Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 171: In the Eijkman test, MacConkey broth tubes are incubated at

A. 25 degree centigrade
B. 52 degree centigrade
C. 44 degree centigrade (Correct Answer)
D. 37 degree centigrade

Explanation: ***44 degree centigrade*** - The **Eijkman test**, used for the detection of **fecal coliforms**, specifically requires incubation at **44°C**. - This elevated temperature selectively inhibits the growth of non-fecal coliforms while allowing the growth of *Escherichia coli* and other fecal coliforms. *25 degree centigrade* - This temperature is too low and would allow the growth of a wide range of **non-fecal coliforms**, compromising the selectivity of the Eijkman test. - It is not a standard incubation temperature for the specific detection of **fecal coliforms**. *52 degree centigrade* - This temperature is too high and would likely inhibit the growth of even **thermotolerant fecal coliforms**, leading to false negative results. - It is generally outside the optimal growth range for most coliforms. *37 degree centigrade* - While 37°C is a common incubation temperature for many bacteria, including most coliforms, it is not sufficiently selective to differentiate **fecal coliforms** from **environmental coliforms** in the Eijkman test. - The higher temperature of **44°C** is crucial for this differentiation.

Question 172: ICMR Delhi and NIV Pune have developed an ELISA kit for COVID-19 antibody detection test known as:

A. Chhaya ELISA
B. Kavach ELISA (Correct Answer)
C. Nishchay ELISA
D. Aarogya ELISA

Explanation: ***Kavach ELISA*** - The **Kavach ELISA** kit was developed by ICMR Delhi and NIV Pune for **COVID-19 antibody detection**. - This kit specifically targets **IgG antibodies** against the SARS-CoV-2 virus, indicating past infection. *Chhaya ELISA* - This is not the name of the **COVID-19 antibody detection kit** developed by ICMR and NIV. - No widely recognized ELISA kit for COVID-19 with the name "Chhaya" has been reported. *Nishchay ELISA* - This is not the correct name for the **COVID-19 antibody detection kit** developed by these Indian institutions. - "Nishchay" is not associated with an official COVID-19 antibody test from ICMR/NIV. *Aarogya ELISA* - The name "Aarogya" is associated with other initiatives (e.g., Aarogya Setu app), but not with an **ICMR/NIV developed ELISA kit** for COVID-19 antibody detection. - This option does not refer to the specific kit developed by the stated organizations.

Question 173: Dark ground microscopy is used to see?

A. Capsule
B. Refractile organisms (Correct Answer)
C. Fimbriae
D. Flagella

Explanation: ***Refractile organisms*** - **Dark-field (dark-ground) microscopy** is the gold standard technique for visualizing **spirochetes**, particularly **Treponema pallidum** (causative agent of syphilis). - These organisms appear as **bright, refractile, motile** structures against a **dark background** due to the scattered light illumination. - Spirochetes are too thin (0.1-0.2 μm) to be seen with conventional bright-field microscopy but become visible with dark-field illumination. - **Clinical application**: Used for direct examination of chancre exudate in primary syphilis and other spirochetal infections (Leptospira, Borrelia). *Capsule* - The **capsule** is best visualized using **negative staining** techniques such as **India ink preparation** or **Anthony's capsule stain**. - In negative staining, the capsule appears as a **clear halo** around the bacterial cell against a dark background. - Dark-field microscopy does not specifically highlight capsules. *Flagella* - **Flagella** require special staining techniques such as **Leifson's stain**, **silver impregnation methods**, or **electron microscopy** for visualization. - While motility can be observed with dark-field microscopy, the technique is not used to visualize flagellar structure itself. - The primary diagnostic use of dark-field is for spirochetes, not flagellar demonstration. *Fimbriae* - **Fimbriae (pili)** are short, hair-like appendages used for bacterial adherence. - These structures are **too fine and small** (3-10 nm diameter) to be resolved by light microscopy, including dark-field. - **Electron microscopy** is required for clear visualization of fimbriae.

Question 174: Following statements are true on dark ground microscopy EXCEPT -

A. Effective for visualizing slender organisms like spirochetes.
B. Uses a dark field condenser with a central circular stop for illumination.
C. Relies on transmitted light for illumination. (Correct Answer)
D. Objects appear self-luminous against a dark background due to scattered light.

