NEET-PG 2013 - Microbiology Practice Questions

Q: Macrophage tropic strains of HIV use which co-receptor?

CCR5. ***CCR5*** - **Macrophage-tropic** HIV strains, also known as **R5 strains**, primarily use the **CCR5 co-receptor** to enter target cells. - These strains are typically involved in the **initial infection** and transmission of HIV. - CCR5-tropic viruses are usually the **predominant strains transmitted** during sexual transmission. *CXCR4* - **T-cell-tropic** HIV strains, or **X4 strains**, preferentially utilize the **CXCR4 co-receptor** for cell entry. - These strains are associated with a **more rapid decline in CD4+ T-cell counts** during later stages of HIV infection. - Emergence of X4 strains is linked to **disease progression**. *CCR3* - While a chemokine receptor, **CCR3** is not a primary co-receptor used by common HIV strains for entry into macrophages or T cells. - CCR3 is primarily involved in **eosinophil chemotaxis** and allergic responses. *CCR2* - **CCR2** is another chemokine receptor but is **not a major co-receptor** for HIV entry. - While some laboratory-adapted strains may show minor usage, it is not clinically significant for macrophage-tropic HIV strains.

Q: How many segments of RNA does the Influenza virus have?

8 segments of single-stranded RNA. ***8 segments of single-stranded RNA*** - The **Influenza virus** is characterized by its segmented genome, which consists of **eight distinct negative-sense single-stranded RNA (ssRNA)** molecules. - This segmentation is crucial for its high mutation rate and ability to undergo **antigenic shift** and **antigenic drift**, leading to new strains. *5 segments of single-stranded RNA* - This option is incorrect because the Influenza virus specifically has **eight segments**, not five. - While it is a single-stranded RNA virus, the number of segments is a key characteristic. *8 segments of double-stranded DNA* - This option is incorrect as Influenza is an **RNA virus**, not a DNA virus, and its genetic material is single-stranded, not double-stranded. - No known influenza viruses have a **double-stranded DNA genome**. *8 segments of single-stranded DNA* - This option is incorrect because Influenza is an **RNA virus**, not a DNA virus. - Its genetic material is composed of **RNA**, specifically negative-sense single-stranded RNA.

Q: Prions are best killed by

Autoclaving at 134°C (for 18 minutes). ***Autoclaving at 134°C (for 18 minutes)*** - **Prions** are highly resistant to conventional sterilization methods, and **autoclaving at 134°C for at least 18 minutes** is the **most effective method for sterilizing reusable medical instruments** contaminated with prions. - This high temperature and pressure protocol (WHO/CDC recommended) helps to denature the misfolded protein structure of prions, reducing their infectivity to safe levels. - **In the context of sterilization and disinfection**, this is the best practical method for surgical instruments that cannot be discarded. *Incineration at high temperatures* - **Incineration at 800-1000°C** achieves complete combustion and is **highly effective** at destroying prions. - However, incineration is used only for **single-use disposable items** and prion-contaminated waste, not for reusable surgical instruments. - In the clinical context of sterilization (implied by this topic), autoclaving is the preferred answer as it applies to reusable equipment. *Sodium hypochlorite solution* - **Sodium hypochlorite** (bleach) at **high concentrations** (20,000 ppm or 2% available chlorine) for extended contact times (1+ hours) can inactivate prions. - However, it is **corrosive to instruments**, damages tissue samples, and requires precise concentration and exposure conditions, making it less practical than autoclaving. *5% formalin solution* - Formalin is **not effective at inactivating prions**; it can actually **preserve and stabilize** prion infectivity. - Formalin cross-links proteins and preserves tissue morphology but does not reliably break down the highly stable **beta-sheet structures** characteristic of prions.

Q: What are the changes in the variable region of immunoglobulins?

Idiotype. ***Idiotype*** - **Idiotype** refers to the unique set of antigenic determinants in the **variable region** of an antibody molecule, specifically within the **hypervariable regions (complementarity-determining regions, CDRs)**. - These unique determinants allow antibodies to recognize specific antigens and are generated by the specific **V(D)J gene rearrangements** in B cells. *Isotype* - **Isotype** refers to the constant region of an antibody, determining its class (e.g., **IgG, IgM, IgA, IgD, IgE**). - This region defines the antibody's effector functions and has nothing to do with the antigen-binding variability. *Allotype* - **Allotype** refers to minor genetic variations within the **constant region** of an antibody molecule within a species. - These variations are due to different alleles inherited from parents and are not associated with the variable region that binds to antigens. *Epitope* - An **epitope** is the specific part of an **antigen** that an antibody or T-cell receptor recognizes and binds to. - It is a feature of the antigen, not a change within the variable region of the immunoglobulin itself.

