Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 231: Sabin Feldman dye test is used for diagnosis of which of the following condition?

A. Botulism
B. Toxoplasmosis (Correct Answer)
C. Sarcoidosis
D. Yellow fever

Explanation: ***Toxoplasmosis*** The **Sabin-Feldman dye test** is a **serological assay** used to detect specific IgG antibodies against *Toxoplasma gondii*, the causative agent of toxoplasmosis. It measures the ability of antibodies in a patient's serum to prevent the cytoplasmic staining of live toxoplasma tachyzoites by methylene blue dye, indicating an **active immune response** to the parasite. This classic test has high sensitivity and specificity for detecting toxoplasma antibodies and is considered the gold standard serological test, though it has been largely replaced by ELISA and IFA in routine practice due to the requirement for live organisms. *Botulism* Botulism is diagnosed through **toxin detection** in serum, stool, or food samples using mouse bioassay, or by culturing *Clostridium botulinum* from clinical specimens. The Sabin-Feldman dye test is not relevant for the diagnosis of botulism, which is a **neuroparalytic disease** caused by botulinum neurotoxin blocking acetylcholine release at neuromuscular junctions. *Sarcoidosis* Sarcoidosis is a multisystem **granulomatous disease** diagnosed primarily by **tissue biopsy** showing non-caseating granulomas along with compatible clinical and radiological findings. Supportive tests include elevated serum ACE levels and Kveim test (historical). There is no serological test like the Sabin-Feldman dye test associated with the diagnosis of sarcoidosis, as it is not an infectious disease requiring antibody detection. *Yellow fever* Yellow fever is a **viral hemorrhagic disease** caused by a flavivirus, diagnosed by detecting viral RNA through RT-PCR or specific IgM antibodies in the acute phase of infection using ELISA or immunofluorescence. The Sabin-Feldman dye test is not used for viral infections like yellow fever, as it specifically targets **toxoplasma antibodies** and has no role in arboviral disease diagnosis.

Question 232: Which culture medium contains potassium tellurite?

A. TCBS medium
B. Monsur medium (Correct Answer)
C. BYCE medium
D. Muller Hinton agar

Explanation: ***Monsur medium*** - **Monsur medium** is a selective culture medium specifically designed for the isolation of *Vibrio cholerae*. - It contains **potassium tellurite** which acts as a selective agent, inhibiting the growth of many gram-positive bacteria and some gram-negative bacteria, while allowing *Vibrio* species to grow. *TCBS medium* - **TCBS (Thiosulfate Citrate Bile Salts Sucrose) agar** is a selective medium for *Vibrio* species, but it does not contain potassium tellurite. - Its selective components include **bile salts** and **thiosulfate**, which inhibit gram-positive bacteria and most enterics. *BYCE medium* - **BYCE (Buffered Charcoal Yeast Extract) agar** is primarily used for the isolation of **Legionella pneumophila**. - It contains **activated charcoal** and **yeast extract** as key components, not potassium tellurite. *Muller Hinton agar* - **Muller Hinton agar** is a general-purpose medium primarily used for **antimicrobial susceptibility testing** (Kirby-Bauer method). - It does not contain potassium tellurite and is not a selective medium for specific pathogens.

Question 233: Which test cannot differentiate endemic and epidemic typhus

A. Complement fixation test
B. Immunofluorescence
C. ELISA
D. Weil-Felix reaction (Correct Answer)

Explanation: ***Weil-Felix reaction*** - The **Weil-Felix reaction** relies on agglutination of *Proteus* antigens, a non-specific test for typhus. - It detects antibodies that cross-react with *Proteus* antigens, and these antibodies are common to both **endemic** and **epidemic typhus**, making it unable to differentiate between them. *Complement fixation test* - The **complement fixation test** can differentiate between typhus types by detecting specific antibodies to *Rickettsia prowazekii* (epidemic) and *Rickettsia typhi* (endemic), as it measures specific antigen-antibody reactions. - It uses specific antigens from each *Rickettsia* species to identify which type of typhus is present. *Immunofluorescence* - **Immunofluorescence assays (IFAs)** are highly specific and can differentiate between endemic and epidemic typhus by detecting specific antibodies against *R. prowazekii* or *R. typhi*. - They allow for the visualization of antibody binding to specific rickettsial antigens, providing differential diagnosis. *ELISA* - **ELISA** (Enzyme-Linked Immunosorbent Assay) tests can be designed with specific antigens from *R. prowazekii* and *R. typhi* to distinguish between epidemic and endemic typhus. - This method offers high sensitivity and specificity in detecting type-specific antibodies.

