Diagnostic Microbiology Practice Questions

Q: Mark the false statement regarding testing of COVID-19.

First line screening assay: N gene assay.. ***First line screening assay: N gene assay.*** * While the **nucleocapsid (N) gene** is a common target for COVID-19 PCR assays, the statement that it is the *first-line screening assay* is often a simplification or outdated, as many assays target multiple genes (e.g., N, E, RdRp) for increased sensitivity and specificity from the outset. * Different health organizations and diagnostic kits have varied recommendations for initial screening targets, but there isn't a universal consensus that the N gene alone is the specific 'first-line screening assay' in all contexts when considering the breadth of available PCR tests. *Confirmatory assay: RdRp gene assay.* * The **RdRp (RNA-dependent RNA polymerase) gene** is a highly specific and conserved target for SARS-CoV-2 detection and is often used in **confirmatory PCR assays**. * Detection of the RdRp gene, sometimes alongside other targets like the E (envelope) gene, helps in confirming the presence of the virus. *Peripheral ground glass opacities on CT is the hallmark feature.* * **Peripheral ground glass opacities (GGOs)** are indeed a **hallmark radiological finding** in COVID-19 pneumonia, seen on CT scans. * These opacities reflect alveolar inflammation and fluid accumulation, especially in the early and moderate stages of the disease. *Most predominant method of diagnosis of COVID-19 is PCR.* * **Reverse transcription-polymerase chain reaction (RT-PCR)** remains the **gold standard and most predominant method** for diagnosing active COVID-19 infection. * PCR tests directly detect viral genetic material, offering high sensitivity and specificity in symptomatic and asymptomatic individuals.

Q: Antemortem diagnosis of rabies is made with:

Corneal impression smear. ***Corneal impression smear*** - A **corneal impression smear** can detect viral antigens in the cornea using fluorescent antibody staining, a method that can be performed on living patients. - This technique provides a relatively rapid and non-invasive way to diagnose rabies **antemortem**. *Inoculation in culture media* - Rabies virus is notoriously difficult to culture in standard cell culture media, making this method impractical and unreliable for **antemortem diagnosis**. - While viral isolation is possible in specialized research settings, it is not a routine diagnostic tool for rabies in living patients. *Negri bodies in hippocampus* - **Negri bodies** are eosinophilic inclusions found in the cytoplasm of neurons, particularly in the hippocampus, which are pathognomonic for rabies. - However, their detection requires **postmortem brain tissue biopsy**, making this a **postmortem diagnostic** method, not antemortem. *Rabies virus specific antibodies* - While the presence of **rabies virus-specific antibodies** (particularly in CSF) can indicate exposure and infection, they often appear late in the disease course. - The detection of antibodies may not be reliable for early **antemortem diagnosis**, especially in naive individuals whose immune response has not yet fully developed.

Q: Auramine and rhodamine staining are used for:

Rapid diagnosis of TB. ***Rapid diagnosis of TB*** - **Auramine and rhodamine** are fluorescent dyes that bind to the **mycolic acid** in the cell walls of mycobacteria. - This method allows for a quicker screening of sputum samples for **Mycobacterium tuberculosis** compared to traditional Ziehl-Neelsen staining, as the fluorescent bacilli are easily identifiable under a fluorescent microscope. *Nocardia* - While *Nocardia* species are **partially acid-fast**, they are typically identified using modified Ziehl-Neelsen or Kinyoun stains, not auramine-rhodamine. - The primary clinical relevance of auramine-rhodamine staining is for **mycobacterial detection**, not *Nocardia*. *Spirochetes* - **Spirochetes** are too thin to be visualized effectively by direct light microscopy with auramine-rhodamine or even Gram stain. - They are typically detected using **darkfield microscopy**, immunofluorescence, or silver stains like **Warthin-Starry stain**. *All of the options.* - This option is incorrect because while *Nocardia* shares some staining characteristics with *Mycobacteria*, auramine-rhodamine is specifically optimized and primarily used for the detection of **acid-fast bacilli** like *Mycobacterium tuberculosis*, not *Nocardia* or spirochetes.