Q: Which immunoglobulin is known to be heat-labile?

IgM. ***IgM*** - **IgM** is known for its **heat lability** and is readily denatured at 56°C within a few minutes. - This characteristic is due to its **pentameric structure** held together by disulfide bonds and J chains, which are sensitive to thermal denaturation. - Heat lability of IgM is clinically important in complement fixation tests and other laboratory assays where heat inactivation is performed. - IgM is the first antibody produced in primary immune response and its heat sensitivity distinguishes it from other immunoglobulins. *IgA* - **IgA** exists in monomeric (serum) and dimeric (secretory) forms and shows moderate stability to heat. - Secretory IgA is relatively stable as it needs to function in harsh mucosal environments, though not as heat-resistant as IgG. - Does not exhibit the pronounced heat lability characteristic of IgM. *IgG* - **IgG** is the most stable immunoglobulin and is highly resistant to heat denaturation. - Can withstand temperatures up to 60-70°C without significant loss of activity. - Its monomeric structure with strong intramolecular bonds provides exceptional thermal stability. - Most abundant antibody in serum and has the longest half-life. *IgE* - **IgE** is actually quite stable to heat and can withstand 56°C for extended periods. - While it has a short half-life in serum (2-3 days), this is due to receptor binding rather than heat instability. - Important in type I hypersensitivity reactions and parasitic infections. - Does not show the characteristic heat lability that defines IgM.

Q: Which immunoglobulin is primarily secreted by the intestine?

IgA. **IgA** - **Secretory IgA** is the dominant immunoglobulin in mucosal secretions, including those of the intestine. - It plays a crucial role in providing **local immunity** by preventing microbial adherence and neutralizing toxins on mucosal surfaces. *IgG* - **IgG** is the most abundant immunoglobulin in serum and plays a major role in systemic immunity, including opsonization and complement activation. - While some IgG is found in secretions, it is not the primary immunoglobulin secreted by the intestine. *IgM* - **IgM** is a pentameric immunoglobulin, primarily found in blood and lymph, where it is very effective in activating the complement system and agglutinating antigens. - Although it can be found at mucosal surfaces in small amounts, it is not the principal secreted antibody in the intestine. *IgD* - **IgD** is primarily found on the surface of naive B lymphocytes, where it functions as a B cell receptor. - Its role in secreted form is minimal, and it is not significantly secreted into the intestine or other bodily fluids.

Q: Rosette formation with sheep RBCs (SRBCs) indicates functioning of -

T-cells. ***T-cells*** - **T-cells** possess specific receptors, like **CD2** on their surface, that can bind to ligands on sheep red blood cells (SRBCs). - This binding leads to the formation of characteristic **rosettes**, where SRBCs cluster around the T-lymphocytes, indicating functional T-cells. *B-cells* - **B-cells** primarily function in **humoral immunity** by producing antibodies and do not typically form rosettes with sheep RBCs. - While B-cells have surface receptors, they are not CD2 and thus do not facilitate this specific type of rosette formation. *Neutrophils* - **Neutrophils** are **phagocytic cells** involved in innate immunity, primarily combating bacterial and fungal infections. - They lack the specific surface receptors (like CD2) required to form rosettes with sheep RBCs. *Monocytes* - **Monocytes** are precursors to macrophages and dendritic cells, involved in phagocytosis and antigen presentation. - They do not possess the necessary surface markers to form rosettes with sheep RBCs.

Q: What is the typical bacterial count in the duodenum?

10^2 per gram. ***10^2 per gram*** - The duodenum has a **relatively low bacterial count** (typically 10^2-10^4 CFU/gram) due to the **acidic environment** from gastric acid and **rapid transit** of contents. - A count of **10^2 CFU/gram** represents the **lower end of the normal range** for the proximal duodenum, where gastric acid effects are strongest. - This sparse bacterial population contrasts sharply with the dense colonization seen in the distal gut. *10^1 per gram* - This represents an **extremely low count** more characteristic of the **stomach**, not the duodenum. - Such minimal bacterial presence is due to the **hostile acidic environment** (pH 1-3) in the stomach. - The duodenum, while having low counts, consistently has higher bacterial densities than this. *10^5 per gram* - This count is characteristic of the **distal small intestine (ileum)**, where bacterial concentrations progressively increase. - A bacterial count of **10^5 per gram in the duodenum** would be considered **abnormal** and suggest **small intestinal bacterial overgrowth (SIBO)**. - SIBO occurs when colonic-type bacteria colonize the small intestine inappropriately. *10^10 per gram* - This bacterial density is typical of the **colon** (which harbors 10^11-10^12 CFU/gram), the most densely colonized part of the human gut. - Such a high count in the duodenum would indicate **severe bacterial overgrowth** or gross contamination. - The colon's anaerobic environment supports this massive bacterial population.

