NEET-PG 2012 — Microbiology

Q: Which of the following statements about Streptococcus is false?

Group A Streptococcus is catalase positive. ***Group A Streptococcus is catalase positive*** - This statement is false because all species of **Streptococcus**, including Group A Streptococcus (GAS), are **catalase-negative**. - The absence of the **catalase enzyme** is a key differentiating feature between streptococci and staphylococci, which are catalase-positive. *Group B causes neonatal meningitis* - **Group B Streptococcus (GBS)**, or *Streptococcus agalactiae*, is a well-known cause of **neonatal meningitis** and sepsis. - Infants can acquire GBS during passage through the birth canal from a colonized mother, leading to severe infections. *Group B lives in female genital tract* - GBS is a common commensal organism found in the **lower gastrointestinal** and **female genital tracts** of up to 30% of healthy women. - Its presence in the female genital tract poses a risk for transmission to neonates during childbirth. *Classification by Lancefield based on carbohydrate antigen* - The **Lancefield grouping system** classifies streptococci based on the presence of specific **carbohydrate antigens** in their cell walls. - This serological classification, developed by Rebecca Lancefield, divides streptococci into groups A, B, C, D, F, and G, which helps in identifying different species and their associated pathologies. [Practice more Microbiology PYQs on OnCourse]

Q: Which of the following statements is true regarding Wuchereria bancrofti?

Tail tip free from nuclei. ***Tail tip free from nuclei*** - The **microfilariae** of *Wuchereria bancrofti* are characterized by a **clean tail tip**, meaning there are no nuclei extending into the very end of the tail. - This feature is crucial for differentiating it from other microfilariae like *Brugia malayi*, which has two distinct nuclei in its tail tip. *Unsheathed* - *Wuchereria bancrofti* microfilariae are **sheathed**, meaning they retain an egg envelope as a loose covering. - An unsheathed microfilaria, like that of *Onchocerca volvulus*, lacks this outer covering. *Non-periodic* - *Wuchereria bancrofti* exhibits **nocturnal periodicity**, meaning its microfilariae are most abundant in the peripheral blood during the night. - Non-periodic microfilariae are found in the blood at any time of day, as seen with *Loa loa*. *None of the options* - This statement is incorrect because "Tail tip free from nuclei" is a true characteristic of *Wuchereria bancrofti* microfilariae. [Practice more Microbiology PYQs on OnCourse]

Q: From which strain of virus is the rabies vaccine prepared?

Fixed Virus. ***Fixed Virus*** - The rabies vaccine is prepared from **fixed virus strains**, which are **attenuated** forms of the rabies virus that have lost their pathogenicity for humans through serial passages in animals or cell cultures. - This attenuation allows the virus to induce an immune response without causing disease, making it safe and effective for vaccination. - Examples include **Pitman-Moore strain**, **Flury strain (HEP/LEP)**, and **SAD strain**. *Street virus* - The **street virus** is the term used for the **wild-type rabies virus** as it occurs naturally in infected animals and causes clinical rabies disease. - Due to its high pathogenicity and ability to cause lethal infection, the street virus is **not used** for vaccine production. *Wild virus* - **Wild virus** is another term referring to the naturally occurring, **virulent rabies virus** found in infected animals. - Like the street virus, it is too pathogenic to be used directly in vaccine preparation. *Pasteur virus* - While **Louis Pasteur** pioneered rabies vaccination, "Pasteur virus" is not a standard terminology for vaccine strains. - The term **"fixed virus"** specifically denotes laboratory-adapted, attenuated strains regardless of their origin. [Practice more Microbiology PYQs on OnCourse]

Q: What is the primary virulence factor of Neisseria gonorrhoeae?

