Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 451: Hepatitis Delta Virus (HDV) is classified as which type of virus based on its genetic material?

A. Single-stranded RNA (SS-RNA) virus (Correct Answer)
B. Single-stranded DNA (SS-DNA) virus
C. Double-stranded RNA (DS-RNA) virus
D. Double-stranded DNA (DS-DNA) virus

Explanation: **Explanation:** Hepatitis Delta Virus (HDV) is a unique, defective human pathogen. It is classified as a **Single-stranded RNA (ssRNA) virus**. Specifically, it contains a small, circular, negative-sense ssRNA genome. It is considered "defective" because it lacks the genetic information to produce its own envelope protein; it requires the presence of Hepatitis B Virus (HBV) to provide the Hepatitis B Surface Antigen (HBsAg) for its packaging and transmission. **Analysis of Options:** * **Option A (Correct):** HDV has a circular, single-stranded RNA genome. It is the only member of the genus *Deltavirus*. * **Option B & D (Incorrect):** Hepatitis B (HBV) is the only Hepatitis virus that contains DNA (partially double-stranded). HDV is strictly an RNA virus. * **Option C (Incorrect):** While the HDV RNA molecule undergoes extensive base-pairing to form a rod-like structure that *resembles* double-stranded DNA, its actual genetic classification is single-stranded RNA. Reoviruses (like Rotavirus) are the classic examples of DS-RNA viruses. **High-Yield Clinical Pearls for NEET-PG:** * **Dependency:** HDV can only replicate in the presence of HBV. * **Co-infection:** Simultaneous infection with HBV and HDV (usually presents as acute hepatitis; low risk of chronicity). * **Super-infection:** HDV infection in a chronic HBV carrier (leads to severe acute hepatitis and a very high risk of chronic liver disease/cirrhosis). * **Genome Feature:** HDV RNA possesses **ribozyme activity** (it can cleave and ligate itself). * **Diagnosis:** Detection of Delta antigen or HDV RNA in serum. Anti-HDV antibodies indicate infection but are not always protective.

Question 452: Which of the following is most sensitive for the diagnosis of HIV?

A. PCR (Correct Answer)
B. bDNA assay
C. NASBA
D. P24 antigen detection

Explanation: **Explanation:** The correct answer is **PCR (Polymerase Chain Reaction)**. **1. Why PCR is the correct answer:** PCR is a target amplification technique that can detect as few as 1–10 copies of HIV-RNA or DNA per milliliter of blood. It is the most sensitive method because it exponentially amplifies the genetic material of the virus itself. In clinical practice, **RT-PCR (Reverse Transcriptase PCR)** is the gold standard for measuring viral load and is the earliest marker to become positive (as early as 5–10 days post-exposure), making it superior for diagnosing acute HIV infection and neonatal HIV. **2. Why the other options are incorrect:** * **bDNA (Branched DNA) assay:** This is a **signal amplification** technique. While highly specific and useful for monitoring viral load, it has a higher limit of detection (less sensitive) compared to PCR. * **NASBA (Nucleic Acid Sequence-Based Amplification):** This is an isothermal amplification method. While sensitive, it is generally considered slightly less robust and less widely used than PCR in standard diagnostic settings. * **P24 antigen detection:** This detects a viral protein, not genetic material. There is a "window period" where p24 levels may be below the detection limit. It is significantly less sensitive than PCR, especially in the very early stages of infection. **Clinical Pearls for NEET-PG:** * **Window Period:** The time between infection and detectability. PCR has the shortest window period (~10 days), followed by p24 (~16 days), and ELISA (~21-28 days). * **Diagnosis in Infants:** PCR (DNA PCR) is the investigation of choice for diagnosing HIV in infants born to HIV-positive mothers, as maternal IgG antibodies can persist for up to 18 months, making ELISA unreliable. * **Screening vs. Confirmation:** ELISA is the standard screening test; Western Blot was historically the confirmatory test, though current protocols often use sensitive Immunoassays (4th Gen).

