Virology — MCQs

Virology Indian Medical PG Practice Questions and MCQs

Question 341: What is the incubation period of Hepatitis B Virus (HBV)?

A. 45 to 180 days (Correct Answer)
B. 6 to 60 days
C. 10 days
D. 10 hours

Explanation: ### Explanation **Correct Answer: A. 45 to 180 days** Hepatitis B Virus (HBV) is a DNA virus characterized by a **long incubation period**, typically ranging from **45 to 180 days** (average 60–90 days). This prolonged period is due to the virus's slow replication cycle and the time required for the host's immune system to mount a response against the hepatocytes expressing viral antigens. In clinical practice, HBsAg (Hepatitis B Surface Antigen) usually appears in the serum 1 to 10 weeks after acute exposure, prior to the onset of symptoms or jaundice. **Analysis of Incorrect Options:** * **B. 6 to 60 days:** This range is more characteristic of **Hepatitis A (HAV)**, which has a shorter incubation period (average 28 days) as it is transmitted via the feco-oral route and replicates rapidly in the liver. * **C. 10 days:** This is too short for any viral hepatitis. Such short incubation periods are typically seen in bacterial gastroenteritis or certain respiratory viruses (e.g., Influenza). * **D. 10 hours:** This timeframe is consistent with **preformed bacterial toxins** (e.g., *Staphylococcus aureus* or *Bacillus cereus* food poisoning), not viral infections. **High-Yield Clinical Pearls for NEET-PG:** * **Incubation Period Mnemonic:** Remember the "Rule of 2s": HAV (2–6 weeks), HBV (2–6 months), HCV (2 weeks to 6 months). * **Window Period:** The interval during which HBsAg has disappeared but anti-HBs has not yet appeared. The only serological marker present here is **Anti-HBc IgM**. * **First Marker to Appear:** HBsAg. * **Indicator of High Infectivity:** HBeAg (reflects active viral replication). * **Hepatitis D:** Requires HBV (HBsAg) for its envelope; it cannot cause infection alone.

Question 342: What is the vaccine strain for yellow fever?

A. 17D (Correct Answer)
B. 5D
C. 4D
D. 2D

Explanation: **Explanation:** **Yellow Fever Vaccine (Strain 17D):** The correct answer is **17D**. The Yellow Fever vaccine is a **live-attenuated vaccine** derived from the wild-type Asibi strain. It was developed by Max Theiler in 1937 (for which he received a Nobel Prize). The 17D strain is highly effective, providing long-lasting immunity (often considered lifelong) after a single subcutaneous dose. It is cultured in **embryonated chicken eggs**, making a history of egg allergy a contraindication. **Analysis of Options:** * **A. 17D (Correct):** This is the globally standardized strain used for all yellow fever vaccinations. It exists in two sub-strains: 17D-204 and 17DD, both of which are clinically equivalent. * **B, C, and D (5D, 4D, 2D):** These are distractors. There are no recognized human vaccines for yellow fever or other major viral pathogens using these specific alphanumeric designations. **High-Yield Clinical Pearls for NEET-PG:** * **Type of Vaccine:** Live-attenuated (cannot be given to immunocompromised individuals or infants <6 months). * **Dosage:** 0.5 ml administered subcutaneously. * **Validity:** International Health Regulations (IHR) now state that a single dose provides lifelong protection; the certificate becomes valid **10 days** after vaccination and lasts for life. * **Contraindications:** Egg hypersensitivity, thymus disorders, and symptomatic HIV/AIDS. * **Council of India Requirement:** Yellow Fever vaccination is mandatory for travelers arriving from or transiting through endemic zones (Africa and South America) to India. * **Vector:** Primarily transmitted by the *Aedes aegypti* mosquito.

Question 343: Which microorganism has been historically used as a weapon in biological terrorism?

A. Smallpox Virus (Correct Answer)
B. Rabies Virus
C. Herpes Virus
D. Influenza C Virus

Explanation: **Explanation:** **Smallpox virus (*Variola major*)** is classified by the CDC as a **Category A Bioterrorism Agent**. These are high-priority pathogens that pose a national security risk because they can be easily disseminated, result in high mortality rates, and have the potential for major public health impact. Smallpox is particularly dangerous because it is highly contagious via respiratory droplets, has a high case-fatality rate (~30%), and since routine vaccination ceased in 1980 following global eradication, the majority of the world's population now lacks immunity. **Analysis of Incorrect Options:** * **B. Rabies Virus:** While fatal if untreated, it is not suitable for bioterrorism because it requires direct inoculation (bites) or organ transplantation for transmission; it does not spread efficiently from person to person. * **C. Herpes Virus:** These viruses (e.g., HSV, VZV) are ubiquitous and generally cause self-limiting or manageable chronic infections. They lack the high mortality and rapid transmission required for a biological weapon. * **D. Influenza C Virus:** Unlike Influenza A, Type C typically causes only mild upper respiratory tract infections and does not cause epidemics or pandemics, making it an ineffective weapon. **High-Yield Clinical Pearls for NEET-PG:** * **CDC Category A Agents:** Remember the mnemonic **"ABC To Search"**: **A**nthrax (*B. anthracis*), **B**otulism (*C. botulinum*), **C**holera/Plague (*Y. pestis*), **T**ularemia (*F. tularensis*), **S**mallpox, and **H**emorrhagic fever viruses (Ebola, Marburg). * **Eradication:** Smallpox is the only human infectious disease to be completely eradicated (declared by WHO in 1980). * **Vaccine:** The vaccine uses **Live Vaccinia virus**, not Variola. It provides cross-protection.