Q: Rapid examination of tubercle bacilli is possible with:

Auramine-Rhodamine fluorescent stain.. ***Auramine-Rhodamine fluorescent stain*** - This method provides a **more rapid** means of examining tubercle bacilli because it allows for the use of lower magnifications (e.g., 20x or 40x), enabling the viewing of a larger field and faster scanning of smears. - The **fluorescent bacilli** appear as bright yellow-green rods against a dark background, making them easier and quicker to detect compared to conventional light microscopy. *Ziehl–Neelsen method for AFB detection* - While it is a standard method for identifying **acid-fast bacilli (AFB)**, it requires higher magnification (100x oil immersion) and more extensive scanning to detect the red bacilli, making it **slower** than fluorescent methods. - It uses a basic fuchsin stain and heat to drive the stain into the waxy mycobacterial cell wall, followed by decolorization with acid-alcohol. *Giemsa stain for blood smears* - This stain is primarily used for identifying **blood parasites** (e.g., malaria) and cellular morphology in **hematological disorders**, and is not suitable for detecting mycobacteria. - It stains nuclear and cytoplasmic components differently, providing morphological details of blood cells and pathogens. *Kinyoun cold acid-fast stain* - Similar to the Ziehl-Neelsen stain, the Kinyoun method is also a conventional acid-fast stain that does not involve heat, but it still requires **high magnification** and **slow scanning** for detection. - It uses a higher concentration of basic fuchsin and a wetting agent to penetrate the cell wall without heating, but it is not considered rapid.

Q: Paul Bunnell Test is used to detect Infectious mononucleosis caused by

Epstein Barr Virus. ***Epstein Barr Virus*** - The **Paul-Bunnell test** detects **heterophile antibodies** which are characteristic of infectious mononucleosis caused by **Epstein-Barr Virus (EBV)**. - These antibodies are IgM class antibodies that agglutinate sheep or horse red blood cells and are not absorbed by guinea pig kidney cells. *Cytomegaloviruses* - **Cytomegalovirus (CMV)** can cause a mononucleosis-like syndrome, but it typically does not induce the production of **heterophile antibodies** detectable by the Paul-Bunnell test. - Diagnosis of CMV infection usually relies on detecting **CMV DNA**, antigens, or specific antibodies. *Toxoplasmosis* - **Toxoplasmosis** is caused by the parasite **Toxoplasma gondii** and can, in some cases, present with lymphadenopathy and fatigue, mimicking infectious mononucleosis. - However, it does not produce **heterophile antibodies** and therefore would not be detected by the Paul-Bunnell test. *None of the above* - This option is incorrect because the **Paul-Bunnell test specifically identifies infectious mononucleosis caused by Epstein-Barr Virus**.

Q: Weil felix reaction for Scrub typhus shows positivity for -

OX-K. ***Correct Answer: OX-K*** - The **Weil-Felix reaction** for Scrub typhus specifically detects antibodies against the **OX-K antigen**, which is derived from *Proteus mirabilis* but shares antigenic determinants with *Orientia tsutsugamushi* (causative agent of Scrub typhus). - A positive **OX-K** reaction indicates the presence of these antibodies, suggesting an active or recent infection with **Scrub typhus**. - This is the **characteristic and specific finding** for Scrub typhus in the Weil-Felix test. *Incorrect: OX-19* - This antigen is used to detect **epidemic typhus** (*Rickettsia prowazekii*) and **murine typhus** (*Rickettsia typhi*) in the Weil-Felix reaction. - It does not show significant cross-reactivity with *Orientia tsutsugamushi*, the causative agent of Scrub typhus. - Would be **negative** in Scrub typhus cases. *Incorrect: OX-2* - The **OX-2 antigen** is primarily used to detect **spotted fever group rickettsiae**, such as *Rickettsia rickettsii* (Rocky Mountain spotted fever). - It is **not relevant** for the diagnosis of Scrub typhus and would typically show a negative result in such cases. *Incorrect: OXK + OX19* - While both antigens are part of the Weil-Felix reaction panel, positivity for **OX-K alone** is characteristic of Scrub typhus. - **OX-19 positivity** points to epidemic or murine typhus, which are different rickettsial diseases. - This combination is not the typical pattern for Scrub typhus.

