Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 161: Paul Bunnell Test is used to detect Infectious mononucleosis caused by

A. Cytomegaloviruses
B. Toxoplasmosis
C. None of the above
D. Epstein Barr Virus (Correct Answer)

Explanation: ***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**.

Question 162: Weil felix reaction for Scrub typhus shows positivity for -

A. OX-19
B. OX-K (Correct Answer)
C. OX-2
D. OXK + OX19

Explanation: ***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.

Question 163: Which of the following statements regarding diagnosis of malaria are true?

A. Thin blood film is used to determine parasite concentration.
B. As the sensitivity of microscopy is low, it is useful to detect parasite load at high concentrations only.
C. Jaswant Singh Bhattacharya (JSB) Stain is used. (Correct Answer)
D. Thick blood film is used to detect plasmodium species causing infection.

Explanation: ***Correct: Jaswant Singh Bhattacharya (JSB) Stain is used.*** - **JSB stain** is a rapid and effective method for staining malaria parasites in blood films, particularly in resource-limited settings where traditional Romanowsky stains might not be readily available. - Its quick staining time (3-5 minutes) and ease of use make it valuable for prompt diagnosis of malaria. - This is the **most clearly correct** statement as JSB stain is definitively used in malaria diagnosis. *Thick blood film is used to detect plasmodium species causing infection.* - A **thick blood film** is primarily used for **detecting** the presence of malaria parasites due to its higher sensitivity in screening larger volumes of blood (concentrates parasites 20-40 times). - However, it is **not ideal for species identification** due to distorted RBC morphology and lysed red blood cells. - The statement is **misleading** - while thick films detect parasites, they are not the preferred method for determining the **specific species**. *Thin blood film is used to determine parasite concentration.* - This statement is **technically correct** - thin blood films ARE used to determine parasite concentration (parasitemia) and for speciation. - However, in the context of this question, **JSB stain is the better answer** as it is more specifically and uniquely associated with malaria diagnosis, whereas thin films have broader applications. - Thin films allow accurate quantification of parasitemia (parasites/µL or percentage of infected RBCs) and species identification due to preserved RBC morphology. *As the sensitivity of microscopy is low, it is useful to detect parasite load at high concentrations only.* - **Incorrect** - Microscopy, particularly with thick blood films, has **high sensitivity** and is considered the gold standard for malaria diagnosis. - Microscopy can detect parasites at concentrations as low as **50-100 parasites/µL** (approximately 0.001% parasitemia). - While operator-dependent, it is certainly not limited to detecting parasites only at high concentrations.

Question 164: 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?

A. Tracheal aspirates
B. Sputum
C. Cough plate culture
D. Nasopharyngeal swab (Correct Answer)

Explanation: ***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.

Question 165: Which of the following statements about Anti-Streptolysin O (ASO) titre is FALSE?

A. In acute glomerulonephritis the titre is low
B. It is highly antigenic
C. ASO titre > 200 indicate rheumatic fever
D. In normal people the titre is > 200 (Correct Answer)

Explanation: ***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.

Question 166: Screening test for HIV infection in a patient prior to the development of antibodies (in window period):

A. Western blot
B. p24 antigen (Correct Answer)
C. ELISA
D. All of the options

Explanation: ***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 167: Which of the following methods can be used to detect congenital rubella infection in the fetus?

A. IgA Antibody in fetal blood
B. IgM antibody in fetal blood (Correct Answer)
C. Fetal hemoglobin
D. T4 cell count

Explanation: ***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.

Question 168: Dark ground microscopy is used for detection of?

A. Chlamydia
B. Spirochetes (Correct Answer)
C. Fungi
D. Virus

Explanation: ***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**.

Question 169: Casoni's test is used in the diagnosis of which of the following infections?

A. Toxocariasis
B. Echinococcus granulosus (Correct Answer)
C. Toxoplasmosis
D. Syphilis

Explanation: ***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**.

Question 170: Which of the following is an enrichment medium?

A. Alkaline peptone water
B. Selenite F broth
C. Monsur's taurocholate Tellurite peptone water
D. All of the options (Correct Answer)

Explanation: **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.

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