cystitis/nephropathy) and JC virus (Progressive Multifocal Leukoencephalopathy). **High-Yield Clinical Pearls for NEET-PG:** * **Koilocytes:** Pathognomonic finding on Pap smear (cells with perinuclear halo and wrinkled "raisinoid" nuclei). * **Vaccination:** Quadrivalent vaccine (Gardasil) covers types 6, 11 (genital warts/condyloma acuminata) and 16, 18 (cervical cancer). * **Epidermodysplasia Verruciformis:** A rare genetic condition associated with HPV-5 and HPV-8, leading to widespread warts and skin cancer.

Question 1145: Infectious mononucleosis is a viral disorder characterized by which of the following statements?

A. It is most prevalent in children less than 14 years old.
B. It is caused by a rhabdovirus.
C. The causative pathogen is Epstein-Barr virus. (Correct Answer)
D. Affected persons respond to treatment with the production of heterophil antibodies.

Explanation: **Explanation:** **Infectious Mononucleosis (IM)**, also known as "Glandular Fever" or the "Kissing Disease," is a clinical syndrome most commonly caused by the **Epstein-Barr Virus (EBV)**, a human herpesvirus (HHV-4). 1. **Why Option C is Correct:** EBV specifically infects B-lymphocytes by binding to the **CD21 receptor** (CR2). The hallmark of the host immune response is the proliferation of **CD8+ T-cells**, which appear as **atypical lymphocytes (Downey cells)** on a peripheral blood smear. 2. **Why Other Options are Incorrect:** * **Option A:** While EBV infection occurs early in developing countries (often asymptomatic), classic clinical IM is most prevalent in **adolescents and young adults (15–24 years)**. * **Option B:** EBV is a **DNA virus** belonging to the *Gammaherpesvirinae* subfamily. Rhabdoviruses (like Rabies) are RNA viruses. * **Option C:** Heterophil antibodies are a **diagnostic marker** produced during the infection (detected by the Paul-Bunnell or Monospot test); they are **not** produced in response to treatment. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Triad:** Fever, pharyngitis (often with exudates), and lymphadenopathy (typically posterior cervical). * **Splenomegaly:** Present in 50% of cases; patients must avoid contact sports to prevent **splenic rupture**. * **Diagnostic Markers:** Paul-Bunnell Test (+ve), Atypical lymphocytes (>10% on smear), and Anti-VCA (Viral Capsid Antigen) antibodies. * **Drug Rash:** Administration of **Ampicillin or Amoxicillin** in a patient with IM characteristically triggers a maculopapular rash. * **Associated Malignancies:** EBV is linked to Burkitt Lymphoma, Nasopharyngeal Carcinoma, and Hodgkin Lymphoma.

Question 1146: What is the causative agent of SARS?

A. Coronavirus (Correct Answer)
B. Picornavirus
C. Myxovirus
D. Retrovirus

Explanation: **Explanation:** **1. Why Coronavirus is Correct:** Severe Acute Respiratory Syndrome (SARS) is caused by the **SARS-associated coronavirus (SARS-CoV)**. Coronaviruses are large, enveloped, positive-sense single-stranded RNA (+ssRNA) viruses characterized by club-shaped surface projections (peplomers) that give them a "crown-like" appearance under electron microscopy. They primarily infect the upper respiratory and gastrointestinal tracts. SARS-CoV specifically utilizes the **ACE2 receptor** to enter host cells, leading to severe atypical pneumonia and respiratory failure. **2. Why the Other Options are Incorrect:** * **Picornavirus:** These are small, non-enveloped RNA viruses. Members include Poliovirus, Rhinovirus (common cold), and Hepatitis A. They do not cause the severe lower respiratory pathology seen in SARS. * **Myxovirus:** This group includes Orthomyxoviruses (Influenza) and Paramyxoviruses (Measles, Mumps, RSV). While they cause respiratory infections, their genomic structure and replication cycles differ significantly from Coronaviruses. * **Retrovirus:** These are enveloped RNA viruses (like HIV) that use reverse transcriptase to convert RNA into DNA. They are associated with chronic infections and immunodeficiency, not acute respiratory syndromes like SARS. **3. High-Yield Clinical Pearls for NEET-PG:** * **Origin:** SARS-CoV is a zoonotic virus; the natural reservoir is the **bats**, and the intermediate host is the **Civet cat**. * **Receptor:** Both SARS-CoV and SARS-CoV-2 (COVID-19) bind to **Angiotensin-Converting Enzyme 2 (ACE2)** receptors. * **Diagnosis:** Gold standard is **RT-PCR**. * **Other Coronaviruses to remember:** MERS-CoV (uses **DPP4 receptor**; intermediate host is the Dromedary camel) and SARS-CoV-2.

