When is the prozone phenomenon seen?
A patient with sore throat has a positive Paul Bunnell test, indicating infectious mononucleosis. The causative organism is?
Which of the following statements is true regarding T cell independent antigens?
What is the correct order of application of reagents in the complete Gram staining procedure?
Which of the following statements about Haemophilus influenzae is true?
Acute Infective Endocarditis with abscess formation is most commonly associated with
Most common bacterial cause of acute parotitis -
An 18-year-old girl presents with watery diarrhea. Most likely causative agent -
Ehrlichia chaffeensis is the causative agent of -
Hanging drop method is used for:
NEET-PG 2012 - Microbiology NEET-PG Practice Questions and MCQs
Question 61: When is the prozone phenomenon seen?
- A. Same concentration of antibody and antigen
- B. Hyperimmune reaction
- C. In antigen excess to antibody
- D. Antibody excess to antigen (Correct Answer)
Explanation: ***Antibody excess to antigen*** - The **prozone phenomenon** occurs when there is a significant **excess of antibodies** relative to the antigen, leading to inhibition of lattice formation. - In this state, too many antibodies bind to individual antigen sites, preventing cross-linking and thus inhibiting visible **agglutination** or **precipitation**. *Same concentration of antibody and antigen* - This scenario typically represents the **zone of equivalence**, where optimal lattice formation and visible reaction (agglutination or precipitation) occur. - It is where the concentrations of antibody and antigen are balanced, leading to maximum complex formation. *In antigen excess to antibody* - This situation is known as the **postzone phenomenon**, where an excess of antigen prevents the formation of stable antibody-antigen complexes. - The antigen saturates the limited antibody sites, resulting in no or minimal visible reaction. *Hyperimmune reaction* - A hyperimmune reaction refers to an **exaggerated immune response**, often resulting from repeated exposure to an antigen. - While it involves high antibody levels, it is a clinical state rather than a specific phenomenon describing antibody-antigen ratios in *in vitro* tests.
Question 62: A patient with sore throat has a positive Paul Bunnell test, indicating infectious mononucleosis. The causative organism is?
- A. EBV (Correct Answer)
- B. Adenovirus
- C. CMV
- D. HSV (Herpes Simplex Virus)
Explanation: ***Correct: EBV*** - The **Paul-Bunnell test** (monospot test) detects **heterophile antibodies**, which are characteristic of acute **Epstein-Barr virus (EBV)** infection. - **EBV** is the primary causative agent of **infectious mononucleosis**, commonly known as "mono." *Incorrect: Adenovirus* - **Adenoviruses** can cause various infections, including **pharyngitis** and **conjunctivitis**, but are not associated with a positive **Paul-Bunnell test** or heterophile antibodies. - While it can cause sore throat, the presence of a **positive Paul-Bunnell test** differentiates it from EBV. *Incorrect: CMV* - **Cytomegalovirus (CMV)** can cause a mononucleosis-like syndrome, but it typically results in a **negative Paul-Bunnell test** (i.e., it is heterophile antibody-negative). - CMV mononucleosis is often seen in individuals who are **immunocompromised** or in infants as a congenital infection. *Incorrect: HSV (Herpes Simplex Virus)* - **Herpes simplex virus (HSV)** causes infections such as **oral herpes (cold sores)** and **genital herpes**, and in some cases, **pharyngitis**. - HSV infection is not associated with a positive **Paul-Bunnell test** or the production of heterophile antibodies.
Question 63: Which of the following statements is true regarding T cell independent antigens?
- A. They primarily activate T-cells.
- B. They primarily activate B-cells. (Correct Answer)
- C. They primarily activate macrophages.
- D. They primarily activate CD8+ T cells.
Explanation: ***Correct: They primarily activate B-cells*** - T-cell independent antigens are typically **polysaccharides** (TI-2) or **lipopolysaccharides** (TI-1) with repeating epitopes that can directly cross-link B cell receptors (BCRs) - This direct binding and cross-linking provide a strong enough signal to activate B cells and induce **antibody production** (mainly IgM) without the need for T cell help - They induce a rapid but limited immune response with minimal memory formation *Incorrect: They primarily activate T-cells* - T-cell independent antigens do not require processing and presentation by **MHC molecules**, which is essential for T cell activation - T cells recognize processed peptides presented by MHC, a mechanism not utilized by T-cell independent antigens - By definition, these antigens activate B cells **without** T cell involvement *Incorrect: They primarily activate macrophages* - While macrophages are antigen-presenting cells, their primary role in adaptive immunity is to process and present antigens to T cells - Macrophages are involved in **phagocytosis** and antigen processing, but are not the primary target cells for T-independent antigens - The key feature of TI antigens is direct B cell activation, not macrophage activation *Incorrect: They primarily activate CD8+ T cells* - **CD8+ T cells** are activated by processed antigens presented on **MHC class I molecules**, typically derived from intracellular pathogens - T-cell independent antigens do not utilize this pathway and are primarily involved in **humoral immunity** through direct B cell activation - TI antigens cannot activate CD8+ T cells as they bypass the T cell-dependent pathway entirely
Question 64: What is the correct order of application of reagents in the complete Gram staining procedure?