Q: A 7-month-old, partially immunized child presented with cough ending in a characteristic whoop. Which of the following is considered the best type of specimen to isolate the organism and confirm the diagnosis?

Nasopharyngeal swab. ***Nasopharyngeal swab*** - A **nasopharyngeal swab** is the **gold standard** for collecting Bordetella pertussis specimens because the organism primarily colonizes the nasopharynx. - The flexible wire swab allows for proper collection from the posterior nasopharynx, ensuring a higher yield for culture or **PCR testing**. *Tracheal-aspirates* - While tracheal aspirates can be used for respiratory infections, they are generally **too invasive** for routine diagnosis of pertussis in an infant. - The organism is primarily in the upper respiratory tract, making a less invasive collection method preferable and effective. *Sputum* - Sputum samples are often difficult to obtain reliably from infants and are prone to **contamination** with oral flora. - The organism is not typically found in sufficient quantities in sputum for optimal diagnostic yield. *Cough plate culture* - **Cough plate culture** involves holding a specialized agar plate in front of the patient's mouth during a cough, which is **difficult and unreliable** in a 7-month-old. - This method also carries a high risk of **contamination** from oropharyngeal flora and is less sensitive than nasopharyngeal sampling.

Q: Which of the following statements about Anti-Streptolysin O (ASO) titre is FALSE?

In normal people the titre is > 200. ***In normal people the titre is > 200*** - Normal ASO titre is typically **less than 200 IU/mL**. A titre above this level suggests a recent **Streptococcal infection**. - A titre > 200 in a normal, asymptomatic individual would be considered elevated and warrant further investigation, not a normal finding. *In acute glomerulonephritis the titre is low* - Acute glomerulonephritis (AGN) can follow Lancefield group A streptococcal infection, and the ASO titre is **often low** because the nephritogenic strains of *Streptococcus* are **weak producers of Streptolysin O**. - Other streptococcal antibodies, like **anti-DNase B**, are more consistently elevated in post-streptococcal glomerulonephritis. *It is highly antigenic* - **Streptolysin O** is indeed a **highly antigenic** exotoxin produced by Group A Streptococcus, which elicits a significant immune response and the production of ASO antibodies. - The detection of these antibodies forms the basis of the ASO titre test, indicating a potent immune reaction to the antigen. *ASO titre > 200 indicate rheumatic fever* - An ASO titre **greater than 200 IU/mL** (or higher in some laboratories, especially in children) is generally indicative of a recent **streptococcal infection** and supports a diagnosis of **acute rheumatic fever** in the presence of other clinical criteria. - The elevated titre reflects the body's immune response to the streptococcal infection that can precede rheumatic fever.

Q: A child presents with complaints of cough. Characteristic inspiratory whoop present. He is not immunised. Sample for investigation is:

Nasopharyngeal swab. ***Nasopharyngeal swab*** - A **nasopharyngeal swab** is the preferred sample for diagnosing **whooping cough** (pertussis) caused by *Bordetella pertussis*, especially for PCR testing. - This method provides the best yield for detecting the bacterium which colonizes the **nasopharynx**. *Sputum culture* - **Sputum culture** is generally not recommended for diagnosing pertussis as *Bordetella pertussis* is a fastidious organism and less likely to be found in sputum. - Sputum collection can also be challenging and less effective in young children. *Tracheal aspiration* - **Tracheal aspiration** is an invasive procedure and is generally reserved for critically ill patients or those with specific indications like ventilator-associated pneumonia, not routine pertussis diagnosis. - The primary site for *Bordetella pertussis* colonization is the nasopharynx, not typically deep within the trachea for initial sampling. *Cough plate culture* - **Cough plate culture** involves exposing a culture medium directly to a patient's cough, but it is an older technique with lower sensitivity and specificity compared to nasopharyngeal swabs and PCR. - Modern diagnostic methods, such as PCR from nasopharyngeal samples, offer faster and more accurate results for pertussis.

