Microbiome and Immune System — MCQs
Which of the following is not considered an antigen-presenting cell?
Which of the following is a type of inflammatory bowel disease primarily affecting the small intestine? a) Coeliac disease b) Tropical sprue c) Regional ileitis d) Cystic fibrosis e) Ulcerative colitis
A clinically depressed farmer complains of extreme weakness, a daily rise and fall in fever, and night sweats. Small gram-negative rods are isolated from blood cultures after a 2-week incubation period. Which of the following organisms is the most likely etiologic agent?
The organism causing pseudomembranous colitis:
Which of the following is not true about innate immunity
Following acute pharyngitis, a patient was on broad spectrum antibiotics. One week later he developed watery diarrhoea with a foul odour and abdominal cramps. Antibiotic induced colitis is confirmed. What is the most common cause of antibiotic induced colitis?
Fecal leucocytes are absent in all of the following, except:
A farmer presents with a subcutaneous wound on his foot with discharge. Microscopy of a white granule from the wound shows Gram-positive filamentous rods. What is the most likely organism?
A 36 years male presented with complaint of productive cough and fever for last 2 months. He has undergone kidney transplantation 2 years back. His sputum examination revealed a gram positive filamentous bacteria that showed acid fastness with modified Ziehl-Neelsen staining (1% H2SO4). The most likely etiological agent is ?
Diagnosis of C. difficile infection is made by which of the following methods?
Microbiome and Immune System Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following is not considered an antigen-presenting cell?
- A. Macrophages
- B. Langerhans cells
- C. Thymocytes (Correct Answer)
- D. B lymphocytes
Explanation: ***Thymocytes*** - Thymocytes are **developing T cells** found in the thymus and do not function as antigen-presenting cells (APCs) [1]. - Unlike APCs, thymocytes are primarily involved in the **maturation** and selection of T lymphocytes. *Langerhans cells* - Langerhans cells are a type of **dendritic cell** found in the skin and are effective antigen-presenting cells to T cells [1]. - They play a crucial role in **immune surveillance** and response to skin infections. *Macrophages* - Macrophages are well-known antigen-presenting cells that engulf pathogens and present antigens to T cells [1]. - They are also involved in **phagocytosis** and secrete various cytokines to modulate immune responses. *M-cells* - M-cells (microfold cells) are specialized epithelial cells that transport antigens from the intestinal lumen to underlying immune cells. - Although not traditional APCs, they play a role in immune surveillance and stimulating **mucosal immunity**. **References:** [1] Kumar V, Abbas AK, et al.. Robbins and Cotran Pathologic Basis of Disease. 9th ed. Diseases of the Immune System, pp. 200, 207-208.
Question 2: Which of the following is a type of inflammatory bowel disease primarily affecting the small intestine? a) Coeliac disease b) Tropical sprue c) Regional ileitis d) Cystic fibrosis e) Ulcerative colitis
- A. Ulcerative colitis
- B. Cystic fibrosis
- C. Tropical sprue
- D. Regional ileitis (Correct Answer)
- E. Coeliac disease
Explanation: ***Regional ileitis*** - **Regional ileitis** is another name for **Crohn's disease** when it primarily affects the **ileum**, which is part of the small intestine [1]. - Crohn's disease is a type of **inflammatory bowel disease (IBD)** [1] characterized by **transmural inflammation** that can affect any part of the gastrointestinal tract, but most commonly involves the small intestine. *Ulcerative colitis* - **Ulcerative colitis** is an **inflammatory bowel disease** that exclusively affects the **large intestine (colon and rectum)** [2]. - Unlike Crohn's disease, it involves continuous inflammation of the mucosa and submucosa, usually starting in the rectum and extending proximally [2]. *Cystic fibrosis* - **Cystic fibrosis** is a **genetic disorder** that affects the lungs, pancreas, liver, and intestine, leading to the production of **thick, sticky mucus**. - While it can cause malabsorption and intestinal issues due to pancreatic insufficiency, it is not primarily an inflammatory bowel disease in itself. *Tropical sprue* - **Tropical sprue** is a malabsorption syndrome thought to be caused by **environmental factors and microbial changes** in the small intestine, typically affecting individuals in tropical regions [3]. - It results in abnormal small intestinal architecture and nutrient malabsorption, but it is not classified as an inflammatory bowel disease like Crohn's or ulcerative colitis. *Coeliac disease* - **Coeliac disease** is an **immune-mediated condition** triggered by the ingestion of **gluten**, leading to damage of the small intestinal villi [3]. - While it affects the small intestine and involves an immune response, it is distinct from inflammatory bowel diseases which are characterized by chronic, relapsing inflammation of the GI tract.
