Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 191: Acute Hepatitis B can be earliest diagnosed by -

A. IgM anti HBc ab
B. Hbs Ag (Correct Answer)
C. Anti HBs ab
D. IgG anti HBc abs

Explanation: ***Hbs Ag*** - **HBsAg** is the earliest serological marker to appear in an acute hepatitis B infection, typically detectable within 1 to 10 weeks after exposure. - Its presence indicates an active infection, either acute or chronic, and is crucial for initial diagnosis. *IgM anti HBc ab* - **IgM anti-HBc antibodies** appear shortly after HBsAg and can persist for 3-6 months, indicating acute or recent infection. - While it is a key marker for acute infection, **HBsAg** generally appears earlier. *Anti HBs ab* - **Anti-HBs antibodies** indicate either recovery from hepatitis B infection with immunity or successful immunization through vaccination. - They are not present during the early acute phase of infection. *IgG anti HBc abs* - **IgG anti-HBc antibodies** persist indefinitely after recovery from acute hepatitis B and also appear in chronic infection. - They are a marker of past or chronic infection but do not indicate the earliest phase of acute infection.

Question 192: Congenital syphilis can be diagnosed by:

A. IgG FTA-ABS
B. TPI
C. IgM FTA-ABS (Correct Answer)
D. VDRL

Explanation: ***IgM FTA-ABS*** - **IgM antibodies** do not cross the placenta, meaning their presence in a newborn's blood indicates direct fetal infection with *Treponema pallidum*. - The **fluorescent treponemal antibody-absorption (FTA-ABS)** test specifically detects antibodies against *Treponema pallidum* antigens, making IgM FTA-ABS highly specific for congenital syphilis. *IgG FTA-ABS* - **IgG antibodies** can cross the placenta from mother to fetus, so a positive IgG result in a neonate may reflect maternal antibodies rather than an active fetal infection. - While IgG FTA-ABS confirms exposure, it does not differentiate between passively acquired maternal antibodies and active congenital infection. *TPI* - **TPI (Treponema pallidum immobilization) test** is an older, highly specific test for syphilis, but it is technically challenging, expensive, and largely replaced by newer serological methods. - TPI detects antibodies that immobilize live *Treponema pallidum*, but it does not differentiate between IgM and IgG, limiting its utility for diagnosing congenital syphilis. *VDRL* - **VDRL (Venereal Disease Research Laboratory) test** is a non-treponemal test that detects antibodies to cardiolipin, a lipid released from damaged host cells and *Treponema pallidum*. - While VDRL can be positive in congenital syphilis, it is not as specific as treponemal tests and can be affected by passive transfer of maternal antibodies or other conditions causing biological false positives.

Question 193: Which of the following is NOT a test for chickenpox?

A. ELISA
B. Immunofluorescence
C. PCR
D. Widal test (Correct Answer)

Explanation: ***Widal test*** - The **Widal test** is specifically used to diagnose **typhoid fever** (enteric fever) by detecting antibodies against *Salmonella Typhi* antigens. - It does not detect the **varicella-zoster virus (VZV)**, which causes chickenpox. *ELISA* - **ELISA (Enzyme-Linked Immunosorbent Assay)** can be used to detect **VZV antibodies (IgM or IgG)** in serum, indicating acute or past infection, respectively. - It is a common serological test for various viral infections, including chickenpox. *Immunofluorescence* - **Direct immunofluorescence (DIF)** can be used to detect **VZV antigens** directly from skin lesion scrapings. - **Indirect immunofluorescence (IIF)** can detect VZV antibodies in a patient's serum. *PCR* - **Polymerase Chain Reaction (PCR)** is a highly sensitive and specific test that detects **VZV DNA** directly from clinical samples like vesicular fluid, scabs, or blood. - It is often used for rapid and definitive confirmation of **chickenpox** or **shingles**, especially in atypical cases or immunocompromised patients.

Question 194: Test done for Mycobacterium tuberculosis based on CMI is

A. GenXpert
B. BACTEC
C. Culture
D. IGRA (Correct Answer)

Explanation: ***IGRA*** - **Interferon-gamma release assays (IGRAs)** measure the host's **cellular immune response** to *Mycobacterium tuberculosis* antigens. - They assess the release of **interferon-gamma** by T cells sensitized to specific mycobacterial antigens, indicating CMI. *GenXpert* - **GeneXpert MTB/RIF** is a **molecular test** that detects *M. tuberculosis* DNA and rifampicin resistance. - While it's a rapid diagnostic tool, it's based on **nucleic acid amplification**, not CMI. *BACTEC* - **BACTEC** is a **radiometric or fluorometric culture system** used for rapid detection and growth of *M. tuberculosis*. - This method assesses bacterial viability and metabolic activity, not the host's cellular immune response. *Culture* - Mycobacterial **culture** involves growing *M. tuberculosis* in specific media to identify its presence. - This is a direct method for detecting the organism, not an assessment of the host's cell-mediated immunity.