Explanation: ***Relies on transmitted light for illumination.*** - **Dark ground microscopy** works by illuminating the specimen with light that is *not directly transmitted through it*, but rather scattered by the specimen. - This method creates a brilliant image of the specimen against a dark background, making the statement that it "relies on transmitted light" incorrect. *Effective for visualizing slender organisms like spirochetes.* - **Dark ground microscopy** is particularly useful for observing **unstained, living and slender organisms** such as **spirochetes** (e.g., *Treponema pallidum*, *Leptospira*, *Borrelia*), which might be difficult to visualize with bright-field microscopy. - The technique enhances the contrast of these delicate structures, making them appear bright against a dark field. *Uses a dark field condenser with a central circular stop for illumination.* - A **dark field condenser** is a key component of this type of microscope, which includes a **central opaque disk (circular stop)**. - This stop blocks direct transmitted light from passing through the specimen, allowing only **obliquely scattered light** to reach the objective lens. *Objects appear self-luminous against a dark background due to scattered light.* - In **dark ground microscopy**, the specimen **scatters light obliquely** that then enters the objective lens, making the object appear bright or "self-luminous." - This effect results from the **light being scattered and refracted** by the specimen at oblique angles, rather than transmitting directly through it, creating a high-contrast image against a dark field.

Question 175: What is injected in tuberculin Mantoux test?

A. Live bacteria
B. Killed bacteria
C. Live attenuated antigen
D. Purified Protein Derivative (PPD) (Correct Answer)

Explanation: ***Purified Protein Derivative (PPD)*** - The Mantoux test involves injecting **Purified Protein Derivative (PPD)**, which constitutes antigens derived from *Mycobacterium tuberculosis*. - This PPD is used to detect a **delayed-type hypersensitivity reaction** in individuals previously exposed to *Mycobacterium tuberculosis* or vaccinated with BCG. *Live bacteria* - Injecting **live bacteria** (e.g., *Mycobacterium tuberculosis*) would cause active infection in the individual, which is not the purpose of a diagnostic test. - The goal of the Mantoux test is to elicit an immune response to bacterial antigens, not to induce disease. *Killed bacteria* - While vaccines can use **killed bacteria**, the Mantoux test specifically uses **purified protein extracts** rather than whole killed bacterial cells. - Using whole killed bacteria might lead to more non-specific or severe local reactions. *Live attenuated antigen* - **Live attenuated antigens** are typically used in vaccines (e.g., BCG vaccine) to stimulate long-term immunity, not for diagnostic skin tests. - The Mantoux test is a diagnostic tool to assess previous immune exposure, not to confer immunity.

Question 176: Selenite F broth is:

A. Enriched medium
B. Selective medium
C. Indicator medium
D. Enrichment medium (Correct Answer)

Explanation: ***Enrichment medium*** - Selenite F broth is a **liquid medium** designed to **promote the growth of specific bacteria** (e.g., *Salmonella* and *Shigella*) while inhibiting the growth of commensal flora. - Its primary purpose is to increase the **number of target organisms** from a clinical sample to a detectable level for subsequent plating on selective media. *Enriched medium* - An enriched medium contains **added nutrients** (e.g., blood, serum, or growth factors) to support the growth of a wide range of fastidious bacteria. - While Selenite F broth has nutrients, its main role is not for general growth but for **selective amplification** of certain pathogens. *Selective medium* - A selective medium contains agents that **inhibit the growth of unwanted microorganisms** while allowing the growth of specifically desired ones. - Though Selenite F broth exhibits selective properties, it is primarily an **enrichment broth** used *before* plating on solid selective media for isolation. *Indicator medium* - An indicator medium contains a substance (e.g., a pH indicator) that changes color in response to microbial metabolic activity, helping to **differentiate between different types of bacteria**. - Selenite F broth does not contain indicators for direct differentiation of colonies; its role is about enhancing numbers, not phenotypic differentiation.

Question 177: A farmer presents to the emergency department with painful inguinal lymphadenopathy and a history of fever and flu-like symptoms. Clinical examination reveals an ulcer on the leg. Which of the following stains should be used to detect suspected bipolar-stained organisms?

A. Albe's stain
B. McFadyean's stain
C. Wayson's stain (Correct Answer)
D. Ziehl-Neelsen stain

Explanation: ***Wayson's stain*** - This stain is specifically used for the detection of **Yersinia pestis**, the causative agent of **plague**, which often presents with **bipolar staining**. - Clinical features like **painful inguinal lymphadenopathy** (buboes), fever, flu-like symptoms, and an ulcer (possibly an inoculation site) are highly suggestive of **plague**. *Albe's stain* - **Albe's stain** is used for demonstrating **bacterial capsules**, not for bipolar-stained organisms. - It would not specifically identify **Yersinia pestis** in this context. *Mc Fayden's stain* - **McFadyen's stain** is primarily used to detect the capsule of **Bacillus anthracis** (anthrax) from smears. - While helpful for anthrax, it is not the specific stain for bipolar staining of **Yersinia pestis**. *Ziehl Nelson stain* - **Ziehl-Neelsen stain** is an **acid-fast stain** used to identify organisms with high mycolic acid content in their cell walls, such as **Mycobacterium tuberculosis**. - It is not suitable for visualizing gram-negative bacteria like **Yersinia pestis** or their bipolar staining characteristics.