- A. Gentian violet → Iodine → Alcohol/Acetone → Safranin (Correct Answer)
- B. Iodine → Gentian violet → Alcohol/Acetone → Safranin
- C. Safranin → Gentian violet → Iodine → Alcohol/Acetone
- D. Gentian violet → Alcohol/Acetone → Iodine → Safranin
Explanation: **Gentian violet → Iodine → Alcohol/Acetone → Safranin** - **Gentian violet** (or crystal violet) is the **primary stain** that colors all cells purple. - **Iodine** acts as a **mordant**, forming a crystal violet-iodine complex within the cell walls. - **Alcohol/Acetone** is the **decolorizer**, washing out the primary stain from Gram-negative cells but not from Gram-positive cells. - Finally, **Safranin** is the **counterstain** that stains decolorized Gram-negative cells pink or red. *Iodine → Gentian violet → Alcohol/Acetone → Safranin* - **Iodine** is a mordant and needs a primary stain (like gentian violet) to bind to and form a complex; applying it first would not effectively stain the cells. - The correct sequence requires the primary stain to be applied before the mordant can fix it. *Safranin → Gentian violet → Iodine → Alcohol/Acetone* - **Safranin** is a counterstain and should be applied last to stain the decolorized Gram-negative cells, not as the first reagent. - Applying reagents out of order would lead to incorrect staining results, as **safranin** is meant to provide contrast after decolorization. *Gentian violet → Alcohol/Acetone → Iodine → Safranin* - **Alcohol/Acetone** (decolorizer) is applied too early in this sequence; it should be used after the mordant (iodine) has formed a complex with the primary stain. - Applying the decolorizer before the mordant would prevent the formation of the crystal violet-iodine complex, leading to incorrect differentiation between Gram-positive and Gram-negative bacteria.
Question 65: Which of the following statements about Haemophilus influenzae is true?
- A. It is not capsulated
- B. Invasive strain causes severe diseases but is less common than non-invasive strains
- C. Encapsulated strains are the most common cause of severe H. influenzae diseases (Correct Answer)
- D. Typically grown on chocolate agar in CO2-enriched environment but not clinically relevant to severe diseases
Explanation: ***Encapsulated strains are the most common cause of severe H. influenzae diseases*** - **Encapsulated strains**, particularly **type b (Hib)**, are responsible for the majority of severe invasive H. influenzae infections including **meningitis**, **epiglottitis**, and **septicemia**. - While the Hib vaccine has dramatically reduced the incidence of type b disease, encapsulated strains remain the primary cause of severe H. influenzae infections when they occur. - Non-encapsulated (nontypeable) strains more commonly cause **mucosal infections** like otitis media and bronchitis, but rarely cause severe invasive disease. *It is not capsulated* - This is incorrect. H. influenzae exists in both **encapsulated** (types a-f) and **non-encapsulated (nontypeable)** forms. - The **encapsulated strains**, especially **type b**, are most virulent and cause severe invasive diseases. *Typically grown on chocolate agar in CO2-enriched environment but not clinically relevant to severe diseases* - While the growth requirements are correct (H. influenzae requires **X factor (hemin)** and **V factor (NAD+)** found in chocolate agar), the second part is completely false. - H. influenzae is highly clinically relevant and causes severe diseases including meningitis and epiglottitis. *Invasive strain causes severe diseases but is less common than non-invasive strains* - While this statement is epidemiologically true (nontypeable strains are more prevalent than encapsulated strains), it doesn't address which type causes severe diseases most commonly. - The question asks which statement is true, not which strain type is more prevalent in the general population.
Question 66: Acute Infective Endocarditis with abscess formation is most commonly associated with
- A. Listeria
- B. Staphylococcus (Correct Answer)
- C. Streptococcus
- D. Enterococcus
Explanation: ***Staphylococcus*** - **_Staphylococcus aureus_** is the most common cause of **acute infective endocarditis (AIE)** and is particularly virulent, leading to rapid valve destruction and **abscess formation**. - Its ability to adhere to damaged endothelium and produce various toxins contributes to its high pathogenicity and propensity for complicated infections. *Listeria* - **_Listeria monocytogenes_** is a gram-positive rod known to cause meningitis and gastrointestinal infections, especially in immunocompromised individuals. - While it can rarely cause endocarditis, it is not typically associated with the majority of AIE cases or abscess formation. *Streptococcus* - **_Streptococcus_ species**, particularly **_Viridans streptococci_**, are commonly associated with **subacute infective endocarditis (SIE)** on previously damaged valves. - They generally cause a more indolent course and are less frequently linked to rapid valve destruction or abscess formation compared to _Staphylococcus aureus_. *Enterococcus* - **_Enterococcus_ species** are a common cause of endocarditis, especially in older patients, those with healthcare-associated infections, or urinary tract procedures. - While they can cause serious infections, **_Staphylococcus aureus_** remains the predominant pathogen for acute infective endocarditis with abscess formation.