Question 453: Which of the following is NOT a marker of the active replicative phase of chronic hepatitis B?

A. HBV DNA
B. HBV DNA polymerase
C. Anti-HBc (Correct Answer)
D. AST & ALT

Explanation: **Explanation:** To identify the replicative phase of Chronic Hepatitis B, we look for markers that signify active viral multiplication and the resulting liver injury. **Why Anti-HBc is the correct answer:** **Anti-HBc (Antibody to Hepatitis B core antigen)** is a marker of **exposure**, not replication. It appears shortly after the onset of infection and persists for life. In chronic infection, **Total Anti-HBc** (primarily IgG) remains positive regardless of whether the virus is actively replicating or in a latent/inactive state. Therefore, it cannot differentiate between the active replicative phase and the inactive carrier state. **Why the other options are markers of replication:** * **HBV DNA:** This is the most sensitive and quantitative marker of viral load. High levels directly indicate active viral replication. * **HBV DNA Polymerase:** This enzyme is required for the synthesis of viral DNA; its presence in the serum correlates directly with active viral multiplication. * **AST & ALT:** These liver transaminases are markers of hepatocellular injury. During the active replicative phase (specifically the "Immune Clearance" phase), the host’s immune system attacks infected hepatocytes, leading to elevated AST and ALT levels. **High-Yield Clinical Pearls for NEET-PG:** * **HBeAg:** The classic serological marker for high infectivity and active replication. Its disappearance and the appearance of **Anti-HBe** (seroconversion) usually signal a transition to the inactive state. * **Window Period:** The interval where HBsAg and Anti-HBs are both negative; **IgM Anti-HBc** is the sole diagnostic marker during this phase. * **HBsAg:** Persistence for **>6 months** defines chronic infection. * **Pre-core Mutants:** Patients may have high HBV DNA but be HBeAg negative; these cases still represent active replication.

Question 454: Recurrent herpes simplex infection occurs due to:

A. Virus residing in the oral mucosa
B. Latent virus in the skin supplying the affected area
C. Latent virus in the nerve ganglia (Correct Answer)
D. None of the above

Explanation: ### Explanation **Correct Answer: C. Latent virus in the nerve ganglia** **Mechanism of Latency and Recurrence:** Herpes Simplex Virus (HSV-1 and HSV-2) exhibits a unique life cycle characterized by **latency**. During the primary infection, the virus replicates in the mucoepithelial cells. From there, it travels via **retrograde axonal transport** along sensory nerve fibers to the sensory nerve ganglia. * **HSV-1** typically remains latent in the **Trigeminal ganglion**. * **HSV-2** typically remains latent in the **Sacral ganglia**. In these ganglia, the viral genome persists in an episomal form without killing the host cell. Upon triggers like stress, UV light, fever, or immunosuppression, the virus undergoes **reactivation**, travels via **anterograde transport** back down the nerve to the skin/mucosa, and causes recurrent lesions (e.g., Herpes Labialis). **Why other options are incorrect:** * **Option A & B:** The virus does not remain latent in the oral mucosa or the skin. While these are the sites of active replication and clinical manifestation, the virus is cleared from these peripheral tissues by the immune system. It only survives long-term by "hiding" in the immunologically privileged site of the nervous system. **High-Yield Clinical Pearls for NEET-PG:** * **Diagnostic Gold Standard:** Viral Culture (though PCR is now more sensitive). * **Tzanck Smear:** Shows **Multinucleated Giant Cells** and **Cowdry Type A** intranuclear inclusion bodies (characteristic but not specific for HSV). * **Drug of Choice:** Acyclovir (inhibits viral DNA polymerase). * **HSV-1** is the most common cause of **Sporadic Viral Encephalitis** (affects the temporal lobe). * **HSV-2** is a common cause of **Aseptic Meningitis**.