Question 1147: Rice watery stool is indicative of which organism?

A. Vibrio cholerae O1 (Correct Answer)
B. Vibrio cholerae O139
C. Vibrio vulnificus
D. Shigella

Explanation: **Explanation:** **Vibrio cholerae O1** is the classic causative agent of **Cholera**, characterized by "rice-water stools." The underlying mechanism involves the **Cholera Toxin (Choleragen)**, an A-B type enterotoxin. The B-subunit binds to GM1 ganglioside receptors on intestinal epithelial cells, allowing the A-subunit to enter and permanently activate **adenylate cyclase**. This leads to a massive increase in intracellular **cAMP**, resulting in the hypersecretion of water and electrolytes (sodium, potassium, bicarbonate) into the intestinal lumen, bypassing the colon's resorptive capacity. **Analysis of Options:** * **Vibrio cholerae O1 (Correct):** Historically and clinically the primary cause of epidemic cholera. It is divided into two biotypes: Classical and El Tor. * **Vibrio cholerae O139:** While it also causes rice-water diarrhea and produced the 1992 epidemic in India, the O1 serogroup remains the "textbook" and most frequent answer for this clinical sign in competitive exams unless specified otherwise. * **Vibrio vulnificus:** This is a halophilic bacterium primarily associated with **cellulitis/necrotizing fasciitis** (via wound infection) and septicemia following the ingestion of raw oysters, rather than epidemic diarrhea. * **Shigella:** Causes **bacillary dysentery**, characterized by frequent, small-volume stools containing **blood and mucus** (inflammatory diarrhea), rather than the profuse watery diarrhea seen in Cholera. **NEET-PG High-Yield Pearls:** * **Rice-water stool:** Non-offensive, colorless, contains mucus flakes and epithelial cells. * **Microscopy:** Shows characteristic **"Darting motility"** (inhibited by specific antisera in the Pfeiffer phenomenon). * **Culture:** Grows on **TCBS agar** (Yellow colonies due to sucrose fermentation). * **Transport Media:** Venkatraman-Ramakrishnan (VR) medium or Cary-Blair medium. * **Treatment:** Prompt rehydration (ORS/IV fluids) is the mainstay; Doxycycline is the drug of choice for adults to reduce shedding.

Question 1148: Which of the following is a DNA virus?

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

Explanation: **Explanation:** The classification of viruses based on their genetic material is a fundamental concept in virology. Most animal viruses contain either DNA or RNA. **Correct Answer: B. Adenovirus** Adenoviruses are non-enveloped, icosahedral viruses containing **double-stranded DNA (dsDNA)**. They are a classic example of DNA viruses and are clinically significant for causing respiratory tract infections (pharyngoconjunctival fever), epidemic keratoconjunctivitis, and hemorrhagic cystitis. **Analysis of Incorrect Options:** * **A. Poliovirus:** A member of the *Picornaviridae* family, it is a single-stranded, positive-sense RNA (+ssRNA) virus. * **C. Parvovirus:** While Parvovirus B19 is indeed a DNA virus, it is unique because it is **single-stranded DNA (ssDNA)**. In many standardized exams, if a question asks for "the" DNA virus among options containing both dsDNA and ssDNA, the focus is often on the standard dsDNA structure unless "ssDNA" is specified. However, in a strict classification sense, both B and C are DNA viruses. In the context of typical NEET-PG patterns, Adenovirus is the prototypical dsDNA virus. * **D. Hepatitis A virus:** Like Poliovirus, it belongs to the *Picornaviridae* family and contains +ssRNA. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for DNA Viruses:** "**HHAPPPPy**" – **H**erpes, **H**epadna, **A**deno, **P**apilloma, **P**olyoma, **P**arvo, and **P**ox. * **Exceptions to remember:** All DNA viruses are double-stranded except **Parvovirus** (ssDNA). All DNA viruses are icosahedral except **Poxvirus** (complex/brick-shaped). * **Replication:** All DNA viruses replicate in the **nucleus**, except **Poxvirus**, which replicates in the cytoplasm (carrying its own DNA-dependent RNA polymerase).

