Cercariae are the infective form of which of the following parasites?
What is the correct sequence of the sexual cycle in malaria?
Which of the following is the most common yeast pathogen?
Coxsackie virus is classified as which type of virus?
The idiotype of an antibody is determined by -
Which of the following is a member of the kingdom Protista?
Katayama fever is caused by which of the following?
Necrotizing fasciitis is caused by -
The gene encoding cholera toxin is carried on -
Unsegmented eggs are in which parasite?
NEET-PG 2012 - Microbiology NEET-PG Practice Questions and MCQs
Question 31: 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 32: What is the correct sequence of the sexual cycle in malaria?
- A. Sporozoites to gametocytes
- B. Gametocytes to gametes
- C. Gametocytes to sporozoites (Correct Answer)
- D. Gametes to zygote
Explanation: ***Gametocytes to sporozoites*** - The sexual cycle begins when a mosquito ingests **gametocytes** during a blood meal. - These gametocytes develop into **gametes**, which fuse to form a **zygote**. The zygote matures into an **oocyst** and then releases **sporozoites**, which migrate to the mosquito's salivary glands, ready to infect a new human host. *Gametocytes to gametes* - This is an initial step within the sexual cycle where gametocytes differentiate into **male and female gametes**, respectively. - However, it's not the complete *sequence* of the sexual cycle, as it omits subsequent crucial stages like fertilization and sporozoite formation. *Sporozoites to gametocytes* - **Sporozoites** are injected into a human host and initiate the asexual cycle by infecting liver cells, then red blood cells. - **Gametocytes** are formed later during the asexual cycle in the human host, ready to be picked up by another mosquito; this sequence describes part of the human infection, not the sexual cycle in the mosquito. *Gametes to zygote* - This step represents **fertilization**, where male and female gametes fuse, forming a **zygote** in the mosquito gut. - While essential, it is only one part of the overall sexual cycle and doesn't encompass the full transformation from gametocytes to infective sporozoites.
Question 33: Which of the following is the most common yeast pathogen?
- A. Candida (Correct Answer)
- B. Mucor
- C. Rhizopus
- D. Cryptococcus
Explanation: ***Candida*** - **Candida** species, particularly *Candida albicans*, are the **most common cause of fungal infections** worldwide, ranging from superficial mucocutaneous infections to invasive systemic candidiasis. - They are part of the normal human microbiota and opportunistic pathogens, thriving in conditions like **immunocompromise**, antibiotic use, or presence of medical devices. *Mucor* - **Mucor** is a genus of mold, not yeast, and is known to cause **mucormycosis**, a severe and rapidly progressive fungal infection. - While dangerous, mucormycosis is **much rarer** compared to candidiasis. *Rhizopus* - **Rhizopus** is also a genus of mold, not yeast, and is another causative agent of **mucormycosis**, particularly in immunocompromised individuals. - Like Mucor, infections caused by Rhizopus are **less common** than those caused by Candida. *Cryptococcus* - **Cryptococcus neoformans** is a significant yeast pathogen, primarily causing **cryptococcosis**, which often manifests as meningoencephalitis in immunocompromised patients. - Although it is an important pathogen, its overall incidence is **lower than that of Candida** species.
Question 34: Coxsackie virus is classified as which type of virus?
- A. Pox virus
- B. Enterovirus (Correct Answer)
- C. Myxovirus
- D. Herpes virus
Explanation: ***Enterovirus*** - **Coxsackie virus** belongs to the genus *Enterovirus* within the family *Picornaviridae*. - Enteroviruses are characterized as **non-enveloped, positive-sense, single-stranded RNA viruses** and typically infect the gastrointestinal tract. *Herpes virus* - Herpes viruses are **enveloped, double-stranded DNA viruses** known for causing latent infections. - Examples include HSV-1 (oral herpes) and VZV (chickenpox), which are distinct from Coxsackie. *Pox virus* - Pox viruses are **large, enveloped, double-stranded DNA viruses** that replicate in the cytoplasm of infected cells. - Smallpox and molluscum contagiosum are caused by pox viruses, which have different genetic and structural characteristics than Coxsackie virus. *Myxovirus* - **Myxovirus** is an older classification that once included viruses now categorized into *Orthomyxoviridae* (e.g., influenza) and *Paramyxoviridae* (e.g., measles, mumps). - These are **enveloped, negative-sense, single-stranded RNA viruses**, a different viral structure and replication strategy compared to Coxsackie virus.