Question 178: Sensitivity of urinary antigen test for Legionella pneumophila serogroup 1 is?

A. 95%
B. 90% (Correct Answer)
C. 80%
D. 99%

Explanation: ***90%*** - The **urinary antigen test** for **Legionella pneumophila serogroup 1** has a sensitivity generally reported in the range of **70-90%**, with **90%** representing the **upper end** of this range in optimal conditions. - This high sensitivity means the test is effective at identifying true positive cases, particularly in severe infections with high bacterial loads. - The test specifically detects **serogroup 1**, which accounts for approximately **70-80%** of all Legionella infections. *95%* - A sensitivity of **95%** is **higher than typically reported** for the Legionella urinary antigen test. - Most studies and clinical references cite sensitivity in the **70-90%** range, making 95% an overestimate. *80%* - A sensitivity of **80%** falls within the commonly reported range but represents the **mid-to-upper range** rather than the maximum sensitivity. - While this is a reasonable estimate, the test can achieve higher sensitivity (**up to 90%**) particularly in severe pneumonia cases. *99%* - A sensitivity of **99%** would indicate an almost perfect test with very few **false negatives**. - This level of sensitivity is **not achieved** by the Legionella urinary antigen test, which has inherent limitations including inability to detect non-serogroup 1 strains and reduced sensitivity in mild infections.

Question 179: Which of the following is an enrichment medium?

A. Egg media
B. Chocolate media
C. Selenite F broth (Correct Answer)
D. Meatextract media

Explanation: ***Selenite F broth*** - This is a classic example of an **enrichment medium**, which contains substances that **inhibit the growth of commensal organisms** while promoting the growth of target pathogens. - It is frequently used for the isolation of **Salmonella** and **Shigella** from stool samples, where they are often outnumbered by normal gut flora. *Egg media* - This typically refers to media like **Lowenstein-Jensen (LJ) medium**, which is an **enriched solid medium** used primarily for the cultivation of **Mycobacterium tuberculosis**. - While it is enriched, it's not specifically a broth designed to selectively increase a small number of pathogens from a mixed population. *Chocolate media* - This is an **enriched, non-selective solid medium** that contains **heated blood**, which lyses red blood cells and releases essential growth factors like **heme (X factor)** and **NAD (V factor)**. - It is used for fastidious bacteria such as **Haemophilus influenzae** and **Neisseria gonorrhoeae**, but it doesn't selectively promote growth by inhibiting other microbes. *Meatextract media* - This refers to a **general-purpose basic medium** in microbiology, often found in formulations like **nutrient broth** or **nutrient agar**. - It provides a source of nutrients for a wide range of non-fastidious bacteria, but it lacks the selective components characteristic of enrichment media.

Question 180: A Giemsa stain of a thin peripheral blood smear is prepared. Which of the following cannot be diagnosed?

A. Coxiella burnettii (Correct Answer)
B. Bartonella henselae
C. Ehrlichia chaffeensis
D. Toxoplasma gondii

Explanation: ***Coxiella burnettii*** - *Coxiella burnettii* causes **Q fever** and is an **obligate intracellular bacterium** that resides primarily in **tissue macrophages** (lungs, liver, bone marrow), not in circulating blood cells. - It is **not found in peripheral blood smears** because it does not infect circulating leukocytes in significant numbers that would allow microscopic visualization. - Diagnosis requires **serology** (most common), **PCR**, or specialized culture in BSL-3 facilities—direct microscopic visualization in blood smears is not possible. *Bartonella henselae* - Causes **Cat scratch disease** and can invade **red blood cells**, making it potentially visible on Giemsa-stained blood smears, particularly in immunocompromised patients with bacillary angiomatosis or bacteremia. - While difficult and not the primary diagnostic method, it *can* be visualized in peripheral blood, unlike *Coxiella*. *Ehrlichia chaffeensis* - Causes **human monocytotropic ehrlichiosis (HME)** and forms characteristic **morulae** (berry-like clusters) within the cytoplasm of **monocytes**. - These morulae are readily visible on **Giemsa-stained peripheral blood smears** and are a key diagnostic finding, making this condition easily diagnosed by this method. *Toxoplasma gondii* - An **intracellular parasite** whose **tachyzoites** can occasionally be found in **peripheral blood leukocytes** during acute infection, especially in immunocompromised patients. - While rare and not the primary diagnostic method (serology/PCR preferred), tachyzoites *can* be observed in blood smears during active parasitemia.

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