Pili (fimbriae). ***Pili (fimbriae)*** - **Pili are the PRIMARY virulence factor** of *Neisseria gonorrhoeae*, essential for **initial attachment and colonization** of urogenital mucosa - Enable bacteria to **adhere to non-ciliated epithelial cells**, resisting mechanical clearance by urination and secretions - Undergo **antigenic variation** to evade host immune responses - Without pili, *N. gonorrhoeae* cannot establish infection *Endotoxin (lipooligosaccharide)* - *N. gonorrhoeae* possesses **LOS (lipooligosaccharide)** which causes inflammation and tissue damage - While important for pathogenesis, it is a **secondary virulence factor** - LOS contributes to symptoms but cannot cause infection without prior colonization via pili *Exotoxin* - *N. gonorrhoeae* does **not produce significant exotoxins** - Pathogenicity is mediated through **adherence factors (pili), LOS, and surface proteins** rather than secreted protein toxins - This is not a mechanism of gonococcal virulence *All of the above are incorrect* - This statement is false as **pili (fimbriae)** are definitively the primary virulence factor for *N. gonorrhoeae* [Practice more Microbiology PYQs on OnCourse]

Q: What is the most common bacterial cause of multiple sinus tracts resulting from an infection of the great toe?

Actinomyces. ***Actinomyces*** - **Actinomycosis** is known for causing **chronic suppurative infections** with **multiple draining sinus tracts** and characteristic **sulfur granules**. - The causative organism is **Actinomyces israelii**, an anaerobic, gram-positive, filamentous bacterium. - While rare, it can affect bones, leading to **osteomyelitis**, especially in the feet, presenting with these multifocal sinus tracts. *Tuberculosis* - **Tuberculosis of bone** is typically a **monoarticular affection**, often affecting larger joints, and does not commonly present with multiple draining sinus tracts like actinomycosis. - It is more common in the spine (**Pott's disease**) and large weight-bearing joints, rather than solely the great toe with multiple sinuses. *Staphylococcus aureus* - **Staphylococcus aureus** is the most common cause of **acute osteomyelitis** and can lead to draining sinuses. - However, it typically causes **single or localized sinus tracts** rather than the extensive, multiple sinus formation associated with actinomycosis. *Pseudomonas aeruginosa* - **Pseudomonas aeruginosa** is often associated with **osteomyelitis following puncture wounds** through footwear. - While it can cause chronic infections, it is not primarily known for forming the characteristic **multiple, long-standing sinus tracts** seen with Actinomyces. [Practice more Microbiology PYQs on OnCourse]

72 Previous Year Questions with Answers & Explanations

Questions

Which of the following statements about Streptococcus is false?

Which of the following statements is true regarding Wuchereria bancrofti?

From which strain of virus is the rabies vaccine prepared?

What is the primary virulence factor of Neisseria gonorrhoeae?

What is the most common bacterial cause of multiple sinus tracts resulting from an infection of the great toe?

Which of the following is the most potent stimulator of Naive T-cells?

Cercariae are the infective form of which of the following parasites?

Which of the following statements is true regarding Diphyllobothrium?

Which of the following is a primary cell line?

Q10

Flame cells are primarily associated with which of the following organisms?

NEET-PG 2012 - Microbiology NEET-PG Practice Questions and MCQs

Question 1: Which of the following statements about Streptococcus is false?

A. Group A Streptococcus is catalase positive (Correct Answer)
B. Group B causes neonatal meningitis
C. Group B lives in female genital tract
D. Classification by Lancefield based on carbohydrate antigen

Explanation: ***Group A Streptococcus is catalase positive*** - This statement is false because all species of **Streptococcus**, including Group A Streptococcus (GAS), are **catalase-negative**. - The absence of the **catalase enzyme** is a key differentiating feature between streptococci and staphylococci, which are catalase-positive. *Group B causes neonatal meningitis* - **Group B Streptococcus (GBS)**, or *Streptococcus agalactiae*, is a well-known cause of **neonatal meningitis** and sepsis. - Infants can acquire GBS during passage through the birth canal from a colonized mother, leading to severe infections. *Group B lives in female genital tract* - GBS is a common commensal organism found in the **lower gastrointestinal** and **female genital tracts** of up to 30% of healthy women. - Its presence in the female genital tract poses a risk for transmission to neonates during childbirth. *Classification by Lancefield based on carbohydrate antigen* - The **Lancefield grouping system** classifies streptococci based on the presence of specific **carbohydrate antigens** in their cell walls. - This serological classification, developed by Rebecca Lancefield, divides streptococci into groups A, B, C, D, F, and G, which helps in identifying different species and their associated pathologies.