Question 234: ELISA test for virulence antigen is used for which type of E. coli?

A. Enterohemorrhagic E. coli (EHEC)
B. Enterotoxigenic E. coli (ETEC)
C. Enteroinvasive E. coli (EIEC) (Correct Answer)
D. Enteroaggregative E. coli (EAEC)

Explanation: ***Enteroinvasive E. coli (EIEC)*** - The **virulence antigen** (also known as the **invasion plasmid antigen**, Ipa) detected by ELISA is a key indicator of EIEC. - EIEC invades the **intestinal epithelial cells**, leading to symptoms similar to **shigellosis**, including dysentery. *Enterotoxigenic E. coli (ETEC)* - ETEC produces **heat-labile (LT)** and/or **heat-stable (ST) toxins**, which are the virulence factors, not an invasion antigen. - It is a common cause of **traveler's diarrhea**, leading to watery stools without significant inflammation. *EHEC* - **Enterohemorrhagic E. coli (EHEC)** produces **Shiga toxins (Stx1 and Stx2)**, which cause bloody diarrhea and can lead to hemolytic uremic syndrome (HUS). - Detection methods typically focus on these toxins or the genes encoding them, rather than an invasion antigen. *EAEC* - **Enteroaggregative E. coli (EAEC)** is characterized by its distinctive **aggregative adherence** to epithelial cells, forming a "stacked brick" pattern. - Its virulence is associated with **adhesins** and **toxins** that cause persistent watery diarrhea.

Question 235: What is the primary method for diagnosing Pneumocystis jirovecii pneumonia?

A. Positive serology
B. Growth on artificial media
C. Sputum examination for trophozoites and cysts under microscope
D. Bronchoalveolar lavage (BAL) with microscopic examination (Correct Answer)

Explanation: **Bronchoalveolar lavage (BAL) with microscopic examination** - **BAL** provides samples directly from the lower respiratory tract, where **Pneumocystis jirovecii** organisms are typically concentrated during infection. - **Microscopic examination** of BAL fluid using specific stains (e.g., Gomori methenamine silver, Giemsa) allows for direct visualization of the characteristic **cysts and trophic forms** of the organism. *Sputum examination for trophozoites and cysts under microscope* - While sputum can sometimes show *Pneumocystis* organisms, its sensitivity is significantly lower than BAL due to the difficulty in obtaining adequate deep lung samples and the organism's sparse presence in sputum. - Spontaneous sputum is often not sufficient for diagnosis, and induced sputum, while better, still lags behind BAL in diagnostic yield. *Positive serology* - **Serology (antibody tests)** for *Pneumocystis jirovecii* are generally not useful for diagnosing active PJP because most healthy individuals have antibodies due to past exposure, and immunosuppressed patients (who are most at risk for PJP) often have impaired antibody responses. - Therefore, a positive serology does not confirm active infection, nor does a negative one rule it out reliably in the immunocompromised. *Growth on artificial media* - *Pneumocystis jirovecii* is an **obligate intracellular parasite** and **cannot be cultured** using standard artificial laboratory media. - This makes direct visualization of the organism from clinical samples (like BAL) or molecular methods (like PCR) the primary diagnostic approaches.

Question 236: What was the Complement Fixation test historically used for?