Question 455: Which of the following causes a fatal encephalitis for which a vaccine is available?

A. Rabies (Correct Answer)
B. Rhinovirus
C. Cytomegalovirus
D. Respiratory syncytial virus

Explanation: **Explanation:** **Rabies virus** (Option A) is the correct answer because it causes an acute, progressive, and almost invariably fatal encephalitis once clinical symptoms appear. It is a Rhabdovirus transmitted typically via the saliva of an infected animal. Crucially, it is a vaccine-preventable disease. Both pre-exposure prophylaxis (for high-risk individuals) and post-exposure prophylaxis (PEP) are highly effective if administered before the onset of symptoms. **Analysis of Incorrect Options:** * **Rhinovirus (Option B):** The primary cause of the common cold. It is restricted to the upper respiratory tract and does not cause encephalitis. There is no vaccine due to the high number of serotypes. * **Cytomegalovirus (Option C):** While CMV can cause encephalitis in immunocompromised patients (e.g., AIDS), it is not "invariably fatal" in the general population. Currently, there is no commercially available vaccine for CMV. * **Respiratory Syncytial Virus (Option D):** A major cause of bronchiolitis and pneumonia in infants. It does not typically cause encephalitis, and while vaccines have recently been approved for specific populations (older adults/maternal use), it does not fit the "fatal encephalitis" profile. **High-Yield NEET-PG Pearls:** * **Negri Bodies:** Pathognomonic intracytoplasmic eosinophilic inclusions found in pyramidal cells of the hippocampus and Purkinje cells of the cerebellum. * **Mechanism:** The virus binds to **Nicotinic Acetylcholine Receptors (nAChR)** at the neuromuscular junction and travels via retrograde axonal transport to the CNS. * **Vaccine Type:** Modern rabies vaccines (e.g., PCECV, HDCV) are **inactivated/killed** vaccines. * **Hydrophobia:** Characteristic symptom due to painful spasms of the laryngeal and pharyngeal muscles when attempting to swallow.

Question 456: Which of the following is caused by street rabies virus?

A. Natural rabies (Correct Answer)
B. Laboratory passage in rabbits
C. Fatal encephalitis in 6 days
D. Negri bodies not seen

Explanation: **Explanation:** Rabies virus exists in two distinct forms based on its source and characteristics: **Street virus** and **Fixed virus**. **1. Why Option A is correct:** The **Street virus** refers to the rabies virus as it exists in nature, isolated from naturally infected animals (e.g., dogs, bats, wolves). It is characterized by a long and variable incubation period (20–60 days), the consistent production of **Negri bodies** (intracytoplasmic inclusion bodies), and high pathogenicity for humans and animals. **2. Why the other options are incorrect:** Options B, C, and D describe the **Fixed virus**, which is a laboratory-modified strain: * **Option B:** Fixed virus is obtained by serial **laboratory passage** (usually in rabbits) until the incubation period becomes "fixed." * **Option C:** The Fixed virus has a short, stable incubation period of **4–6 days**, whereas the Street virus takes much longer. * **Option D:** **Negri bodies are absent** in infections caused by the Fixed virus, whereas they are the hallmark of the Street virus. **Clinical Pearls for NEET-PG:** * **Negri Bodies:** These are pathognomonic for rabies, most commonly found in the **Hippocampus** (Ammon’s horn) and **Cerebellum** (Purkinje cells). * **Vaccine Production:** Only the **Fixed virus** is used for the preparation of rabies vaccines (e.g., Pitman-Moore strain) because it is predictable and non-pathogenic when injected subcutaneously. * **Centripetal Spread:** The virus travels from the bite site to the CNS via **retrograde axonal transport**. * **Hydrophobia:** This classic sign is due to painful spasms of the pharyngeal muscles when attempting to swallow.

Question 457: For the diagnosis of HIV infection in asymptomatic individuals, what is the minimum number of tests required?