Question 3: A clinically depressed farmer complains of extreme weakness, a daily rise and fall in fever, and night sweats. Small gram-negative rods are isolated from blood cultures after a 2-week incubation period. Which of the following organisms is the most likely etiologic agent?
- A. Campylobacter jejuni
- B. Francisella tularensis
- C. Brucella melitensis (Correct Answer)
- D. Salmonella enteritidis
Explanation: ***Brucella melitensis*** - This organism is known to cause **brucellosis**, which presents with **undulant fever** (daily rise and fall), night sweats, and fatigue, consistent with the patient's symptoms. The profession of a **farmer** puts him at higher risk due to exposure to infected livestock. - **Neuropsychiatric manifestations** including depression, fatigue, and malaise are well-recognized features of chronic brucellosis, explaining the patient's clinical depression. - **_Brucella_ species** are characteristic for their slow growth, often requiring **extended incubation periods** (up to 2 weeks) in blood cultures, and appear as small gram-negative rods. *Campylobacter jejuni* - This bacterium is a common cause of **gastroenteritis**, leading to **diarrhea**, abdominal cramps, and fever. While it can cause bacteremia, it typically presents with more prominent gastrointestinal symptoms. - _C. jejuni_ is a **curved or spiral-shaped** gram-negative rod, distinct from the small rods described, and does not typically cause an undulant fever pattern. *Francisella tularensis* - This agent causes **tularemia**, which can manifest with fever, chills, and fatigue, but often includes a characteristic **skin lesion (ulceroglandular)** and prominent lymphadenopathy. - Although it is a small gram-negative rod, the specific **undulant fever pattern** and the farmer's exposure history are more indicative of brucellosis. *Salmonella enteritidis* - This bacterium commonly causes **food poisoning** with symptoms like diarrhea, fever, and vomiting. While it can lead to bacteremia, it is less likely to present with the prolonged, **undulant fever** seen in brucellosis. - **_Salmonella_ species** are typically readily isolated from blood cultures within a few days, unlike the prolonged incubation needed for _Brucella_.
Question 4: The organism causing pseudomembranous colitis:
- A. Clostridium difficile (Correct Answer)
- B. Clostridium botulinum
- C. Clostridium tetani
- D. Clostridium perfringens
Explanation: ***Clostridium difficile*** - This bacterium is the primary cause of **pseudomembranous colitis**, an inflammation of the colon characterized by the formation of membranes on the mucosal surface. - It produces **toxins A and B** which damage the intestinal lining, leading to severe diarrhea, abdominal pain, and fever. *Clostridium botulinum* - This organism is responsible for **botulism**, a severe form of food poisoning or wound infection. - It produces a potent **neurotoxin** that causes flaccid paralysis by blocking acetylcholine release at neuromuscular junctions. *Clostridium tetani* - This bacterium causes **tetanus**, characterized by muscle spasms and rigidity. - It produces the **tetanospasmin neurotoxin** which inhibits inhibitory neurotransmitters in the spinal cord. *Clostridium perfringens* - Primarily known for causing **gas gangrene** (myonecrosis) and certain types of food poisoning. - It produces various **toxins**, including alpha-toxin, which contribute to tissue destruction and gas formation.
Question 5: Which of the following is not true about innate immunity
- A. Memory is seen (Correct Answer)
- B. It is relatively non specific
- C. It is first line of defence
- D. It is present prior to antigenic exposure
Explanation: ***Memory is seen*** - Innate immunity is characterized by a **lack of immunological memory**, meaning it does not \"remember\" previous encounters with pathogens to mount a stronger, faster response. - This feature is a hallmark of **adaptive (acquired) immunity**, which develops memory cells after initial exposure. *It is relatively non specific* - Innate immune responses are **non-specific** and target broad categories of pathogens, rather than specific antigens. - It recognizes conserved structures on pathogens, known as **Pathogen-Associated Molecular Patterns (PAMPs)**, shared by many different microbes. *It is first line of defence* - Innate immunity serves as the **body's immediate and primary defense** against invading pathogens. - It provides rapid protection through physical barriers, cellular components, and soluble factors, often preventing infection before it takes hold. *It is present prior to antigenic exposure* - The components of innate immunity are **pre-existing and fully functional** before any exposure to pathogens or antigens. - This readiness allows for an **instantaneous response** upon microbial invasion, without requiring prior sensitization.
