Diagnostic Microbiology Practice Questions

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: 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.

Q: Which of the following methods can be used to detect congenital rubella infection in the fetus?

IgM antibody in fetal blood. ***IgM antibody in fetal blood*** - **IgM antibodies** are the first antibodies produced in response to an infection and do not cross the **placental barrier**. - Their presence in fetal blood indicates that the fetus has mounted its own immune response to an infection, such as **rubella**. *IgA Antibody in fetal blood* - **IgA antibodies** are primarily found in mucous secretions and are not routinely used for diagnosing congenital infections in fetal blood. - While IgA can be produced by the fetus, **IgM** is the more definitive marker for acute fetal infection. *Fetal hemoglobin* - **Fetal hemoglobin (HbF)** is a normal component of fetal blood and its presence is not indicative of an infection. - HbF levels can be used to assess fetal anemia or certain hemoglobinopathies, but not infectious diseases. *T4 cell count* - **T4 cell count** (CD4+ T cells) is a measure of immune system function, often used in conditions like HIV. - It does not directly detect the presence of the rubella virus or antibodies against it.

Q: Dark ground microscopy is used for detection of?

Spirochetes. ***Spirochetes*** - **Darkfield microscopy** is particularly useful for visualizing **spirochetes** due to their slender, spiral shape and motility, which are difficult to see with conventional light microscopy. - This technique is commonly employed for diagnosing infections like **syphilis**, caused by *Treponema pallidum*, by detecting the characteristic corkscrew-like movements of the bacteria. *Chlamydia* - **Chlamydia** are obligate intracellular bacteria and are too small to be effectively visualized as individual organisms using **darkfield microscopy**. - Detection of *Chlamydia* typically relies on **NAAT (Nucleic Acid Amplification Tests)**, **ELISA**, or **immunofluorescence staining**. *Fungi* - **Fungi** are generally larger organisms with distinct cell walls and can be visualized using **brightfield microscopy** after staining (e.g., KOH mount, India ink). - While some fungal elements might be seen, **darkfield microscopy** is not the primary or most effective method for their detection or detailed identification. *Virus* - **Viruses** are sub-microscopic and cannot be directly visualized with any form of **light microscopy**, including darkfield microscopy, due to their extremely small size. - Their detection requires sophisticated methods such as **electron microscopy**, **PCR**, or **antigen/antibody tests**.

Q: Casoni's test is used in the diagnosis of which of the following infections?

Echinococcus granulosus. ***Echinococcus granulosus*** - The **Casoni's test** is an intradermal skin test historically used to detect antigens of **Echinococcus granulosus**, the causative agent of **hydatid disease**. - A **positive reaction** indicates previous exposure to Echinococcus antigens, leading to a local hypersensitivity reaction. *Toxocariasis* - Diagnosis of **toxocariasis** (visceral larva migrans) primarily relies on **serological tests** like ELISA to detect antibodies against *Toxocara* antigens. - While Casoni's test involves an immune reaction, it is not specific or used for **Toxocara** detection. *Toxoplasmosis* - **Toxoplasmosis** is diagnosed through **serological tests** measuring IgG and IgM antibodies to *Toxoplasma gondii*, or by PCR for detecting parasitic DNA. - The Casoni's test has no role in the diagnosis of **Toxoplasma** infection. *Syphilis* - Diagnosis of **syphilis** involves **nontreponemal tests** (e.g., VDRL, RPR) for screening and **treponemal tests** (e.g., TPPA, FTA-ABS) for confirmation. - The Casoni's test is entirely unrelated to the diagnostic methods for **syphilis**.

Q: Which of the following is an enrichment medium?

All of the options. **All of the options** - **Alkaline peptone water**, **Selenite F broth**, and **Monsur's taurocholate Tellurite peptone water** are all examples of enrichment media. - **Enrichment media** are liquid media that favor the growth of a particular microorganism while inhibiting the growth of others, often by containing specific nutrients or inhibitory substances. *Alkaline peptone water* - This is a common **enrichment broth** used for the isolation of *Vibrio cholerae*. - Its **alkaline pH** inhibits the growth of many non-Vibrio species, while Vibrio thrives. *Selenite F broth* - This medium is specifically designed for the isolation of **Salmonella** and some **Shigella** species from stool samples. - **Selenite** inhibits the growth of coliforms and other gram-negative bacteria, allowing Salmonella and Shigella to multiply. *Monsur's taurocholate Tellurite peptone water* - This specialized enrichment medium is used for the isolation of **Vibrio cholerae**. - The presence of **taurocholate** and **tellurite** selectively inhibits commensal flora, favoring *Vibrio* growth.

Question 1

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 2

Weil felix reaction for Scrub typhus shows positivity for -

Accepted Answer

OX-K

Answer

OX-19

Answer

OX-2

Answer

OXK + OX19

Question 3

Which of the following statements regarding diagnosis of malaria are true?

Accepted Answer

Jaswant Singh Bhattacharya (JSB) Stain is used.

Answer

Thin blood film is used to determine parasite concentration.

Answer

As the sensitivity of microscopy is low, it is useful to detect parasite load at high concentrations only.

Answer

Thick blood film is used to detect plasmodium species causing infection.

Question 4

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 5

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 6

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

Question 7

Which of the following methods can be used to detect congenital rubella infection in the fetus?

Accepted Answer

IgM antibody in fetal blood

Answer

IgA Antibody in fetal blood

Answer

Fetal hemoglobin

Answer

T4 cell count

Question 8

Dark ground microscopy is used for detection of?

Accepted Answer

Spirochetes

Answer

Chlamydia

Answer

Fungi

Answer

Virus

Question 9

Casoni's test is used in the diagnosis of which of the following infections?

Accepted Answer

Echinococcus granulosus

Answer

Toxocariasis

Answer

Toxoplasmosis

Answer

Syphilis

Question 10

Which of the following is an enrichment medium?

Accepted Answer

All of the options

Answer

Alkaline peptone water

Answer

Selenite F broth

Answer

Monsur's taurocholate Tellurite peptone water

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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