Question 35: The idiotype of an antibody is determined by -
- A. Fc region
- B. Hinge region
- C. Carboxy end
- D. Amino end (Correct Answer)
Explanation: ***Amino end*** - The **idiotypic class** of an antibody is determined by the unique amino acid sequences within the **variable regions** of both the heavy and light chains, located at the **amino-terminal end** of the antibody. - These variable regions form the **antigen-binding sites**, giving each antibody its specific binding capabilities and thus its idiotype. *Fc region* - The **Fc (fragment crystallizable) region** is the tail region of an antibody that interacts with cell surface receptors and other immune system molecules. - It determines the **isotype (class)** of the antibody (e.g., IgG, IgM) and mediates effector functions but does not define the idiotype. *Hinge region* - The **hinge region** is a flexible segment in the middle of the heavy chains of IgG, IgA, and IgD antibodies. - It allows flexibility between the antigen-binding (Fab) arms, facilitating bivalent binding, but does not determine the idiotype. *Carboxy end* - The **carboxy end** (C-terminus) of the heavy and light chains typically corresponds to the constant regions of the antibody. - While it contributes to the antibody's overall structure and effector functions (especially the Fc region), it does not contain the unique sequences that define the **idiotype**.
Question 36: Which of the following is a member of the kingdom Protista?
- A. Fungi
- B. Protozoa (Correct Answer)
- C. Bacteria
- D. Virus
Explanation: ***Protozoa*** - **Protozoa** are single-celled eukaryotic organisms that are heterotrophic and typically motile, fitting the classification within the kingdom Protista. - Protista is a **diverse kingdom** encompassing various eukaryotic organisms that are not animals, plants, or fungi, and protozoa represent the animal-like protists. - Examples include **Amoeba, Plasmodium, Giardia**, and Entamoeba. *Virus* - **Viruses** are not classified within any kingdom as they are **acellular** (not made of cells). - They are obligate intracellular parasites that require a host cell to replicate. - Lack cellular machinery and metabolic processes that define living organisms. *Fungi* - **Fungi** belong to their own distinct kingdom, Fungi, and are not classified under Protista. - They are **heterotrophic eukaryotes** that absorb nutrients and have cell walls made of chitin. - Examples include yeasts, molds, and mushrooms. *Bacteria* - **Bacteria** are prokaryotic organisms, meaning they lack a membrane-bound nucleus and other membrane-bound organelles. - They belong to the kingdom **Monera** (or domain Bacteria in modern classification), separate from eukaryotic kingdoms like Protista. - They have peptidoglycan cell walls and reproduce by binary fission.
Question 37: Katayama fever is caused by which of the following?
- A. F. hepatica
- B. C. sinensis
- C. A. lumbricoides
- D. S. japonicum (Correct Answer)
Explanation: ***Correct: S. japonicum*** - Katayama fever, also known as **acute schistosomiasis**, is a systemic hypersensitivity reaction to the migrating schistosomula and oviposition of eggs that primarily occurs in infections with **_Schistosoma japonicum_** or _S. mansoni_. - It presents with fever, chills, cough, diarrhea, abdominal pain, hepatosplenomegaly, and eosinophilia, typically 2-8 weeks after exposure to contaminated water. - S. japonicum tends to cause the most severe form of Katayama fever. *Incorrect: F. hepatica* - **_Fasciola hepatica_** causes fascioliasis, an infection of the bile ducts and liver, which can present with fever and eosinophilia, but it does not typically cause the acute systemic reaction known as Katayama fever. - The disease is usually acquired by ingesting **metacercariae** on aquatic vegetation or in contaminated water. *Incorrect: C. sinensis* - **_Clonorchis sinensis_** is the Chinese liver fluke, causing clonorchiasis, an infection primarily of the bile ducts. - Symptoms are often mild or asymptomatic but can include abdominal pain, indigestion, diarrhea, and jaundice in heavy infections, without the distinct acute systemic syndrome of Katayama fever. *Incorrect: A. lumbricoides* - **_Ascaris lumbricoides_** is a roundworm that causes ascariasis, primarily affecting the gastrointestinal tract. - While it can cause pulmonary symptoms during larval migration (Löffler's syndrome), it does not cause Katayama fever, which is specific to schistosomiasis.