Question 6: Following acute pharyngitis, a patient was on broad spectrum antibiotics. One week later he developed watery diarrhoea with a foul odour and abdominal cramps. Antibiotic induced colitis is confirmed. What is the most common cause of antibiotic induced colitis?
- A. Esch. coli
- B. Clostridium difficile (Correct Answer)
- C. Campylobacter jejuni
- D. Salmonella
Explanation: ***Clostridium difficile*** - **Clostridium difficile** infection is the most common cause of **antibiotic-induced colitis** [1], leading to symptoms like **watery diarrhea, foul odor**, and **abdominal cramps** [1]. - Antibiotics disrupt the normal gut flora, allowing **C. difficile** to overgrow and produce toxins that damage the colon [1]. *Esch. coli* - While some strains of *E. coli* can cause **diarrhea** (e.g., ETEC, EHEC), it is not the primary cause of antibiotic-induced colitis [2]. - **Esch. coli** diarrhea is typically acquired through contaminated food or water, not antibiotic use [2]. *Campylobacter jejuni* - *Campylobacter jejuni* typically causes **gastroenteritis** with **bloody diarrhea** and **fever**, often from consuming contaminated poultry. - It is not directly associated with **antibiotic-induced colitis** as the primary causative agent. *Salmonella* - **Salmonella** infections usually result from contaminated food and present with **fever, abdominal cramps**, and **diarrhea** (which can be bloody). - It is not the most common pathogen responsible for **antibiotic-induced colitis**.
Question 7: Fecal leucocytes are absent in all of the following, except:
- A. Cryptosporidiosis
- B. Clostridium perfringes infection
- C. Giardiasis
- D. Campylobacter infection (Correct Answer)
Explanation: ***Campylobacter infection*** - This infection causes **inflammatory diarrhea**, leading to the presence of **fecal leucocytes** as a response to intestinal tissue invasion. - The inflammatory process results in disruption of the intestinal mucosa, attracting **neutrophils** and other inflammatory cells to the stool. *Giardiasis* - **Giardia lamblia** infection typically causes **non-inflammatory diarrhea** by interfering with nutrient absorption in the small intestine. - Due to the non-invasive nature of the pathogen, **fecal leucocytes** are generally **absent** in the stool. *Cryptosporidiosis* - **Cryptosporidium parvum** primarily causes **non-inflammatory watery diarrhea** by adhering to and damaging the microvilli of the intestinal epithelium. - While it can cause flattening of the villi, it does not typically lead to significant tissue invasion or the presence of **fecal leucocytes**. *Clostridium perfringens infection* - This bacterium causes **food poisoning** mainly through the production of **toxins** that affect the intestinal lining. - The diarrhea is typically **non-inflammatory**, and **fecal leucocytes** are usually **absent** because there is no significant host immune cell invasion.
Question 8: A farmer presents with a subcutaneous wound on his foot with discharge. Microscopy of a white granule from the wound shows Gram-positive filamentous rods. What is the most likely organism?
- A. Staphylococcus aureus
- B. Histoplasma
- C. Nocardia (Correct Answer)
- D. Sporothrix
Explanation: ***Nocardia*** - The presence of **white granules** in the discharge, along with **Gram-positive, filamentous rods**, is highly characteristic of *Nocardia* infection, often forming **sulfur granules** (though not always yellow). - *Nocardia* is a common soil bacterium, making it a likely pathogen in a **farmer with a subcutaneous wound** related to environmental exposure. *Staphylococcus aureus* - While *Staphylococcus aureus* can cause skin infections and abscesses, it presents as **Gram-positive cocci in clusters**, not filamentous rods. - It does not typically form **granules** in the discharge in the same manner as *Nocardia*. *Histoplasma* - *Histoplasma* is a **dimorphic fungus** that causes systemic infections, often acquired by inhaling spores. - It would appear as **yeast forms** in tissue or cultures, not Gram-positive filamentous rods, and is not typically associated with subcutaneous wounds forming granules. *Sporothrix* - *Sporothrix schenckii* causes **sporotrichosis**, characterized by a **subcutaneous nodule** that progresses along lymphatic channels. - It is a **dimorphic fungus** (yeast in tissue, mold in culture) and would not appear as Gram-positive filamentous rods on microscopy.