Question 344: Acquired immunodeficiency syndrome (AIDS) primarily affects which type of immune cell?

A. T-helper cells (Correct Answer)
B. T-suppressor cells
C. T-cytotoxic cells
D. B cells

Explanation: **Explanation:** The Human Immunodeficiency Virus (HIV), the causative agent of AIDS, primarily targets cells expressing the **CD4 receptor** on their surface. The most critical of these are the **T-helper (Th) cells**. 1. **Mechanism of Entry:** HIV’s envelope glycoprotein **gp120** binds specifically to the CD4 molecule. It also requires co-receptors (CCR5 or CXCR4) for entry. Once inside, the virus replicates and eventually leads to the depletion of these cells through direct viral lysis and apoptosis. 2. **Consequence:** T-helper cells are the "conductors" of the immune system. Their depletion leads to a collapse of both cell-mediated and humoral immunity, predisposing the patient to opportunistic infections and malignancies. **Analysis of Incorrect Options:** * **T-suppressor (CD8+) and T-cytotoxic (CD8+) cells:** These cells do not possess the CD4 receptor. While their function is eventually impaired due to the lack of "help" from Th cells, they are not the primary targets of HIV infection. In fact, early in the disease, the CD8 count may temporarily rise. * **B cells:** HIV does not infect B cells directly as they lack CD4 receptors. However, B cell dysfunction occurs secondary to the loss of T-helper signals, leading to paradoxical hypergammaglobulinemia but a poor response to new antigens. **NEET-PG High-Yield Pearls:** * **Diagnostic Marker:** A CD4 count **<200 cells/mm³** is the defining threshold for AIDS. * **Infection of other cells:** HIV also infects **Macrophages and Dendritic cells** (which act as reservoirs) and **Microglial cells** in the brain (leading to AIDS Dementia Complex). * **Coreceptors:** **CCR5** is used by M-tropic strains (early infection); **CXCR4** is used by T-tropic strains (late stage). Individuals with a **CCR5-Δ32 mutation** are resistant to HIV infection.

Question 345: Which of the following flaviviruses is closely related to the Russian spring-summer encephalitis virus?

A. Dengue
B. Chikungunya
C. Yellow fever
D. Kyasanur Forest Disease (KFD) (Correct Answer)

Explanation: **Explanation:** The **Russian spring-summer encephalitis (RSSE)** virus and **Kyasanur Forest Disease (KFD)** virus both belong to the **Tick-borne encephalitis (TBE) complex** within the *Flaviviridae* family. They share significant antigenic similarities and are both transmitted by the bite of infected ticks (specifically *Ixodes* for RSSE and *Haemaphysalis spinigera* for KFD). **Why Kyasanur Forest Disease (KFD) is correct:** KFD is often referred to as "Monkey Fever" and is endemic to the Western Ghats of India. It is antigenically closely related to RSSE. Both viruses cause a biphasic illness characterized by fever and hemorrhagic manifestations or neurological involvement. Because of this close relationship, the RSSE vaccine has historically been used to provide cross-protection against KFD. **Why other options are incorrect:** * **Dengue and Yellow Fever:** While these are also Flaviviruses, they belong to the **Mosquito-borne** group (transmitted by *Aedes aegypti*). They are antigenically distinct from the Tick-borne encephalitis complex. * **Chikungunya:** This is an **Alphavirus** (family *Togaviridae*), not a Flavivirus. It is transmitted by *Aedes* mosquitoes and primarily causes severe arthralgia. **High-Yield Clinical Pearls for NEET-PG:** * **Vector for KFD:** *Haemaphysalis spinigera* (Hard tick). * **Reservoirs for KFD:** Monkeys (Langurs and Bonnet macaques) are the common amplifiers; their death is often a sentinel event for an outbreak. * **TBE Complex Members:** Includes RSSE, KFD, Omsk Hemorrhagic Fever, and Powassan virus. * **Diagnosis:** PCR is the gold standard in the early phase; IgM ELISA is used later.

