NEET-PG 2013 - Microbiology Practice Questions

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: Cytolytic activity of membrane attack complex is modulated by ?

Factor S (vitronectin). ***Correct Option: Factor S (vitronectin)*** - Vitronectin (S-protein) is a **plasma protein** that directly modulates the **cytolytic activity of the membrane attack complex (MAC)**. - It binds to the **C5b-7 complex** in the fluid phase, preventing its insertion into target cell membranes and thereby blocking the formation of the complete, functional MAC. - By inhibiting membrane insertion of C5b-7, vitronectin prevents the subsequent binding of **C8 and C9**, which are essential for the cytolytic pore formation. - This is a **direct modulation** of MAC's cytolytic activity at the MAC assembly stage. *Incorrect Option: Factor H* - Factor H is a regulatory protein that controls the **alternative pathway** of complement activation by promoting degradation of **C3b**. - By degrading C3b, Factor H prevents formation of **C5 convertase**, thereby reducing downstream MAC formation. - However, Factor H acts **early in the complement cascade** and does not directly modulate the cytolytic activity of already-formed MAC components. - Its effect is on **preventing MAC formation**, not on modulating MAC's cytolytic function itself. *Incorrect Option: Factor I* - Factor I is a **serine protease** that cleaves and inactivates C3b and C4b, requiring cofactors like Factor H or C4bp. - Like Factor H, it regulates complement activation **upstream** of MAC formation. - It does not directly interact with or modulate the cytolytic activity of the MAC. *Incorrect Option: Factor B* - Factor B is a component of the **alternative pathway C3 convertase** (C3bBb). - It **promotes complement activation** rather than modulating MAC's cytolytic activity. - Factor B functions early in the cascade and has no direct role in regulating MAC function.

Q: Which of the following statements about interleukin-1 is false?

IL-1 inhibits IL-2 production by T-cells.. ***IL-1 inhibits IL-2 production by T-cells*** - This statement is false because **IL-1** actually **enhances the production of IL-2** by T-cells, which is crucial for T-cell proliferation and immune response. - **IL-1 acts synergistically with IL-6 and TNF-α** to promote inflammation and immune cell activation, where IL-2 plays a key role. *The primary source of IL-1 is the monocyte-macrophage system* - This statement is true; **monocytes and macrophages** are the main producers of **IL-1α and IL-1β** upon activation by various stimuli. - Other cells, such as neutrophils, dendritic cells, and endothelial cells, can also produce IL-1, but monocytes and macrophages are the predominant source. *IL-1 is an endogenous pyrogen* - This statement is true; **IL-1** is a potent **endogenous pyrogen** that acts on the hypothalamus to induce fever, a hallmark of the acute phase response. - It triggers prostaglandin synthesis in the hypothalamus, leading to an elevation in the body's thermoregulatory set point. *IL-1 promotes acute phase protein synthesis in the liver* - This statement is true; **IL-1** is a key mediator that stimulates **hepatocytes** to produce **acute phase proteins**, such as C-reactive protein and serum amyloid A. - This hepatic response is part of the innate immune system's effort to control infection and inflammation.

Q: Which of the following is a superantigen ?

TSST. ***TSST*** - **Toxic Shock Syndrome Toxin-1 (TSST-1)** is a classic example of a superantigen produced by *Staphylococcus aureus*. - Superantigens **bind directly to MHC class II molecules and T-cell receptors (TCRs)** outside of the antigen-binding groove, leading to non-specific activation of a large percentage of T cells and a massive release of cytokines. *Cholera toxin* - **Cholera toxin** is an exotoxin produced by *Vibrio cholerae* that causes massive fluid secretion in the intestine by **activating adenylate cyclase** in enterocytes. - It functions by **ADP-ribosylating the Gs alpha subunit**, leading to constitutive activation of cyclic AMP production, but it is not a superantigen. *Diphtheria toxin* - **Diphtheria toxin**, produced by *Corynebacterium diphtheriae*, inhibits protein synthesis in eukaryotic cells by **ADP-ribosylating elongation factor-2 (EF-2)**. - This action leads to cell death and the characteristic pseudomembrane formation in diphtheria, but it does not act as a superantigen. *Vero-cytoxin* - **Vero-cytoxin** (also known as Shiga toxin or Shiga-like toxin) is produced by *E. coli* O157:H7 and other Shiga toxin-producing *E. coli* (STEC). - It inhibits protein synthesis by **cleaving ribosomal RNA**, primarily causing damage to intestinal cells and renal endothelial cells, but it is not a superantigen.

Q: What is the primary use of the Hybridoma technique?