Question 195: Which of the following is best for ante-mortem diagnosis of rabies?

A. Immunofluorescence of corneal impressions
B. Isolation of virus from saliva
C. Immunofluorescence of skin biopsy (Correct Answer)
D. Antirabies antibodies in blood

Explanation: ***Immunofluorescence of skin biopsy*** - This method involves taking a **skin biopsy** from the **nuchal area** (nape of the neck) and staining it with **fluorescently labeled antibodies** to detect **rabies viral antigens** in cutaneous nerves. - It is considered the most reliable ante-mortem diagnostic test for rabies due to its high specificity and sensitivity in detecting viral nucleocapsid protein. *Immunofluorescence of corneal impressions* - While this method can detect rabies antigens, it generally has **lower sensitivity** compared to skin biopsy. - The procedure can be technically challenging and may yield **false negatives**, especially in early stages of the disease. *Isolation of virus from saliva* - **Viral isolation from saliva** is a possible method, but it is **less sensitive** and **more time-consuming** than immunological detection. - The shedding of rabies virus in saliva can be **intermittent**, leading to potential false negatives. *Antirabies antibodies in blood* - The presence of **antirabies antibodies in the blood** usually indicates either prior vaccination or a late stage of infection where the immune system has begun to respond. - These antibodies are often **undetectable in the early stages** of rabies infection, making this test unreliable for early ante-mortem diagnosis.

Question 196: All are true about culture media except:

A. LJ medium is used for tubercle bacilli
B. Loeffler's serum slope is used for Corynebacterium diphtheriae
C. The best medium for anaerobes is chocolate agar (Correct Answer)
D. Blood agar supports fastidious organisms

Explanation: ***The best medium for anaerobes is chocolate agar*** - **Chocolate agar** is an enriched medium used for the isolation of **fastidious organisms** like *Haemophilus influenzae* and *Neisseria* species, but it is not optimized for anaerobic growth. - Anaerobes require **anaerobic specific media** (e.g., thioglycollate broth, blood agar with reducing agents) and conditions (e.g., anaerobic jar) for optimal growth. *LJ medium is used for tubercle bacilli* - **Lowenstein-Jensen (LJ) medium** is a primary isolation medium specifically formulated for the growth of **mycobacteria**, including *Mycobacterium tuberculosis*. - It contains **malachite green**, which inhibits the growth of most other bacteria, and nutrients like **egg and asparagine** to support mycobacterial growth. *Loeffler's serum slope is used for Corynebacterium diphtheriae* - **Loeffler's serum slope** is an enrichment medium used to isolate and presumptively identify *Corynebacterium diphtheriae*. - It enhances the production of **metachromatic granules (Babes-Ernst granules)** by *C. diphtheriae*, which are visible upon staining. *Blood agar supports fastidious organisms* - **Blood agar** is an enriched medium containing 5% sheep blood, providing essential growth factors for many bacteria, including some **fastidious organisms**. - It is used to detect **hemolytic reactions**, which are important for differentiating various bacterial species.

Question 197: Type of light used in dark ground microscopy:

A. Polarized light
B. Reflected light
C. Oblique light (Correct Answer)
D. Transmitted light

Explanation: ***Oblique light*** - **Dark-ground microscopy** (also known as darkfield microscopy) uses a special condenser that blocks direct light, allowing only **oblique light** to illuminate the specimen. - This creates a **dark background** against which the specimen appears bright, as it's the only object scattering or refracting the oblique light into the objective lens. *Polarized light* - **Polarized light microscopy** uses two polarizers to analyze birefringence in materials, primarily used to visualize crystalline structures or fibers. - It does not produce the characteristic dark background with brightly illuminated specimens seen in dark-ground microscopy. *Reflected light* - **Reflected light microscopy** (or epi-illumination) is typically used for opaque specimens, where light hits the specimen from above and reflects into the objective. - This method is not suitable for transparent microorganisms often viewed with dark-ground microscopy and produces a bright, not dark, field of view. *Transmitted light* - **Transmitted light microscopy** is the most common type, where light passes directly through the specimen from below, illuminating the entire field of view. - This results in a bright field and is distinct from the technique used in dark-ground microscopy, where direct light is blocked.

Question 198: A 9-year-old child presented to OPD with complaints of high-grade fever, vomiting, and one episode of seizure. CSF examination was done and Gram staining of the culture showed lanceolate-shaped gram-positive diplococci. What is the probable causative agent?