A. 1
B. 2
C. 3 (Correct Answer)
D. 4

Explanation: ### Explanation **Correct Answer: C (3)** The diagnosis of HIV in asymptomatic individuals follows the **National AIDS Control Organization (NACO)** guidelines, which are based on the WHO strategy for HIV testing. **1. Why 3 is the correct answer:** In asymptomatic individuals, the prevalence of HIV is generally low. To ensure high specificity and avoid false positives, NACO recommends **Strategy III**. This strategy requires the serum sample to be tested with **three different kits** (using different antigens or different principles, such as ELISA or Rapid tests). * **Test 1:** Highly sensitive (Screening). If positive, proceed to Test 2. * **Test 2:** Highly specific. If positive, proceed to Test 3. * **Test 3:** Confirmatory. If all three are positive, the individual is diagnosed as HIV-positive. **2. Why other options are incorrect:** * **Option A (1):** A single test is only used for **screening** (e.g., blood bank safety) where high sensitivity is required to catch every possible case, even at the cost of false positives. * **Option B (2):** Two tests (Strategy II) are used for individuals with **clinical symptoms** of AIDS or opportunistic infections. Since the pre-test probability is higher in symptomatic patients, two positive tests are sufficient for diagnosis. * **Option D (4):** There is no standard diagnostic strategy requiring four different antibody tests for routine diagnosis. **Clinical Pearls for NEET-PG:** * **Strategy I:** 1 test (Screening/Transfusion safety). * **Strategy II:** 2 tests (Symptomatic patients). * **Strategy III:** 3 tests (Asymptomatic individuals/Surveillance). * **Window Period:** The time between infection and the appearance of detectable antibodies (usually 2–8 weeks). * **Gold Standard for Infants (<18 months):** HIV DNA PCR (as maternal IgG antibodies can persist and cause false positives in ELISA). * **First test to become positive:** p24 antigen (detected by 4th generation ELISA).

Question 458: Enterovirus 72 is also known as which of the following?

A. Hepatitis A (Correct Answer)
B. Hepatitis E
C. Hepatitis G
D. Hepatitis C

Explanation: **Explanation:** The correct answer is **Hepatitis A (Option A)**. **Why Hepatitis A is correct:** Hepatitis A virus (HAV) was originally classified as **Enterovirus 72** within the *Picornaviridae* family because it shares several characteristics with other enteroviruses, such as being a small, non-enveloped, positive-sense single-stranded RNA virus that is transmitted via the fecal-oral route. Although it has since been reclassified into its own genus, ***Hepatovirus***, the term "Enterovirus 72" remains a high-yield synonym frequently tested in postgraduate medical exams. **Why the other options are incorrect:** * **Hepatitis E (Option B):** This belongs to the *Hepeviridae* family (Genus: *Orthohepevirus*). While it is also transmitted via the fecal-oral route, it is genetically distinct from enteroviruses. * **Hepatitis G (Option C):** Now known as GB virus C, it belongs to the *Flaviviridae* family and is parenterally transmitted. * **Hepatitis C (Option D):** This is a member of the *Flaviviridae* family (Genus: *Hepacivirus*). It is an enveloped virus primarily transmitted through blood and body fluids. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Fecal-oral route (commonly associated with contaminated water or shellfish). * **Incubation Period:** Short (approx. 2–6 weeks). * **Clinical Feature:** It never causes chronic hepatitis or a carrier state. * **Diagnosis:** Detection of **IgM anti-HAV** is the gold standard for acute infection. * **Vaccination:** Inactivated vaccines (e.g., Havrix) provide long-term immunity.

Question 459: What is a characteristic of chickenpox?

A. It is commonly seen in a congenital form.
B. It may be severe in a newborn infected by the mother in late pregnancy. (Correct Answer)
C. It affects the limbs more than the trunk.
D. It may cause pneumonitis.