Monoclonal antibodies. ***Monoclonal antibodies*** - The **hybridoma technique** is primarily used to produce **monoclonal antibodies (MAbs)**, which are highly specific antibodies derived from a single B-cell clone. - These antibodies recognize a **single epitope** on an antigen, providing exceptional specificity and uniformity. - The technique involves **fusing a B-lymphocyte** (antibody-producing cell) with a **myeloma cell** (immortal cancer cell) to create a hybridoma that continuously produces identical antibodies. - This is the **gold standard** for producing large quantities of identical, highly specific antibodies for diagnostic and therapeutic use. *Specific antibodies* - While monoclonal antibodies are indeed specific, this term is **too vague** and could refer to any antibody with specificity, including polyclonal antibodies. - **Polyclonal antibodies** are also specific but are produced through conventional immunization, not the hybridoma technique. - The defining characteristic of the hybridoma technique is that it produces **monoclonal** (single clone) antibodies, not just "specific" ones. *Antigen* - An **antigen** is a molecule that elicits an immune response and is used to immunize animals during antibody production. - However, antigens are the **input** for antibody production, not the **product** of the hybridoma technique. *Cytokines* - **Cytokines** are signaling molecules involved in immune cell communication and regulation. - They are not produced by the hybridoma technique, which is specifically designed for **antibody production**.

Q: An adolescent male developed vomiting and diarrhea 1 hour after having food from a restaurant. The most likely pathogen is?

Staphylococcus aureus. ***Staphylococcus aureus*** - The rapid onset of symptoms (within 1 hour) strongly suggests **pre-formed toxin ingestion**, which is characteristic of *Staphylococcus aureus* food poisoning. - While the typical incubation period is **1-6 hours** (average 2-4 hours), onset within 1 hour can occur with **high toxin loads** in contaminated food. - **Vomiting** is often the predominant symptom, occurring shortly after consuming contaminated food, which distinguishes it from other bacterial causes. *Clostridium perfringens* - Onset of symptoms caused by *Clostridium perfringens* is typically **8-16 hours** after ingestion, which is much longer than observed here. - It primarily causes **diarrhea and abdominal cramps** due to toxin production in the intestine, with minimal vomiting. *Vibrio parahaemolyticus* - Symptoms usually appear **4-96 hours** (average 12-24 hours) after consuming contaminated seafood, which is a longer incubation period than described. - It typically causes **watery diarrhea**, abdominal cramps, nausea, and occasional vomiting, but not within 1 hour. *Salmonella* - The incubation period for *Salmonella* infection is typically **6-72 hours** (average 12-36 hours), making it highly unlikely for symptoms to appear within 1 hour. - **Diarrhea, fever, and abdominal cramps** are common with *Salmonella*, but rapid-onset vomiting from pre-formed toxin is not its mechanism.

Q: The outer covering of diatoms is made of?

Silica. ***Correct: Silica*** - The cell walls of diatoms are primarily composed of **hydrated amorphous silica (SiO2·nH2O)**. - This rigid, intricate outer covering is known as a **frustule**, which provides structural support and protection. - Diatoms are uniquely characterized by their intricate silica cell walls, making them easily identifiable under microscopy. *Incorrect: Magnesium* - **Magnesium (Mg)** is an important metal and a component of chlorophyll, essential for photosynthesis. - While diatoms do contain magnesium for metabolic processes, it is not the primary structural component of their outer covering. *Incorrect: Hydrocarbons* - **Hydrocarbons** are organic compounds consisting entirely of hydrogen and carbon, commonly found in fossil fuels. - Diatom cell walls are inorganic (mineral-based), not organic hydrocarbon structures. *Incorrect: None of the options* - This option is incorrect because **silica** is listed among the options and is the correct answer. - Diatom frustules are definitively composed of silica.

Q: Which of the following bacteria is known to exhibit antigenic variation?

Borrelia. ***Borrelia*** - *Borrelia* species, particularly *Borrelia burgdorferi* (causing **Lyme disease**), are known for extensive **antigenic variation** of their outer surface proteins (Osps), especially OspC. - This variation helps the bacteria evade the host's immune response, contributing to persistent infection. *Yersinia* - While *Yersinia* species produce various virulence factors, including proteins that interfere with immune cell function, they are not primarily known for the type of rapid and extensive **antigenic variation**seen in *Borrelia*. - Their immune evasion strategies often involve modifying host cell signaling pathways and resisting phagocytosis. *Bordetella* - *Bordetella pertussis*, causative agent of **whooping cough**, varies its expression of adhesins and toxins through **phase variation**, which is a form of phenotypic switching. - However, this is distinct from the frequent and sequential changes in surface antigens (antigenic variation) observed in *Borrelia*. *Brucella* - *Brucella* species are **intracellular pathogens** that primarily evade the immune system by surviving and replicating within host cells. - They do not typically engage in significant **antigenic variation** of their surface components as a primary immune evasion mechanism.