A. A historical test for syphilis (Wassermann test) - now obsolete (Correct Answer)
B. A historical test for typhoid fever diagnosis
C. A historical test for viral infections (e.g., influenza)
D. A historical test for rickettsial diseases

Explanation: ***A historical test for syphilis (Wassermann test) - now obsolete*** - The **Wassermann test** was the classic and most famous example of a complement fixation test, developed in 1906 to diagnose **syphilis** by detecting antibodies against *Treponema pallidum* antigens. - This test is now largely **obsolete** due to its lack of specificity and sensitivity compared to modern serological tests like **VDRL, RPR, TPHA, and FTA-ABS**. - The Wassermann test was the **gold standard** for syphilis diagnosis for several decades and remains the most historically significant application of complement fixation testing. *Incorrect - Typhoid fever diagnosis* - While complement fixation tests can detect antibodies against various pathogens, the primary historical test for **typhoid fever** was the **Widal test**, which detects agglutinating antibodies. - The Widal test is based on **agglutination**, not complement fixation, and targets *Salmonella typhi* O and H antigens. *Incorrect - Viral infections* - Complement fixation tests were indeed used to some extent for **viral infections** including influenza, but they were not the most prominent or historically significant application like the Wassermann test for syphilis. - For many viral infections, other serological tests like **hemagglutination inhibition** or **neutralization assays** were more commonly employed. *Incorrect - Rickettsial diseases* - Complement fixation tests could be applied to rickettsial diseases, but the **Weil-Felix test** (an agglutination-based test) was more commonly and historically used for **rickettsial diseases** like typhus due to its relative simplicity in detecting cross-reacting antibodies. - The Weil-Felix test detects antibodies against *Proteus* species that cross-react with rickettsial antigens.

Question 237: Test used for screening for urinary tract infection is

A. Paul-Bunnell test
B. Fenton's test
C. Nitrite test (Correct Answer)
D. Sodium nitroprusside test

Explanation: ***Nitrite test*** - The **nitrite test** is a common screening tool for UTIs because many **Gram-negative bacteria**, which are frequent causes of UTIs, produce an enzyme called **nitrate reductase**. - This enzyme converts urinary nitrates (normally present) into nitrites, indicating bacterial presence in the urine. - It is a **rapid, inexpensive screening test** that can be performed as part of routine urinalysis using a dipstick. *Paul-Bunnell test* - The **Paul-Bunnell test** is used to detect **heterophile antibodies** associated with **infectious mononucleosis** (caused by Epstein-Barr virus), not urinary tract infections. - This test identifies antibodies that agglutinate sheep red blood cells. *Fenton's test* - **Fenton's test** is not a standard diagnostic test used in clinical microbiology or for UTI screening. - This is a distractor option. In clinical practice, other urinalysis tests like **leukocyte esterase test** or **microscopy** are used alongside the nitrite test for comprehensive UTI evaluation. *Sodium nitroprusside test* - The **sodium nitroprusside test** is primarily used to detect the presence of **ketones** in the urine, a sign of conditions like diabetic ketoacidosis. - It is also used to detect **cystine** in the urine to screen for cystinuria. - This test is unrelated to bacterial infection screening.

Question 238: What is the confirmatory test for Syphilis?

A. VDRL
B. Rapid plasma reagin test
C. All of the options
D. FTA-ABS (Correct Answer)

Explanation: ***FT-ABS*** - The **Fluorescent Treponemal Antibody Absorption (FTA-ABS)** test specifically detects antibodies against *Treponema pallidum* antigens and is used as a **confirmatory test** for syphilis. - It is a **treponemal test**, meaning it directly targets antibodies produced in response to the syphilis bacterium, making it highly specific and reliable for confirming a diagnosis. *VDRL* - The **Venereal Disease Research Laboratory (VDRL)** test is a **non-treponemal test** used for screening for syphilis. - It detects antibodies to cardiolipin, a lipid released from damaged cells, and can produce **false positives** due to other medical conditions. *Rapid plasma reagin test* - The **Rapid Plasma Reagin (RPR)** test is another **non-treponemal screening test** for syphilis, similar to VDRL. - While it's quicker and easier to perform, it also measures non-specific antibodies and requires a confirmatory treponemal test due to potential **false positives**. *All of the options* - While VDRL and RPR are useful **screening tests**, they are not confirmatory due to their non-specific nature and potential for false positives. - Only **treponemal tests** like FTA-ABS or TP-PA ( *Treponema pallidum* particle agglutination assay) are considered confirmatory for syphilis.

