INI-CET 2023 — Microbiology

Q: A person handling cat feces is at risk of transmitting an infection. Which of the following is the infective stage of the organism transmitted through cat feces?

Oocyst. ***Oocyst*** - The **oocyst** is the infective stage of *Toxoplasma gondii* that is shed in cat feces. - Cats are the definitive host where sexual reproduction occurs in the intestinal epithelium, producing oocysts. - Humans can become infected by ingesting these **oocysts** directly from contaminated cat litter or soil, or indirectly through contaminated food or water. - Oocysts become infective (sporulated) after 1-5 days in the environment. *Bradyzoite* - **Bradyzoites** are slow-growing forms of *Toxoplasma gondii* found within tissue cysts, particularly in muscle and brain tissue. - While they can be infective if undercooked meat containing cysts is consumed, they are not present in cat feces. - This represents a different route of transmission (foodborne via tissue cysts). *Tachyzoite* - **Tachyzoites** are rapidly multiplying forms of *Toxoplasma gondii* responsible for acute infection and tissue damage. - They are found within host cells during active infection and are not shed in cat feces. - They can cross the placenta causing congenital toxoplasmosis. *Gametocyte* - While **gametocytes** (sexual stages) do develop in the cat's intestinal epithelium during *Toxoplasma gondii* reproduction, they are not the stage shed in feces. - The product of sexual reproduction—the **oocyst**—is what gets excreted and serves as the infective stage. - In contrast, gametocytes of *Plasmodium* species remain in blood and are relevant for malaria transmission. [Practice more Microbiology PYQs on OnCourse]

Q: A man has undergone renal transplant and is taking immunosuppressant drug. On biopsy there was presence of budding cells with pseudohyphae. Identify the organism?

Invasive candidiasis. ***Invasive candidiasis*** - The presence of **budding cells** and **pseudohyphae** on biopsy is a classic histological finding for *Candida* species. - Individuals who have undergone **renal transplant** and are on **immunosuppressant drugs** are at high risk for opportunistic fungal infections, including invasive candidiasis. *Pneumocystis* - *Pneumocystis jirovecii* typically causes pneumonia and is characterized by cysts or trophic forms in lung tissue, not budding cells and pseudohyphae. - While common in immunocompromised patients, its microscopic morphology is distinctly different from *Candida*. *Invasive aspergillosis* - *Aspergillus* species are characterized by **septate hyphae with acute angle branching** (typically 45-degree angles) on microscopy. - They do not form budding cells or pseudohyphae, which are characteristic of *Candida*. *Histoplasmosis* - *Histoplasma capsulatum* appears as **small, oval-shaped yeast cells** (2-4 µm) often found within macrophages. - It does not form pseudohyphae or large budding cells as described in the question. [Practice more Microbiology PYQs on OnCourse]

Q: A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

Stenotrophomonas. ***Stenotrophomonas*** - *Stenotrophomonas maltophilia* is intrinsically **resistant to azithromycin** and other macrolides due to its efflux pumps. - It is known to be sensitive to **ticarcillin-clavulanic acid** (a beta-lactam/beta-lactamase inhibitor combination) and **cotrimoxazole** (trimethoprim-sulfamethoxazole), which are common treatment options. *Pseudomonas* - *Pseudomonas aeruginosa* is generally **resistant to macrolides like azithromycin** but can exhibit varying sensitivity to antibiotics. - However, it often shows resistance to cotrimoxazole and ticarcillin-clavulanic acid is not a first-line agent, and it would typically be sensitive to other antipseudomonal beta-lactams (e.g., piperacillin-tazobactam, carbapenems). *Staphylococcus* - Many *Staphylococcus* species, particularly **MRSA**, are resistant to azithromycin. - However, *Staphylococcus* species are typically sensitive to other antibiotics like **vancomycin**, and are generally not sensitive to ticarcillin-clavulanic acid, and their susceptibility to cotrimoxazole varies depending on the species and resistance mechanisms. *Mycoplasma* - *Mycoplasma* species are **intrinsically resistant to beta-lactam antibiotics like ticarcillin** because they lack a cell wall, which is the target of these drugs. - They are typically sensitive to macrolides like azithromycin and also to cotrimoxazole. [Practice more Microbiology PYQs on OnCourse]

Q: CD40 deficiency in a person signifies?