Q: Screening test for HIV infection in a patient prior to the development of antibodies (in window period):

p24 antigen. ***p24 antigen*** - The **p24 antigen** test detects a structural protein of HIV, which becomes detectable before the development of antibodies (during the **window period**). - This test is crucial for early detection of HIV infection, especially when antibody tests might still be negative due to the time lag in immune response. *Western blot* - The **Western blot** is a confirmatory test for HIV, detecting specific antibodies to various HIV proteins. - It becomes positive only after the development of antibodies, making it unsuitable for detecting infection during the **window period**. *ELISA* - **ELISA** (Enzyme-Linked Immunosorbent Assay) is a common screening test for HIV, primarily detecting **HIV antibodies**. - Like Western blot, it relies on antibody presence and thus will be negative during the **window period**. *All of the options* - This option is incorrect because both **Western blot** and **ELISA** detect antibodies, making them unsuitable for screening during the **window period**. - Only the **p24 antigen** test can detect HIV infection before antibody seroconversion.

Question 1

Mark the false statement regarding testing of COVID-19.

Accepted Answer

First line screening assay: N gene assay.

Answer

Confirmatory assay: RdRp gene assay.

Answer

Peripheral ground glass opacities on CT is the hallmark feature.

Answer

Most predominant method of diagnosis of COVID-19 is PCR.

Question 2

Antemortem diagnosis of rabies is made with:

Accepted Answer

Corneal impression smear

Answer

Inoculation in culture media

Answer

Negri bodies in hippocampus

Answer

Rabies virus specific antibodies

Question 3

Auramine and rhodamine staining are used for:

Accepted Answer

Rapid diagnosis of TB

Answer

Nocardia

Answer

Spirochetes

Answer

All of the options

Question 4

Rapid examination of tubercle bacilli is possible with:

Accepted Answer

Auramine-Rhodamine fluorescent stain.

Answer

Ziehl–Neelsen method for AFB detection.

Answer

Giemsa stain for blood smears.

Answer

Kinyoun cold acid-fast stain.

Question 5

Paul Bunnell Test is used to detect Infectious mononucleosis caused by

Accepted Answer

Epstein Barr Virus

Answer

Cytomegaloviruses

Answer

Toxoplasmosis

Answer

None of the above

Question 6

Weil felix reaction for Scrub typhus shows positivity for -

Accepted Answer

OX-K

Answer

OX-19

Answer

OX-2

Answer

OXK + OX19

Question 7

A 7-month-old, partially immunized child presented with cough ending in a characteristic whoop. Which of the following is considered the best type of specimen to isolate the organism and confirm the diagnosis?

Accepted Answer

Nasopharyngeal swab

Answer

Tracheal aspirates

Answer

Sputum

Answer

Cough plate culture

Question 8

Which of the following statements about Anti-Streptolysin O (ASO) titre is FALSE?

Accepted Answer

In normal people the titre is > 200

Answer

In acute glomerulonephritis the titre is low

Answer

It is highly antigenic

Answer

ASO titre > 200 indicate rheumatic fever

Question 9

A child presents with complaints of cough. Characteristic inspiratory whoop present. He is not immunised. Sample for investigation is:

Accepted Answer

Nasopharyngeal swab

Answer

Sputum culture

Answer

Tracheal aspiration

Answer

Cough plate culture

Question 10

Screening test for HIV infection in a patient prior to the development of antibodies (in window period):

Accepted Answer

p24 antigen

Answer

Western blot

Answer

ELISA

Answer

All of the options

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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