Question 9: A 36 years male presented with complaint of productive cough and fever for last 2 months. He has undergone kidney transplantation 2 years back. His sputum examination revealed a gram positive filamentous bacteria that showed acid fastness with modified Ziehl-Neelsen staining (1% H2SO4). The most likely etiological agent is ?
- A. Blastomyces dermatitidis
- B. Actinomyces israelii
- C. Nocardia asteroides (Correct Answer)
- D. Cryptosporidium parvum
Explanation: ***Nocardia asteroides*** - This patient, being an **immunocompromised kidney transplant recipient**, is highly susceptible to **opportunistic infections**. *Nocardia* species are **gram-positive, filamentous, branched bacteria** that are **weakly acid-fast** (positive with modified Ziehl-Neelsen staining, typically 1% H2SO4), commonly causing **pulmonary infections** with productive cough and fever. - Pulmonary nocardiosis can mimic tuberculosis or other fungal infections, and the acid-fast staining characteristic helps differentiate it from non-acid-fast filamentous bacteria like *Actinomyces*. *Blastomyces dermatitidis* - This is a **dimorphic fungus** that causes **blastomycosis**, an endemic infection in certain geographic regions, which is usually diagnosed by visualization of broad-based budding yeasts or culture. - It would not appear as a **gram-positive filamentous bacterium** with acid-fast properties in sputum. *Actinomyces israelii* - *Actinomyces israelii* is a **gram-positive, filamentous bacterium** that causes **actinomycosis**, often characterized by chronic abscesses, sinus tracts, and "sulfur granules." - Unlike *Nocardia*, *Actinomyces* species are **not acid-fast**, which rules it out given the staining results. *Cryptosporidium parvum* - This is a **protozoan parasite** that causes **cryptosporidiosis**, primarily manifesting as **gastroenteritis** (diarrhea), especially in immunocompromised individuals. - It would not present as a **filamentous bacterial form in sputum**, nor would it be diagnosed by Gram stain and acid-fast modified Ziehl-Neelsen staining in this context.
Question 10: Diagnosis of C. difficile infection is made by which of the following methods?
- A. Stool microscopy for pseudomembranes
- B. Culture
- C. Toxin gene detection by polymerase chain reaction (PCR) (Correct Answer)
- D. Enzyme-linked immunosorbent assay (ELISA)
Explanation: ***Toxin gene detection by polymerase chain reaction (PCR)*** - **PCR for toxin genes (tcdA and tcdB)** is the most sensitive and specific method for diagnosing **Clostridioides difficile infection (CDI)**, directly detecting the genetic material responsible for the pathology. - This method is superior because it identifies the presence of toxigenic C. difficile, which is crucial for determining clinical significance and guiding treatment. *Stool microscopy for pseudomembranes* - While **pseudomembranes** are a hallmark of severe CDI, their detection requires **endoscopy** and is not a direct diagnostic test for the pathogen itself. - Furthermore, their absence does not rule out CDI, as pseudomembranes may not form in all cases, especially milder ones. *Culture* - **Culture for C. difficile** can identify the presence of the organism, but it does not differentiate between toxigenic and non-toxigenic strains. - Many individuals can be **colonized with non-toxigenic C. difficile** without having an active infection, leading to false positives if culture alone is used for diagnosis. *Enzyme - linked immunosorbent assay (ELISA)* - ELISA tests primarily detect **C. difficile toxins A and B** or **glutamate dehydrogenase (GDH)** antigen in stool. - While rapid, ELISA for toxins A/B has **lower sensitivity** than PCR, potentially missing cases, and GDH detection alone only indicates the presence of C. difficile (toxigenic or non-toxigenic), requiring further toxin testing for confirmation.
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