Question 239: Which of the following culture media is not used for antibiotic susceptibility testing?

A. Tetrathionate-F (Correct Answer)
B. Blood agar
C. Chocolate agar
D. Muller-Hinton agar

Explanation: ***Tetrathionate-F*** - This is a **selective enrichment broth** primarily used for isolating *Salmonella* species from stool samples. - Its **liquid form and selective nature** make it completely unsuitable for antibiotic susceptibility testing, which requires a solid medium to allow for clear zones of inhibition around antibiotic disks. - Enrichment broths are never used for susceptibility testing—they are used only for isolation and culture enrichment. *Blood agar* - **Blood agar** is an enriched, non-selective medium used for primary isolation of a wide range of bacteria, especially hemolytic organisms. - Blood agar **IS actually used for antibiotic susceptibility testing** of certain fastidious organisms, particularly **Streptococci** (including *S. pneumoniae*) and other organisms that require blood supplementation. - CLSI guidelines recommend 5% sheep blood agar or blood-supplemented Mueller-Hinton agar for testing streptococci and pneumococci. *Chocolate agar* - **Chocolate agar** is an enriched medium used to grow fastidious organisms like *Haemophilus influenzae* and *Neisseria* species. - For antibiotic susceptibility testing of *Haemophilus*, **Haemophilus Test Medium (HTM)** is used, which is essentially Mueller-Hinton agar supplemented with factors present in chocolate agar. - Chocolate agar itself can be used for susceptibility testing of fastidious organisms when appropriately supplemented. *Mueller-Hinton agar* - **Mueller-Hinton agar** is the **gold standard medium** recommended by CLSI for routine antibiotic susceptibility testing (Kirby-Bauer disk diffusion method). - It has a well-defined composition with optimal pH (7.2-7.4), adequate thickness, and appropriate cation concentrations. - It ensures consistent antibiotic diffusion and reliable zone-of-inhibition measurements for most non-fastidious bacteria.

Question 240: Presumptive diagnosis of meningococcal meningitis is made earliest by -

A. Complement Fixation Test (CFT)
B. Latex agglutination test
C. Polymerase chain reaction (PCR) (Correct Answer)
D. Cerebrospinal fluid culture

Explanation: ***Polymerase chain reaction (PCR)*** - **PCR** can detect bacterial **DNA** directly from cerebrospinal fluid (CSF) samples, providing a highly **sensitive and specific** diagnosis, typically within **2-6 hours**. - It is the **most reliable method for early presumptive diagnosis**, especially in patients who have received **prior antibiotic therapy**, where culture may be negative and antigen tests less sensitive. - **Note**: While latex agglutination is faster (minutes), PCR offers superior sensitivity and specificity for definitive presumptive diagnosis. *Latex agglutination test* - **Latex agglutination** provides the **fastest results** (minutes to 1 hour), detecting bacterial **capsular polysaccharide antigens** in CSF. - However, it has **lower sensitivity** (50-90%) compared to PCR, and can yield false negatives, particularly after antibiotic administration or with low bacterial loads. - Results must often be confirmed with more definitive tests. *Cerebrospinal fluid culture* - While the **gold standard** for definitive diagnosis, CSF culture requires **24-48 hours** for bacterial growth, making it slower than molecular methods for initial presumptive diagnosis. - **Prior antibiotic treatment** significantly reduces sensitivity, with culture becoming negative within hours of treatment initiation. *Complement Fixation Test (CFT)* - **CFT** detects antibodies against pathogens, which develop **later in the course of infection** (days to weeks after symptom onset). - This test is **not suitable for early presumptive diagnosis** of acute bacterial meningitis and is rarely used in current clinical practice.

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