Question 2: Which of the following statements is true regarding Wuchereria bancrofti?

A. Unsheathed
B. Tail tip free from nuclei (Correct Answer)
C. Non-periodic
D. None of the options

Explanation: ***Tail tip free from nuclei*** - The **microfilariae** of *Wuchereria bancrofti* are characterized by a **clean tail tip**, meaning there are no nuclei extending into the very end of the tail. - This feature is crucial for differentiating it from other microfilariae like *Brugia malayi*, which has two distinct nuclei in its tail tip. *Unsheathed* - *Wuchereria bancrofti* microfilariae are **sheathed**, meaning they retain an egg envelope as a loose covering. - An unsheathed microfilaria, like that of *Onchocerca volvulus*, lacks this outer covering. *Non-periodic* - *Wuchereria bancrofti* exhibits **nocturnal periodicity**, meaning its microfilariae are most abundant in the peripheral blood during the night. - Non-periodic microfilariae are found in the blood at any time of day, as seen with *Loa loa*. *None of the options* - This statement is incorrect because "Tail tip free from nuclei" is a true characteristic of *Wuchereria bancrofti* microfilariae.

Question 3: From which strain of virus is the rabies vaccine prepared?

A. Street virus
B. Fixed Virus (Correct Answer)
C. Wild virus
D. Pasteur virus

Explanation: ***Fixed Virus*** - The rabies vaccine is prepared from **fixed virus strains**, which are **attenuated** forms of the rabies virus that have lost their pathogenicity for humans through serial passages in animals or cell cultures. - This attenuation allows the virus to induce an immune response without causing disease, making it safe and effective for vaccination. - Examples include **Pitman-Moore strain**, **Flury strain (HEP/LEP)**, and **SAD strain**. *Street virus* - The **street virus** is the term used for the **wild-type rabies virus** as it occurs naturally in infected animals and causes clinical rabies disease. - Due to its high pathogenicity and ability to cause lethal infection, the street virus is **not used** for vaccine production. *Wild virus* - **Wild virus** is another term referring to the naturally occurring, **virulent rabies virus** found in infected animals. - Like the street virus, it is too pathogenic to be used directly in vaccine preparation. *Pasteur virus* - While **Louis Pasteur** pioneered rabies vaccination, "Pasteur virus" is not a standard terminology for vaccine strains. - The term **"fixed virus"** specifically denotes laboratory-adapted, attenuated strains regardless of their origin.

Question 4: What is the primary virulence factor of Neisseria gonorrhoeae?

A. All of the above are incorrect
B. Pili (fimbriae) (Correct Answer)
C. Endotoxin (lipooligosaccharide)
D. Exotoxin

Explanation: ***Pili (fimbriae)*** - **Pili are the PRIMARY virulence factor** of *Neisseria gonorrhoeae*, essential for **initial attachment and colonization** of urogenital mucosa - Enable bacteria to **adhere to non-ciliated epithelial cells**, resisting mechanical clearance by urination and secretions - Undergo **antigenic variation** to evade host immune responses - Without pili, *N. gonorrhoeae* cannot establish infection *Endotoxin (lipooligosaccharide)* - *N. gonorrhoeae* possesses **LOS (lipooligosaccharide)** which causes inflammation and tissue damage - While important for pathogenesis, it is a **secondary virulence factor** - LOS contributes to symptoms but cannot cause infection without prior colonization via pili *Exotoxin* - *N. gonorrhoeae* does **not produce significant exotoxins** - Pathogenicity is mediated through **adherence factors (pili), LOS, and surface proteins** rather than secreted protein toxins - This is not a mechanism of gonococcal virulence *All of the above are incorrect* - This statement is false as **pili (fimbriae)** are definitively the primary virulence factor for *N. gonorrhoeae*

Question 5: What is the most common bacterial cause of multiple sinus tracts resulting from an infection of the great toe?