IgM increase. ***IgM increase*** - A deficiency in **CD40**, or its ligand **CD40L** (found on T helper cells), disrupts **T-cell-dependent B cell activation** and **class switching**. - Without proper signaling through CD40/CD40L, B cells cannot undergo **isotype switching** from **IgM** to IgG, IgA, or IgE, leading to elevated IgM levels and deficiencies in other antibody classes. *IgG increase* - **IgG levels** would likely be **decreased** in CD40 deficiency due to the impaired ability of B cells to undergo **class switching** from IgM to other antibody isotypes. - The primary role of CD40/CD40L interaction is to facilitate this class switching process. *T cell absent* - **CD40 deficiency** does not directly cause the absence of **T cells**; rather, it affects the ability of T cells to adequately activate B cells. - T-cell absence or severe dysfunction would be indicative of a different primary immunodeficiency, such as **SCID (Severe Combined Immunodeficiency)**. *B cell absent* - **CD40 deficiency** does not result in the absence of **B cells**; B cells are present but are dysfunctional in terms of antibody class switching. - Conditions like **X-linked agammaglobulinemia (XLA)** are characterized by the absence or severe deficiency of B cells. [Practice more Microbiology PYQs on OnCourse]

Q: CD 40 marker is absent in the person. Which of the following would be seen?

Impaired B cell function. ***Impaired B cell function*** - The **CD40 receptor** on B cells is crucial for receiving co-stimulatory signals from **CD40 ligand (CD40L)**, primarily expressed on activated T cells. - Absence of CD40 on B cells prevents proper **T-cell dependent antibody class switching** and germinal center formation, leading to impaired B cell activation, immunoglobulin production, and immune responses. - This condition is seen in **Hyper-IgM Syndrome Type 3** (very rare autosomal recessive disorder). *Impaired Macrophage function* - While macrophages express CD40, its absence would primarily affect their ability to be fully activated by T cells and present antigens, but the most direct and profound impact of absent CD40 is on B cells themselves. - Macrophages have other activation pathways not directly dependent on CD40. *Impaired NK cell function* - **Natural killer (NK) cells** primarily recognize and kill target cells lacking MHC class I molecules or those expressing activating ligands, independent of CD40 signaling. - NK cell function is not directly regulated by the CD40-CD40L interaction. *Impaired T cell function* - While **T cells express CD40L** (the ligand for CD40), the question specifies the absence of the **CD40 marker** itself, which is expressed on B cells, not T cells. - T cell function involves antigen recognition, activation, and cytokine production, which are not directly mediated by CD40 expression on T cells. - T-cell function would be indirectly affected due to the lack of proper B cell help and antigen presentation, but the direct impact of absent CD40 is on the cell expressing it (B cells). [Practice more Microbiology PYQs on OnCourse]

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A person handling cat feces is at risk of transmitting an infection. Which of the following is the infective stage of the organism transmitted through cat feces?

A man has undergone renal transplant and is taking immunosuppressant drug. On biopsy there was presence of budding cells with pseudohyphae. Identify the organism?

A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

CD40 deficiency in a person signifies?

CD 40 marker is absent in the person. Which of the following would be seen?

With the lack of CD40 in B cells, which immunological abnormality is seen?

Which of the following microorganisms will be resistant to meropenem and aminoglycosides but sensitive to piperacillin tazobactam and cotrimoxazole?

INI-CET 2023 - Microbiology INI-CET Practice Questions and MCQs

Question 1: A person handling cat feces is at risk of transmitting an infection. Which of the following is the infective stage of the organism transmitted through cat feces?

A. Bradyzoite
B. Tachyzoite
C. Gametocyte
D. Oocyst (Correct Answer)

Explanation: ***Oocyst*** - The **oocyst** is the infective stage of *Toxoplasma gondii* that is shed in cat feces. - Cats are the definitive host where sexual reproduction occurs in the intestinal epithelium, producing oocysts. - Humans can become infected by ingesting these **oocysts** directly from contaminated cat litter or soil, or indirectly through contaminated food or water. - Oocysts become infective (sporulated) after 1-5 days in the environment. *Bradyzoite* - **Bradyzoites** are slow-growing forms of *Toxoplasma gondii* found within tissue cysts, particularly in muscle and brain tissue. - While they can be infective if undercooked meat containing cysts is consumed, they are not present in cat feces. - This represents a different route of transmission (foodborne via tissue cysts). *Tachyzoite* - **Tachyzoites** are rapidly multiplying forms of *Toxoplasma gondii* responsible for acute infection and tissue damage. - They are found within host cells during active infection and are not shed in cat feces. - They can cross the placenta causing congenital toxoplasmosis. *Gametocyte* - While **gametocytes** (sexual stages) do develop in the cat's intestinal epithelium during *Toxoplasma gondii* reproduction, they are not the stage shed in feces. - The product of sexual reproduction—the **oocyst**—is what gets excreted and serves as the infective stage. - In contrast, gametocytes of *Plasmodium* species remain in blood and are relevant for malaria transmission.