Question 346: A patient presented with features of acute hepatitis following a blood transfusion a few weeks ago. Which of the following serological markers indicates the acute phase of hepatitis B infection?

A. Anti HBc positive, HBsAg positive (Correct Answer)
B. Anti HBe positive, HBsAg positive
C. Anti HBs positive, HBsAg negative
D. HBeAg negative, HBsAg negative

Explanation: ### Explanation **1. Why Option A is Correct:** In the acute phase of Hepatitis B Virus (HBV) infection, the first detectable serum marker is **HBsAg** (Hepatitis B surface Antigen). Shortly after, the body produces antibodies against the core antigen (**Anti-HBc**). Specifically, **IgM Anti-HBc** is the hallmark of acute infection. Since HBsAg indicates the presence of the virus and Anti-HBc (IgM) confirms a recent primary response, the combination of **HBsAg (+) and Anti-HBc (+)** is diagnostic of acute hepatitis B. **2. Analysis of Incorrect Options:** * **Option B (Anti-HBe +, HBsAg +):** This pattern typically indicates a later stage of infection or chronic hepatitis B with low viral replication (seroconversion from HBeAg to Anti-HBe). It does not define the acute phase. * **Option C (Anti-HBs +, HBsAg -):** This indicates **immunity**. If Anti-HBc is also present, it signifies recovery from a natural infection; if only Anti-HBs is present, it signifies successful vaccination. * **Option D (HBeAg -, HBsAg -):** This represents a state where the virus is not actively replicating or has been cleared. It is inconsistent with a patient currently presenting with acute hepatitis symptoms. **3. NEET-PG High-Yield Pearls:** * **Window Period:** The interval when HBsAg has disappeared but Anti-HBs has not yet appeared. During this time, **IgM Anti-HBc** is the *only* diagnostic marker present. * **Infectivity Marker:** **HBeAg** is the best indicator of high viral replication and high infectivity. * **Chronic Infection:** Defined by the persistence of HBsAg for >6 months. * **Post-Transfusion Hepatitis:** While HBV can be transmitted via blood, **Hepatitis C (HCV)** was historically the most common cause of post-transfusion hepatitis before rigorous screening; however, HBV remains a high-yield topic for serological interpretation.

Question 347: All of the following viruses contain double-stranded DNA, except:

A. Parvovirus (Correct Answer)
B. Papilloma virus
C. Herpes virus
D. Pox virus

Explanation: **Explanation:** The fundamental rule of virology is that most DNA viruses are double-stranded (dsDNA) and most RNA viruses are single-stranded (ssRNA). However, there are two high-yield exceptions frequently tested in NEET-PG: **Parvoviridae** (the only medically important ssDNA virus) and **Reoviridae** (the only medically important dsRNA virus). 1. **Why Parvovirus is correct:** Parvovirus B19 belongs to the Parvoviridae family. It is a small, non-enveloped virus characterized by a **single-stranded DNA (ssDNA)** genome. This makes it the unique exception among the DNA viruses listed. 2. **Why the other options are incorrect:** * **Papilloma virus:** A member of the Papovaviridae family; it contains circular dsDNA. * **Herpes virus:** A large, enveloped virus containing linear dsDNA. * **Pox virus:** The largest and most complex virus; despite replicating in the cytoplasm (an exception for DNA viruses), it contains a linear dsDNA genome. **Clinical Pearls for NEET-PG:** * **Parvovirus B19:** Causes **Erythema Infectiosum** (Fifth disease), characterized by the "slapped-cheek" rash. It targets erythroid progenitor cells, leading to **Aplastic Crisis** in patients with chronic hemolytic anemias (e.g., Sickle Cell Disease) and **Hydrops Fetalis** if contracted during pregnancy. * **Mnemonic for DNA Viruses:** "**HHAPPPPy**" (Herpes, Hepadna, Adeno, Papilloma, Polyoma, Pox, Parvo). All are dsDNA except **Parvo** (ssDNA). All are icosahedral except **Pox** (complex). All replicate in the nucleus except **Pox** (cytoplasm).

Question 348: What characterizes the latent phase of HIV infection?