A. Tuberculosis
B. Pseudomonas aeruginosa
C. Staphylococcus aureus
D. Actinomyces (Correct Answer)

Explanation: ***Actinomyces*** - **Actinomycosis** is known for causing **chronic suppurative infections** with **multiple draining sinus tracts** and characteristic **sulfur granules**. - The causative organism is **Actinomyces israelii**, an anaerobic, gram-positive, filamentous bacterium. - While rare, it can affect bones, leading to **osteomyelitis**, especially in the feet, presenting with these multifocal sinus tracts. *Tuberculosis* - **Tuberculosis of bone** is typically a **monoarticular affection**, often affecting larger joints, and does not commonly present with multiple draining sinus tracts like actinomycosis. - It is more common in the spine (**Pott's disease**) and large weight-bearing joints, rather than solely the great toe with multiple sinuses. *Staphylococcus aureus* - **Staphylococcus aureus** is the most common cause of **acute osteomyelitis** and can lead to draining sinuses. - However, it typically causes **single or localized sinus tracts** rather than the extensive, multiple sinus formation associated with actinomycosis. *Pseudomonas aeruginosa* - **Pseudomonas aeruginosa** is often associated with **osteomyelitis following puncture wounds** through footwear. - While it can cause chronic infections, it is not primarily known for forming the characteristic **multiple, long-standing sinus tracts** seen with Actinomyces.

Question 6: Which of the following is the most potent stimulator of Naive T-cells?

A. Macrophages
B. B-cell
C. Mature dendritic cells (Correct Answer)
D. Follicular dendritic cells

Explanation: ***Mature dendritic cells*** - **Mature dendritic cells** are the most potent professional antigen-presenting cells (APCs) for activating **naive T cells** due to their efficient antigen processing, presentation abilities, and high expression of costimulatory molecules (e.g., CD80, CD86) and MHC-peptide complexes. - Activated by pathogens or inflammatory signals, they migrate to secondary lymphoid organs where they initiate primary immune responses by presenting antigens to and activating naive T cells. *Follicular dendritic cells* - **Follicular dendritic cells** primarily present intact antigens to **B cells** in germinal centers of secondary lymphoid organs, playing a crucial role in B cell maturation, selection, and antibody production. - They lack MHC class II molecules and thus cannot directly present antigens to naive T cells. *Macrophages* - While **macrophages** are professional APCs, they are generally less efficient than mature dendritic cells at activating **naive T cells**, especially in the initiation of primary immune responses. - They are more involved in presenting antigens to already activated T cells and clearing pathogens, often acting as secondary APCs. *B-cell* - **B cells** can act as APCs, but they are generally less efficient than **dendritic cells** in activating **naive T cells**, especially for the primary immune response. - Their primary role in antigen presentation is to present processed antigens to **helper T cells** to receive costimulation for their own activation and differentiation into plasma cells, often after being activated themselves.

Question 7: Cercariae are the infective form of which of the following parasites?

A. Schistosoma hematobium (Correct Answer)
B. Paragonimus westermani
C. Fasciola hepatica
D. Taenia solium

Explanation: ***Schistosoma hematobium*** - **Cercariae** are the motile, free-swimming larval stage of **Schistosoma** species, including *S. hematobium* - They **actively penetrate human skin** to initiate infection, making cercariae the infective form for humans *Paragonimus westermani* - The infective form for humans is **metacercariae**, which are ingested by consuming insufficiently cooked **crabs or crayfish** - Cercariae develop into metacercariae in secondary intermediate hosts (crustaceans) *Fasciola hepatica* - Humans are infected by ingesting **metacercariae** present on aquatic vegetation, such as **watercress** - Cercariae encyst on plants to form metacercariae, not directly infective *Taenia solium* - The infective forms are **cysticerci** in undercooked pork (for adult tapeworm) or **embryonated eggs** (causing cysticercosis) - This cestode has a different life cycle and does **not** involve cercariae

Question 8: Which of the following statements is true regarding Diphyllobothrium?