Question 1149: Which of the following tests is NOT indicated for a patient with acute viral hepatitis?

A. IgM anti-HAV
B. IgM anti-HBcAg
C. Anti-HCV antibody
D. Anti-HDV antibody (Correct Answer)

Explanation: **Explanation:** In the diagnostic workup of **acute viral hepatitis**, the goal is to identify the causative agent using markers that appear during the early phase of infection. **Why Option D is the Correct Answer:** **Anti-HDV antibody** (Total or IgG) is not indicated as a primary screening test for acute hepatitis. Hepatitis D Virus (HDV) is a defective virus that requires the presence of Hepatitis B (HBsAg) to replicate. Testing for HDV is only indicated if a patient is already known to be HBsAg positive or is experiencing a severe "super-infection" or "co-infection" scenario. Furthermore, in the acute phase, HDV-RNA or IgM anti-HDV are the preferred markers; total anti-HDV antibodies often appear later in the disease course. **Analysis of Incorrect Options:** * **A. IgM anti-HAV:** This is the gold standard for diagnosing **Acute Hepatitis A**. IgM appears early and lasts for 3–6 months. * **B. IgM anti-HBcAg:** This is the most reliable marker for **Acute Hepatitis B**, especially during the "window period" when HBsAg may have disappeared but anti-HBs has not yet developed. * **C. Anti-HCV antibody:** While HCV-RNA is the earliest marker, Anti-HCV is the standard screening test for **Hepatitis C**. In acute settings, it may be negative initially (seroconversion takes 6–8 weeks), but it remains a primary indicated test in the hepatitis panel. **High-Yield Clinical Pearls for NEET-PG:** * **Window Period Marker:** IgM anti-HBc is the only positive marker during the HBV window period. * **Hepatitis E:** In pregnant women, HEV carries a high mortality rate (up to 20%) due to fulminant hepatic failure. * **Co-infection vs. Super-infection:** Co-infection (HBV + HDV simultaneously) usually resolves; Super-infection (HDV on chronic HBV) often leads to chronic HDV and rapid progression to cirrhosis.

Question 1150: Which of the following statements regarding rabies is true?

A. Incubation period depends on the site of the bite. (Correct Answer)
B. Diagnosis is by eosinophilic intranuclear inclusions.
C. It is a DNA virus.
D. It is caused only by dogs.

Explanation: ### Explanation **Correct Answer: A. Incubation period depends on the site of the bite.** Rabies virus is a neurotropic virus that travels from the site of inoculation to the Central Nervous System (CNS) via **retrograde axonal transport**. Because the virus moves at a relatively constant speed (approx. 50–100 mm/day), the distance between the bite site and the brain determines the incubation period. Bites on the face or neck have a significantly shorter incubation period compared to bites on the leg. **Analysis of Incorrect Options:** * **B. Diagnosis is by eosinophilic intranuclear inclusions:** This is incorrect. Rabies is characterized by **Negri bodies**, which are **intracytoplasmic** (not intranuclear) eosinophilic inclusions found typically in the pyramidal cells of the hippocampus and Purkinje cells of the cerebellum. * **C. It is a DNA virus:** Rabies is caused by the Rabies virus, which belongs to the genus *Lyssavirus* and family *Rhabdoviridae*. It is a **negative-sense, single-stranded RNA virus**. * **D. It is caused only by dogs:** While domestic dogs are the most common source of transmission in India, rabies can be transmitted by any warm-blooded mammal, including cats, monkeys, bats, foxes, and wolves. **High-Yield Clinical Pearls for NEET-PG:** * **Shape:** Bullet-shaped virus with a lipoprotein envelope containing G-spikes. * **Incubation Period:** Usually 1–3 months (range: <7 days to >1 year). * **Hydrophobia:** Pathognomonic sign caused by spasms of the muscles of deglutition when attempting to swallow water. * **Post-Exposure Prophylaxis (PEP):** Includes wound toilet, Rabies Vaccine (IDRV/IM), and Rabies Immunoglobulin (RIG) for Category III bites. * **Street Virus vs. Fixed Virus:** "Street virus" is the wild strain; "Fixed virus" is the attenuated strain used for vaccine production (e.g., Semple vaccine).

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