Explanation: **Explanation:** Chickenpox (Varicella-Zoster Virus) presentation varies significantly based on the timing of infection and the host's immune status. **Why Option B is Correct:** Neonatal varicella is most severe when the mother develops the rash between **5 days before and 2 days after delivery**. In this window, the virus crosses the placenta (viremia), but the mother has not yet produced or transferred protective IgG antibodies to the fetus. This results in a high viral load in the newborn without passive immunity, leading to severe disseminated disease with a high mortality rate (up to 30%). **Analysis of Incorrect Options:** * **Option A:** While "Congenital Varicella Syndrome" exists (characterized by limb hypoplasia and scarring), it is actually **rare**, occurring in only about 1-2% of cases where the mother is infected during the first 20 weeks of pregnancy. * **Option C:** The rash of chickenpox is **centripetal**. It typically starts on the trunk and spreads to the face and limbs, with the highest density of lesions found on the trunk (unlike Smallpox, which is centrifugal). * **Option D:** While varicella *can* cause pneumonitis (especially in adults and immunocompromised patients), Option B is considered the "more" characteristic clinical scenario emphasized in neonatal medicine and classic microbiology textbooks regarding maternal-fetal transmission risks. **NEET-PG High-Yield Pearls:** * **Rash Morphology:** Described as "Dewdrops on a rose petal" (vesicles on an erythematous base). * **Pleomorphism:** Lesions in different stages (papules, vesicles, crusts) are seen simultaneously in the same area. * **Tzanck Smear:** Shows Multinucleated Giant Cells with Cowdry Type A intranuclear inclusion bodies. * **Treatment:** Oral Acyclovir for healthy older children/adults; IV Acyclovir for disseminated disease or pregnancy. Varicella-Zoster Immunoglobulin (VZIG) is indicated for exposed neonates if the mother is infected in the perinatal window.

Question 460: Herpes simplex virus is what type of nucleic acid?

A. Single stranded DNA
B. Double stranded DNA (Correct Answer)
C. Single stranded RNA
D. Double stranded RNA

Explanation: **Explanation:** **1. Why Double-stranded DNA (dsDNA) is correct:** Herpes Simplex Virus (HSV) belongs to the **Herpesviridae** family. All members of this family are characterized by a large, linear, **double-stranded DNA** genome contained within an icosahedral capsid, which is further surrounded by a proteinaceous tegument and a lipid envelope. In virology, the "HHAPPPy" mnemonic (Herpes, Hepadna, Adeno, Papilloma, Polyoma, Pox) is a high-yield way to remember the DNA virus families, all of which are double-stranded except for Parvoviridae. **2. Why the other options are incorrect:** * **Single-stranded DNA (ssDNA):** This is characteristic of the **Parvoviridae** family (e.g., B19 virus). Parvoviruses are the only medically important DNA viruses that are single-stranded. * **Single-stranded RNA (ssRNA):** This is the most common configuration for RNA viruses (e.g., Influenza, HIV, Hepatitis C). HSV is a DNA virus and does not use an RNA genome. * **Double-stranded RNA (dsRNA):** This is rare and specifically identifies the **Reoviridae** family (e.g., Rotavirus). **3. High-Yield Clinical Pearls for NEET-PG:** * **Latency:** A hallmark of HSV is the ability to establish lifelong latency in sensory nerve ganglia (Trigeminal ganglia for HSV-1; Sacral ganglia for HSV-2). * **Diagnosis:** The gold standard for CNS infections is PCR. Historically, the **Tzanck smear** was used to identify multinucleated giant cells and **Cowdry Type A** intranuclear inclusion bodies. * **Encephalitis:** HSV-1 is the most common cause of sporadic fatal viral encephalitis, typically involving the **temporal lobes**. * **Treatment:** Acyclovir is the drug of choice, which acts by inhibiting the viral DNA polymerase.

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