A. Humans are the only definitive host
B. Vitamin B12 deficiency always occurs in infection
C. Fish are definitive hosts
D. The operculated egg is a diagnostic feature (Correct Answer)

Explanation: ***The operculated egg is a diagnostic feature*** - *Diphyllobothrium latum*, also known as the **fish tapeworm**, produces characteristic **operculated eggs** that are oval-shaped with an operculum (cap) at one end and a small knob at the other. - The presence of these **unembryonated eggs** in stool samples is the primary diagnostic method for diphyllobothriasis. *Humans are the only definitive host* - While humans are common definitive hosts, other **fish-eating mammals** such as bears, dogs, and cats can also serve as definitive hosts for *Diphyllobothrium latum*. - The definitive host is where the **adult worm resides** and reproduces sexually. *Vitamin B12 deficiency always occurs in infection* - **Vitamin B12 deficiency (megaloblastic anemia)** is a known complication of *Diphyllobothrium latum* infection, as the worm competes for B12 in the host's intestine. - However, it does **not occur in all infected individuals**; it is estimated to affect a significant minority, typically those with heavy worm burdens or prolonged infection, and can be influenced by dietary intake. *Fish are definitive hosts* - Fish (specifically freshwater fish like pike, perch, and salmon) act as **second intermediate hosts** for *Diphyllobothrium latum*, carrying the **plerocercoid larvae**. - **Humans and other fish-eating mammals** are the definitive hosts, where the plerocercoid larvae mature into adult tapeworms in the small intestine.

Question 9: Which of the following is a primary cell line?

A. Chick embryo fibroblasts (Correct Answer)
B. Hela cells
C. Vero cells
D. WI-38

Explanation: ***Chick embryo fibroblasts*** - Primary cell lines are directly derived from **tissues** and have a limited lifespan in culture before undergoing senescence. - **Chick embryo fibroblasts** are isolated directly from chick embryos and propagated for a limited number of passages, making them a true primary cell culture. *Hela cells* - HeLa cells are a well-known example of a **continuous cell line**, meaning they can be cultured indefinitely. - They were originally derived from a cervical cancer patient and are considered **immortalized**. *Vero cells* - Vero cells are an **immortalized cell line** derived from the kidney of an African green monkey. - They are used extensively in virology and vaccine production due to their ability to be propagated for many passages. *WI-38* - WI-38 is a **diploid human cell strain** derived from lung tissue. - While they have a finite lifespan similar to primary cells, they represent a **cell strain** that has been subcultured and characterized, with more homogeneous growth characteristics than fresh primary cultures.

Question 10: Flame cells are primarily associated with which of the following organisms?

A. Protozoa
B. Nematodes
C. Trematodes (Correct Answer)
D. None of the options

Explanation: ***Correct: Trematodes*** - **Flame cells** are specialized **excretory/osmoregulatory structures** found in **Platyhelminthes (flatworms)**, which include **Trematodes** (flukes) and Cestodes (tapeworms). - These cells have a **tuft of cilia** that beat in a flickering motion (resembling a flame), driving fluid through tubules for waste removal and osmoregulation. - **Trematodes** are the most appropriate answer among the given options as they are medically important flatworms with flame cells. *Incorrect: Protozoa* - Protozoa are **single-celled organisms** that use **contractile vacuoles** for osmoregulation, not flame cells. - Examples include Entamoeba, Giardia, and Plasmodium species. *Incorrect: Nematodes* - **Nematodes** (roundworms) belong to phylum Nematoda and possess a distinct excretory system with **renette cells** or **H-shaped/tubular excretory canals**, not flame cells. - Examples include Ascaris, Enterobius, and hookworms. *Incorrect: None of the options* - This is incorrect because **Trematodes** do possess flame cells and is the correct answer among the choices provided.