Q: Most common organism causing ventilator associated pneumonia -

Pseudomonas. ***Pseudomonas*** - **Pseudomonas aeruginosa** is one of the most common causes of **ventilator-associated pneumonia (VAP)**, particularly in **late-onset VAP** (≥5 days) and in patients with prolonged mechanical ventilation, prior antibiotic exposure, or underlying lung disease. - Its ability to form **biofilms** and its intrinsic antibiotic resistance contribute to its prevalence in hospital-acquired infections. - Along with **Staphylococcus aureus** (especially MRSA), Pseudomonas is consistently among the leading causes of VAP in ICU settings. *Legionella* - **Legionella** is a less common cause of VAP and is typically associated with contaminated water sources, manifesting as **Legionnaires' disease**. - It usually causes severe, rapidly progressive pneumonia and is often harder to culture than other bacteria. *Pneumococcus* - **Streptococcus pneumoniae (Pneumococcus)** is the most common cause of **community-acquired pneumonia (CAP)**, but it is less frequently implicated in VAP. - While it can cause severe pneumonia and may be seen in **early-onset VAP**, its incidence in late-onset VAP is lower compared to Gram-negative rods like Pseudomonas. *Coagulase negative staphylococcus* - **Coagulase-negative Staphylococci** (e.g., *Staphylococcus epidermidis*) are common **contaminants** in cultures and primarily cause device-related infections, such as those associated with central venous catheters. - They are rarely a primary cause of VAP, as they typically have low virulence in the respiratory tract.

Q: Which fungus is commonly known as golden yellow jelly fungus?

Tremella mesenterica. ***Tremella mesenterica*** - This fungus is commonly referred to as **golden yellow jelly fungus** or **witch's butter** due to its distinctive golden-yellow, gelatinous, and brain-like appearance. - It is a **jelly fungus** that typically grows on dead hardwood branches, especially after rain, and is known for its pliable, quivering texture. *T. tonsurans* - This refers to **Trichophyton tonsurans**, a dermatophytic fungus primarily known for causing **tinea capitis** (ringworm of the scalp). - Its common name relates to its effect on hair, causing breakage and a "black dot" appearance, rather than a golden yellow, jelly-like form. *Epidermophyton floccosum* - This is a dermatophytic fungus that specifically causes infections of the **skin and nails**, particularly **tinea pedis** (athlete's foot) and **tinea cruris** (jock itch). - It does not produce a fruiting body and is not described as a jelly-like fungus. *T. mentagrophytes* - This refers to **Trichophyton mentagrophytes**, another common dermatophyte responsible for various superficial fungal infections, including **tinea pedis**, **tinea corporis**, and **tinea unguium**. - Its clinical presentation is not that of a golden yellow jelly fungus.

Question 1

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

Accepted Answer

T-cells

Answer

B-cells

Answer

Neutrophils

Answer

Monocytes

Question 2

Cytolytic activity of membrane attack complex is modulated by ?

Accepted Answer

Factor S (vitronectin)

Answer

Factor I

Answer

Factor B

Answer

Factor H

Question 3

Which of the following statements about interleukin-1 is false?

Accepted Answer

IL-1 inhibits IL-2 production by T-cells.

Answer

IL-1 is an endogenous pyrogen.

Answer

The primary source of IL-1 is the monocyte-macrophage system.

Answer

IL-1 promotes acute phase protein synthesis in the liver.

Question 4

Which of the following is a superantigen ?

Accepted Answer

TSST

Answer

Cholera toxin

Answer

Diphtheria toxin

Answer

Vero-cytoxin

Question 5

What is the primary use of the Hybridoma technique?

Accepted Answer

Monoclonal antibodies

Answer

Antigen

Answer

Specific antibodies

Answer

Cytokines

Question 6

An adolescent male developed vomiting and diarrhea 1 hour after having food from a restaurant. The most likely pathogen is?

Accepted Answer

Staphylococcus aureus

Answer

Clostridium perfringens

Answer

Vibrio parahaemolyticus

Answer

Salmonella

Question 7

The outer covering of diatoms is made of?

Accepted Answer

Silica

Answer

Magnesium

Answer

Hydrocarbons

Answer

None of the options

Question 8

Which of the following bacteria is known to exhibit antigenic variation?

Accepted Answer

Borrelia

Answer

Yersinia

Answer

Bordetella

Answer

Brucella

Question 9

Most common organism causing ventilator associated pneumonia -

Accepted Answer

Pseudomonas

Answer

Legionella

Answer

Pneumococcus

Answer

Coagulase negative staphylococcus

Question 10

Which fungus is commonly known as golden yellow jelly fungus?

Accepted Answer

Tremella mesenterica

Answer

T. tonsurans

Answer

Epidermophyton floccosum

Answer

T. mentagrophytes

NEET-PG 2013 — Microbiology