Q: Which gene of Hepatitis B virus (HBV) is most commonly associated with mutations causing antiviral drug resistance?

P gene. ***P gene*** - The **P gene** (polymerase gene) of HBV encodes the viral reverse transcriptase which is essential for viral replication. - Mutations in the P gene can lead to **antiviral drug resistance**, particularly to nucleos(t)ide analogues. *X gene* - The **X gene** encodes the X protein (HBx), a **transcriptional transactivator** involved in viral replication and pathogenesis. - While important for viral function, it is not the primary target for antiviral therapy, and mutations are less frequently associated with drug resistance. *S gene* - The **S gene** encodes the **surface antigens (HBsAg)**, which are crucial for viral entry and immune evasion. - Mutations in the S gene can lead to **vaccine escape mutants** or alter HBsAg detection, but not directly responsible for antiviral resistance. *C gene* - The **C gene** encodes the **core protein (HBcAg)** and the precore protein (HBeAg). - These proteins are involved in **viral particle assembly** and immune modulation, but mutations in this gene are not typically associated with resistance to antiviral drugs.

Q: Which of the following statements about malaria transmission is correct?

Individuals harboring gametocytes can transmit malaria.. ***Individuals harboring gametocytes can transmit malaria.*** - **Gametocytes** are the sexual stage of the malaria parasite that circulate in the human bloodstream and are infectious to mosquitos. - When an *Anopheles* mosquito feeds on an infected human, it ingests these gametocytes, allowing the parasite's life cycle to continue in the mosquito vector, leading to transmission. *P. vivax always completely fills the infected RBC with schizonts.* - While *P. vivax* does infect **reticulocytes** (young RBCs) and can enlarge them, the **schizonts** typically occupy a significant portion but not always completely fill the host cell. - The infected RBCs are often enlarged to about 1.5 to 2 times their normal size and contain numerous **Schüffner's dots**. *Malaria can only be transmitted through blood transfusions.* - The primary mode of malaria transmission is through the bite of an **infected female *Anopheles* mosquito**. - While **blood transfusions** can transmit malaria, it is a less common and secondary route compared to vector-borne transmission. *All stages of P. falciparum are commonly seen in peripheral blood smears.* - In *P. falciparum* infections, only the **ring forms** and **gametocytes** are commonly observed in the peripheral blood smear. - The more mature asexual stages (trophozoites and schizonts) typically sequester in the capillaries of internal organs, where they are not readily visible in peripheral circulation.

Question 1

Macrophage tropic strains of HIV use which co-receptor?

Accepted Answer

CCR5

Answer

CXCR4

Answer

CCR3

Answer

CCR2

Question 2

How many segments of RNA does the Influenza virus have?

Accepted Answer

8 segments of single-stranded RNA

Answer

5 segments of single-stranded RNA

Answer

8 segments of double-stranded DNA

Answer

8 segments of single-stranded DNA

Question 3

Prions are best killed by

Accepted Answer

Autoclaving at 134°C (for 18 minutes)

Answer

Incineration at high temperatures

Answer

5% formalin solution

Answer

Sodium hypochlorite solution

Question 4

What are the changes in the variable region of immunoglobulins?

Accepted Answer

Idiotype

Answer

Isotype

Answer

Epitope

Answer

Allotype

Question 5

Which immunoglobulin is known to be heat-labile?

Accepted Answer

IgM

Answer

IgA

Answer

IgG

Answer

IgE

Question 6

Which immunoglobulin is primarily secreted by the intestine?

Accepted Answer

IgA

Answer

IgG

Answer

IgM

Answer

IgD

Question 7

Rosette formation with sheep RBCs (SRBCs) indicates functioning of -

Accepted Answer

T-cells

Answer

B-cells

Answer

Neutrophils

Answer

Monocytes

Question 8

What is the typical bacterial count in the duodenum?

Accepted Answer

10^2 per gram

Answer

10^1 per gram

Answer

10^10 per gram

Answer

10^5 per gram

Question 9

Which gene of Hepatitis B virus (HBV) is most commonly associated with mutations causing antiviral drug resistance?

Accepted Answer

P gene

Answer

X gene

Answer

S gene

Answer

C gene

Question 10

Which of the following statements about malaria transmission is correct?

Accepted Answer

Individuals harboring gametocytes can transmit malaria.

Answer

P. vivax always completely fills the infected RBC with schizonts.

Answer

Malaria can only be transmitted through blood transfusions.

Answer

All stages of P. falciparum are commonly seen in peripheral blood smears.

NEET-PG 2013 — Microbiology