Question 67: Most common bacterial cause of acute parotitis -
- A. Staphylococcus Aureus (Correct Answer)
- B. Klebsiella
- C. Streptococcus Viridans
- D. Streptococcus Pneumoniae
Explanation: ***Staphylococcus Aureus*** - **_Staphylococcus aureus_** is the most frequent bacterial pathogen isolated in cases of **acute bacterial parotitis**. [2] - It often causes ascending infection from the oral cavity, leading to inflammation and suppuration of the parotid gland. [1] *Streptococcus Pneumonia* - While **_Streptococcus pneumoniae_** can cause various infections, it is not the primary cause of acute bacterial parotitis. - Its infections more commonly manifest as **pneumonia**, otitis media, or meningitis. *Klebsiella* - **_Klebsiella_** species are typically associated with **nosocomial infections**, particularly urinary tract infections and pneumonia. - They are a relatively uncommon cause of acute parotitis compared to _S. aureus_. *Streptococcus Viridans* - **_Streptococcus viridans_** group bacteria are common commensal organisms of the oral cavity and are often implicated in **dental caries** and **endocarditis**. - They are not a usual cause of acute bacterial parotitis.
Question 68: An 18-year-old girl presents with watery diarrhea. Most likely causative agent -
- A. Rotavirus
- B. V. cholerae (Correct Answer)
- C. Salmonella
- D. Shigella
Explanation: ***V. cholerae*** - *Vibrio cholerae* is a classic cause of **acute, severe watery diarrhea** that can lead to rapid dehydration. - While other agents can cause watery diarrhea, *V. cholerae* is primarily associated with large-scale outbreaks of this symptom. *Rota virus* - While rotavirus causes **watery diarrhea**, it primarily affects **infants and young children** and is less common as the most likely cause in an 18-year-old in many settings due to widespread vaccination programs. - The diarrhea, though watery, is often accompanied by **fever and vomiting**. *Salmonella* - *Salmonella* typically causes **inflammatory diarrhea** (dysentery-like symptoms with blood/mucus in stool) or **food poisoning**, with diarrhea that may be watery but is often not as profuse or severe as cholera. - It is more commonly associated with **fever and abdominal cramps**. *Shigella* - *Shigella* causes **bacillary dysentery**, characterized by **bloody, mucoid stools**, abdominal cramps, and fever. - It is not typically associated with solely profuse watery diarrhea.
Question 69: Ehrlichia chaffeensis is the causative agent of -
- A. HME (Correct Answer)
- B. Glandular fever
- C. HGE
- D. None of the options
Explanation: ***HME (Human Monocytic Ehrlichiosis)*** - **Ehrlichia chaffeensis** is the primary causative agent of **Human Monocytic Ehrlichiosis**, a tick-borne illness. - This bacterium primarily infects **monocytes** and macrophages, leading to characteristic intracellular inclusions called **morulae**. *Glandular fever* - Glandular fever, also known as **infectious mononucleosis**, is predominantly caused by the **Epstein-Barr virus (EBV)**. - It is characterized by fever, sore throat, lymphadenopathy, and fatigue, and is not caused by bacteria. *HGE (Human Granulocytic Ehrlichiosis)* - **Human Granulocytic Ehrlichiosis (HGE)** is caused by **Anaplasma phagocytophilum**, not *Ehrlichia chaffeensis*. - HGE primarily targets **neutrophils** (granulocytes), differentiating it from HME which targets monocytes. *None of the options* - This option is incorrect because *Ehrlichia chaffeensis* is indeed the causative agent of HME, as described above.
Question 70: Hanging drop method is used for:
- A. Toxoplasma
- B. Cryptosporidium
- C. Trichomonas (Correct Answer)
- D. Plasmodium
Explanation: ***Trichomonas*** - The **hanging drop method** is a highly effective technique for visualizing the characteristic **motility** of *Trichomonas vaginalis*. - This method allows for the observation of living, unstained organisms directly from clinical samples, making it valuable for rapid diagnosis. *Toxoplasma* - **Toxoplasma gondii** is an intracellular parasite best identified through serological tests for **antibodies** or molecular diagnostics like **PCR**. - It does not exhibit characteristic motility in a hanging drop preparation that would aid in its direct identification. *Cryptosporidium* - **Cryptosporidium** species are typically identified by detecting **oocysts** in stool samples, often using **acid-fast staining** or **immunofluorescence assays**. - Their small size and lack of distinctive motility under a hanging drop method make this technique unsuitable for their diagnosis. *Plasmodium* - **Plasmodium** species, the causative agents of malaria, are diagnosed by visualizing **parasites within red blood cells** on **Giemsa-stained blood smears**. - The hanging drop method would not effectively identify these intracellular parasites for malaria diagnosis.