A. Haemophilus influenzae
B. Streptococcus agalactiae
C. Neisseria meningitidis
D. Streptococcus pneumoniae (Correct Answer)

Explanation: ***Streptococcus pneumoniae*** - The description of **lanceolate-shaped gram-positive diplococci** in CSF is characteristic of *S. pneumoniae*. - This bacterium is a common cause of **bacterial meningitis** in children and can present with high fever, vomiting, and seizures. *Haemophilus influenzae* - This is a **gram-negative coccobacillus**, which would appear as small, pleomorphic rods rather than lanceolate-shaped diplococci on Gram stain. - While it causes meningitis, its Gram stain morphology is distinct from *S. pneumoniae*. *Streptococcus agalactiae* - *S. agalactiae* (Group B Streptococcus) is a **gram-positive coccus**, but it typically appears in **chains** and is a major cause of neonatal meningitis, not usually in a 9-year-old child. - Its morphology on Gram stain would not be described as lanceolate diplococci. *Neisseria meningitidis* - *N. meningitidis* is a **gram-negative diplococcus** and would appear as kidney-bean shaped or flattened paired cocci, not gram-positive. - Though a common cause of meningitis, the Gram stain morphology described rules it out.

Question 199: All tests are used to detect live bacteria, except:

A. Gram staining
B. ELISA for antibodies (Correct Answer)
C. Blood culture
D. Direct microscopy with vital stains

Explanation: ***Correct: ELISA for antibodies*** - **ELISA (Enzyme-linked immunosorbent assay)** for antibodies detects the **host's immune response** to an infection, not the bacteria themselves - This test identifies **antibodies** (IgM, IgG, IgA) produced by the immune system in response to bacterial antigens - **Does NOT detect bacteria** (live or dead) - it detects the immunological memory of exposure - Antibody presence indicates past or current exposure but tells us nothing about the presence of live organisms *Incorrect: Gram staining* - **Gram staining** is primarily a **morphological identification tool** that visualizes bacteria under microscopy - While it stains both live and dead bacteria equally, it is **used clinically to detect bacteria in specimens** (CSF, pus, sputum) - In the context of bacterial detection methods, seeing bacteria on Gram stain from a clinical specimen indicates bacterial presence and guides immediate therapy - Though not a specific viability test, it demonstrates bacterial presence in the sample being examined *Incorrect: Blood culture* - **Blood culture** involves inoculating blood into growth media and incubating to allow bacterial multiplication - **Only viable (live) bacteria will grow** in culture media - this is the gold standard for detecting live bacteria in bloodstream infections - Growth in culture definitively confirms the presence of living, metabolically active bacteria *Incorrect: Direct microscopy with vital stains* - **Vital stains** (e.g., acridine orange, fluorescein diacetate) are dyes that differentiate living cells from dead cells - These stains rely on **metabolic activity** or **intact cell membrane** to distinguish viable organisms - Used in direct microscopy to specifically identify **live bacteria** based on their ability to take up or exclude certain dyes

Question 200: Which test is most specific for active treponemal infection?

A. VDRL
B. FTA-ABS
C. RPR
D. Dark field microscopy (Correct Answer)

Explanation: ***Dark field microscopy*** - Directly visualizes **live *Treponema pallidum* spirochetes** from active lesions (chancres, condyloma lata) - Most specific test for **active treponemal infection** as it demonstrates the **presence of viable organisms** - Gold standard for early primary syphilis when active lesions are present - Requires active lesion and skilled microscopy, but provides immediate diagnosis of active infection *FTA-ABS* - Fluorescent Treponemal Antibody Absorption test detects **antibodies** to *Treponema pallidum* - Highly specific for treponemal infection but remains **positive for life** after infection - Cannot distinguish between **active and past infection** - unsuitable for diagnosing active disease - Confirmatory test for treponemal exposure, not active infection *RPR* - Rapid Plasma Reagin is a **non-treponemal test** detecting antibodies to cardiolipin - Useful for screening and monitoring treatment response (titers decline with treatment) - **Not specific** to *Treponema pallidum* - false positives occur in other conditions - Cannot definitively diagnose treponemal infection *VDRL* - Venereal Disease Research Laboratory test is another **non-treponemal test** detecting anticardiolipin antibodies - Used for screening and CSF testing in neurosyphilis - **Not specific** to treponemal infection - false positives common - Less specific than treponemal tests for confirming syphilis

Practice by Chapter

Specimen Collection and Transport

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Microscopy in Microbiology

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Culture Methods and Media

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Bacterial Identification Techniques

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Antimicrobial Susceptibility Testing

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Serological Diagnosis

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Molecular Diagnostic Methods

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Rapid Diagnostic Tests

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Point-of-Care Testing

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Automation in Microbiology Laboratory

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Quality Control in Diagnostic Microbiology

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Interpretation of Microbiological Reports

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