Question 2: A man has undergone renal transplant and is taking immunosuppressant drug. On biopsy there was presence of budding cells with pseudohyphae. Identify the organism?

A. Invasive candidiasis (Correct Answer)
B. Pneumocystis
C. Invasive aspergillosis
D. Histoplasmosis

Explanation: ***Invasive candidiasis*** - The presence of **budding cells** and **pseudohyphae** on biopsy is a classic histological finding for *Candida* species. - Individuals who have undergone **renal transplant** and are on **immunosuppressant drugs** are at high risk for opportunistic fungal infections, including invasive candidiasis. *Pneumocystis* - *Pneumocystis jirovecii* typically causes pneumonia and is characterized by cysts or trophic forms in lung tissue, not budding cells and pseudohyphae. - While common in immunocompromised patients, its microscopic morphology is distinctly different from *Candida*. *Invasive aspergillosis* - *Aspergillus* species are characterized by **septate hyphae with acute angle branching** (typically 45-degree angles) on microscopy. - They do not form budding cells or pseudohyphae, which are characteristic of *Candida*. *Histoplasmosis* - *Histoplasma capsulatum* appears as **small, oval-shaped yeast cells** (2-4 µm) often found within macrophages. - It does not form pseudohyphae or large budding cells as described in the question.

Question 3: A peculiar organism has resistance to azithromycin, but is sensitive to ticarcillin, clavulanic acid, cotrimoxazole. Identify the organism?

A. Pseudomonas
B. Staphylococcus
C. Stenotrophomonas (Correct Answer)
D. Mycoplasma

Explanation: ***Stenotrophomonas*** - *Stenotrophomonas maltophilia* is intrinsically **resistant to azithromycin** and other macrolides due to its efflux pumps. - It is known to be sensitive to **ticarcillin-clavulanic acid** (a beta-lactam/beta-lactamase inhibitor combination) and **cotrimoxazole** (trimethoprim-sulfamethoxazole), which are common treatment options. *Pseudomonas* - *Pseudomonas aeruginosa* is generally **resistant to macrolides like azithromycin** but can exhibit varying sensitivity to antibiotics. - However, it often shows resistance to cotrimoxazole and ticarcillin-clavulanic acid is not a first-line agent, and it would typically be sensitive to other antipseudomonal beta-lactams (e.g., piperacillin-tazobactam, carbapenems). *Staphylococcus* - Many *Staphylococcus* species, particularly **MRSA**, are resistant to azithromycin. - However, *Staphylococcus* species are typically sensitive to other antibiotics like **vancomycin**, and are generally not sensitive to ticarcillin-clavulanic acid, and their susceptibility to cotrimoxazole varies depending on the species and resistance mechanisms. *Mycoplasma* - *Mycoplasma* species are **intrinsically resistant to beta-lactam antibiotics like ticarcillin** because they lack a cell wall, which is the target of these drugs. - They are typically sensitive to macrolides like azithromycin and also to cotrimoxazole.

Question 4: CD40 deficiency in a person signifies?

A. IgG increase
B. T cell absent
C. IgM increase (Correct Answer)
D. B cell absent

Explanation: ***IgM increase*** - A deficiency in **CD40**, or its ligand **CD40L** (found on T helper cells), disrupts **T-cell-dependent B cell activation** and **class switching**. - Without proper signaling through CD40/CD40L, B cells cannot undergo **isotype switching** from **IgM** to IgG, IgA, or IgE, leading to elevated IgM levels and deficiencies in other antibody classes. *IgG increase* - **IgG levels** would likely be **decreased** in CD40 deficiency due to the impaired ability of B cells to undergo **class switching** from IgM to other antibody isotypes. - The primary role of CD40/CD40L interaction is to facilitate this class switching process. *T cell absent* - **CD40 deficiency** does not directly cause the absence of **T cells**; rather, it affects the ability of T cells to adequately activate B cells. - T-cell absence or severe dysfunction would be indicative of a different primary immunodeficiency, such as **SCID (Severe Combined Immunodeficiency)**. *B cell absent* - **CD40 deficiency** does not result in the absence of **B cells**; B cells are present but are dysfunctional in terms of antibody class switching. - Conditions like **X-linked agammaglobulinemia (XLA)** are characterized by the absence or severe deficiency of B cells.

Question 5: CD 40 marker is absent in the person. Which of the following would be seen?