Question 38: Necrotizing fasciitis is caused by -
- A. Beta hemolytic streptococci (Correct Answer)
- B. Pneumococcus
- C. Staphylococcus aureus
- D. Clostridium perfringens
Explanation: ***Beta hemolytic streptococci*** - **Group A Streptococcus (GAS)**, specifically *Streptococcus pyogenes*, is the most common cause of **Type II necrotizing fasciitis**. - Its virulence factors, like **exotoxins**, contribute to rapid tissue destruction and systemic toxicity. *Staphylococcus aureus* - While *S. aureus* can cause severe skin and soft tissue infections, it is more commonly associated with **cellulitis**, **abscesses**, and **septic arthritis**. - It can be a co-pathogen in **polymicrobial (Type I) necrotizing fasciitis** but is less frequent as a sole cause compared to GAS for Type II. *Pneumococcus* - *Streptococcus pneumoniae* (Pneumococcus) is primarily known for causing respiratory infections like **pneumonia**, **otitis media**, and **meningitis**. - It is not a typical causative agent of necrotizing fasciitis. *Clostridium perfringens* - This bacterium is the primary cause of **gas gangrene** (clostridial myonecrosis), a severe form of necrotizing soft tissue infection involving muscle tissue. - While also a flesh-eating bacterium, its clinical presentation and typical affected tissues differ from those of necrotizing fasciitis caused by streptococci.
Question 39: The gene encoding cholera toxin is carried on -
- A. Chromosomal DNA
- B. Extrachromosomal plasmid
- C. Bacteriophage (Correct Answer)
- D. Transposon
Explanation: ***Bacteriophage*** - The gene encoding **cholera toxin (ctxA and ctxB)** is carried on the genome of a **lysogenic bacteriophage** known as CTXf. - This phage integrates its DNA into the *Vibrio cholerae* chromosome, allowing for toxin production. *Chromosomal DNA* - While the **phage DNA (containing the cholera toxin gene)** integrates into the *Vibrio cholerae* chromosome, the toxin itself is **not directly encoded by the core bacterial chromosomal DNA** but by the integrated phage DNA. - Many bacterial virulence factors are encoded on the main chromosome, but cholera toxin is a specific exception. *Extrachromosomal plasmid* - **Plasmids** are extrachromosomal DNA molecules that can carry virulence genes, but the cholera toxin gene is **not typically found on a plasmid** in *Vibrio cholerae*. - Examples of plasmid-encoded toxins include some enterotoxins in *E. coli*. *Transposon* - **Transposons** are "jumping genes" that can move within and between DNA molecules, but they are generally **mobile genetic elements** that carry genes, not the direct source of the cholera toxin gene. - While transposons can sometimes contribute to the movement of virulence genes, the cholera toxin gene specifically originates from a bacteriophage.
Question 40: Unsegmented eggs are in which parasite?
- A. Ancylostoma
- B. Necator americanus
- C. Dracunculus
- D. Trichuris trichiura (Correct Answer)
Explanation: ***Trichuris trichiura*** - *Trichuris trichiura* (whipworm) eggs are typically **unembryonated** or **unsegmented** when passed in feces. - Upon defecation, the eggs require a period of **development in soil** to become infective. *Ancylostoma* - **Hookworm (Ancylostoma)** eggs are typically **segmented** (possessing a 2-8 cell stage) when passed in feces. - They develop into **rhabditiform larvae** in the soil. *Necator americanus* - **Hookworm (Necator americanus)** eggs are also typically **segmented** (possessing a 2-8 cell stage) when passed in feces. - Like *Ancylostoma*, they require further development in soil to become infective. *Dracunculus* - *Dracunculus medinensis* (Guinea worm) does not lay eggs; instead, it releases **larvae** from the skin blister of the host into water. - The larvae are then ingested by **cyclops** (copepods) to continue their life cycle.