A. Viral replication (Correct Answer)
B. Sequestration in lymphoid tissue
C. Infectivity
D. Non-infectivity

Explanation: The concept of "latency" in HIV is often a point of confusion. In HIV infection, **Clinical Latency** does not equate to **Biological Latency**. ### **Explanation of the Correct Answer** **A. Viral Replication:** During the clinical latent phase (also known as the asymptomatic or chronic phase), the virus is **not** dormant. There is persistent, vigorous viral replication occurring primarily within the lymphoid organs. Billions of new virions are produced and destroyed daily. The "latency" refers only to the absence of outward clinical symptoms, while a dynamic equilibrium exists between viral load and the host's immune response (the viral set point). ### **Analysis of Incorrect Options** * **B. Sequestration in lymphoid tissue:** While the virus is highly concentrated in the lymph nodes during this phase, it is not "sequestered" in a static sense. It is actively replicating and circulating in the plasma, albeit at lower levels than during acute infection or AIDS. * **C & D. Infectivity vs. Non-infectivity:** HIV remains **highly infectious** during the latent phase. Even if the patient is asymptomatic, the virus is present in blood, semen, and vaginal secretions. Therefore, "Non-infectivity" is factually incorrect. ### **NEET-PG High-Yield Pearls** * **The Viral Set Point:** The steady-state level of viremia achieved during the latent phase is a strong predictor of the rate of disease progression to AIDS. * **Cellular Reservoir:** True biological latency (where the virus is integrated but not replicating) occurs in **Memory CD4+ T cells**. These reservoirs are the primary obstacle to a functional cure. * **Duration:** Without ART, the clinical latent phase typically lasts 8–10 years. * **Lymph Node Architecture:** During this phase, there is follicular hyperplasia in the lymph nodes; exhaustion of this architecture leads to the terminal stage (AIDS).

Question 349: Which method is used for virus quantification?

A. Egg inoculation
B. Hemadsorption
C. Plaque assay (Correct Answer)
D. Electron microscopy

Explanation: **Explanation:** **Plaque Assay** is the gold standard method for **virus quantification** (specifically for determining viral infectivity). It is a biological assay where a confluent monolayer of host cells is infected with serial dilutions of a virus. Each infectious viral particle (virion) that infects a cell leads to a localized area of cell lysis or cytopathic effect (CPE) called a **"plaque."** By counting these plaques, the concentration of the virus is calculated as **Plaque Forming Units (PFU) per ml**. **Analysis of Incorrect Options:** * **Egg Inoculation:** This is a method for **virus isolation and cultivation** (e.g., Influenza virus), not for precise quantification. * **Hemadsorption:** This is a technique used to **detect** the presence of hemagglutinating viruses (like Orthomyxoviruses) on the surface of infected cells before CPE appears. It identifies the virus but does not quantify the viral load. * **Electron Microscopy:** While it can visualize and count total viral particles (including non-infectious ones), it is primarily used for **morphological identification** and is not the standard clinical method for quantification due to low sensitivity and high cost. **High-Yield NEET-PG Pearls:** * **Quantification vs. Detection:** Plaque assay measures **infectious** titers, whereas qPCR (Real-time PCR) measures **viral genome copies** (most common clinical method for HIV/HCV viral load). * **Pock Assay:** A variation of the plaque assay used for viruses like Poxvirus on the chorioallantoic membrane (CAM) of embryonated eggs. * **Cytopathic Effect (CPE):** Structural changes in host cells caused by viral invasion (e.g., syncytia formation in RSV/Measles, Negri bodies in Rabies).

Question 350: What is the commonest hepatotropic virus that causes an increased chronic carrier state?

A. Hepatitis E Virus (HEV)
B. Hepatitis A Virus (HAV)
C. Hepatitis B Virus (HBV)
D. Hepatitis C Virus (HCV) (Correct Answer)

Explanation: **Explanation:** The correct answer is **Hepatitis C Virus (HCV)**. The question asks for the virus with the highest propensity to progress to a **chronic carrier state** once an individual is infected. 1. **Why HCV is correct:** While Hepatitis B is more common globally in terms of total numbers, HCV is notorious for its high rate of chronicity. Approximately **75%–85%** of patients infected with HCV fail to clear the virus and develop chronic infection. This is primarily due to the high mutation rate of the virus (lack of proofreading by RNA polymerase), which allows it to evade the host immune response. 2. **Why the other options are incorrect:** * **Hepatitis A (HAV) and Hepatitis E (HEV):** These are transmitted via the feco-oral route and cause **acute hepatitis only**. They do not lead to chronic carrier states or chronic liver disease (Exception: HEV can cause chronicity in severely immunocompromised individuals, but this is rare and not the "commonest"). * **Hepatitis B (HBV):** While HBV causes chronic infection, the rate depends on the age of acquisition. In adults, only about **5%–10%** become chronic carriers. Although the absolute number of HBV carriers is high, the *likelihood* of an individual infection becoming chronic is significantly lower than with HCV. **High-Yield NEET-PG Pearls:** * **Highest Chronicity Rate:** HCV (80%) > HBV (5% in adults, 90% in neonates). * **HCV Screening:** Anti-HCV antibodies (ELISA); **Confirmatory/Gold Standard:** HCV RNA (PCR). * **HCV Genotypes:** Genotype 3 is the most common in India. * **Fulminant Hepatitis in Pregnancy:** Classically associated with **HEV**. * **DNA Virus:** HBV is the only DNA hepatitis virus; all others (A, C, D, E) are RNA viruses.

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Hepatitis Viruses

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Virology — MCQs

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