A. Impaired Macrophage function
B. Impaired NK cell function
C. Impaired B cell function (Correct Answer)
D. Impaired T cell function

Explanation: ***Impaired B cell function*** - The **CD40 receptor** on B cells is crucial for receiving co-stimulatory signals from **CD40 ligand (CD40L)**, primarily expressed on activated T cells. - Absence of CD40 on B cells prevents proper **T-cell dependent antibody class switching** and germinal center formation, leading to impaired B cell activation, immunoglobulin production, and immune responses. - This condition is seen in **Hyper-IgM Syndrome Type 3** (very rare autosomal recessive disorder). *Impaired Macrophage function* - While macrophages express CD40, its absence would primarily affect their ability to be fully activated by T cells and present antigens, but the most direct and profound impact of absent CD40 is on B cells themselves. - Macrophages have other activation pathways not directly dependent on CD40. *Impaired NK cell function* - **Natural killer (NK) cells** primarily recognize and kill target cells lacking MHC class I molecules or those expressing activating ligands, independent of CD40 signaling. - NK cell function is not directly regulated by the CD40-CD40L interaction. *Impaired T cell function* - While **T cells express CD40L** (the ligand for CD40), the question specifies the absence of the **CD40 marker** itself, which is expressed on B cells, not T cells. - T cell function involves antigen recognition, activation, and cytokine production, which are not directly mediated by CD40 expression on T cells. - T-cell function would be indirectly affected due to the lack of proper B cell help and antigen presentation, but the direct impact of absent CD40 is on the cell expressing it (B cells).

Question 6: With the lack of CD40 in B cells, which immunological abnormality is seen?

A. Total lack of NK cells
B. Lack of CD8 mediated cytotoxicity
C. Inability of neutrophil against infections
D. Decreased IgG and increase in IgM (Correct Answer)

Explanation: ***Decreased IgG and increase in IgM*** - The interaction between **CD40 on B cells** and **CD40L (CD154) on T helper cells** is crucial for **B cell activation**, proliferation, and **class switch recombination** (CSR). - Without this interaction, B cells cannot undergo CSR, leading to a failure to produce **IgG, IgA, or IgE**, while **IgM levels remain high** because IgM production is the initial default. *Total lack of NK cells* - **Natural Killer (NK) cells** are part of the innate immune system and their development is largely independent of CD40-CD40L signaling. - The absence of CD40 on B cells primarily affects adaptive humoral immunity, not NK cell numbers or function. *Lack of CD8 mediated cytotoxicity* - **CD8+ T cells** mediate cytotoxicity against infected or cancerous cells and their activation is primarily dependent on antigen presentation by **MHC class I molecules** and costimulation, not directly on B cell CD40. - While B cells can act as APCs, their CD40 interaction is more critical for T helper cell help for humoral responses. *Inability of neutrophil against infections* - **Neutrophils** are phagocytic cells important in innate immunity, and their function is largely independent of CD40 on B cells. - Neutrophil activity relies on pathogen recognition, phagocytosis, and degranulation, which are not directly regulated by the B cell CD40-CD40L pathway.

Question 7: Which of the following microorganisms will be resistant to meropenem and aminoglycosides but sensitive to piperacillin tazobactam and cotrimoxazole?

A. Pseudomonas
B. Acinetobacter
C. Burkholderia cepacia
D. Stenotrophomonas (Correct Answer)

Explanation: ***Stenotrophomonas maltophilia*** - *Stenotrophomonas maltophilia* exhibits **intrinsic resistance to carbapenems (like meropenem)** due to the presence of L1 and L2 metallo-beta-lactamases and chromosomally encoded beta-lactamases. - It is **resistant to aminoglycosides** via aminoglycoside-modifying enzymes and efflux pump mechanisms. - **Trimethoprim-sulfamethoxazole (cotrimoxazole) is the drug of choice** with consistent susceptibility, making it the first-line treatment. - **Susceptibility to piperacillin-tazobactam is variable** - while some isolates may show in vitro susceptibility, clinical efficacy is inconsistent and it is not considered a reliable first-line agent. Among the options given, this organism best fits the described pattern. *Pseudomonas aeruginosa* - **Generally susceptible to carbapenems (meropenem) and aminoglycosides**, which are important therapeutic options. - Does not match the resistance pattern described in the question. *Acinetobacter baumannii* - Shows **multidrug resistance including carbapenems and aminoglycosides** in most clinical isolates. - However, typically also **resistant to piperacillin-tazobactam and cotrimoxazole**, making it inconsistent with the described susceptibility pattern. *Burkholderia cepacia complex* - Exhibits **intrinsic resistance to multiple antibiotics** including aminoglycosides and often carbapenems. - **Variable and often resistant to piperacillin-tazobactam**, and susceptibility to cotrimoxazole is inconsistent. - Does not reliably match the described antibiotic profile.