Diagnostic Microbiology Practice Questions

Q: Congenital syphilis can be diagnosed by:

IgM FTA-ABS. ***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.

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

Widal test. ***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.

Q: Craigie tube is used to differentiate between:

Motile and non-motile strains. ***Motile and non-motile strains*** - The **Craigie tube method** is a specific microbiological technique designed to demonstrate bacterial motility. - A small tube is inserted into a semi-solid agar medium; motile bacteria will grow out of the inserted tube into the surrounding medium, while non-motile bacteria will only grow within the tube. *Capsulated and non-capsulated strains* - Differentiation between capsulated and non-capsulated strains typically involves **negative staining** (e.g., India ink) or specific **serological tests**, not the Craigie tube. - The presence of a capsule is a virulence factor, but the Craigie tube does not directly assess this structure. *Virulent and avirulent strains* - Distinguishing virulent from avirulent strains usually requires **animal inoculation studies**, assessment of specific **virulence factors**, or molecular methods. - The Craigie tube tests for motility, which is a characteristic that may or may not correlate with virulence depending on the specific bacterial species. *Rough and smooth strains* - Rough and smooth colony morphology is observed visually on agar plates and is related to the presence or absence of **O-antigen** in Gram-negative bacteria. - This characteristic is assessed by direct colony observation, often related to phase variation, and is not determined using a Craigie tube.

Q: Test done for Mycobacterium tuberculosis based on CMI is

IGRA. ***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.

Q: Tuberculin unit is:

1 unit of PPD 3. ***1 unit of PPD 3*** - A **tuberculin unit** is defined as the amount of **purified protein derivative (PPD) containing 5 tuberculin units**, which corresponds to 0.0001 mg of PPD. - While often used interchangeably with **5 TU**, the definition of a single tuberculin unit refers to its biological activity, and in the context of PPD, **1 TU** is a specific measure of potency. *0.0001 mg* - This value represents the mass of **purified protein derivative (PPD) in 5 tuberculin units**, not a single tuberculin unit itself. - It is typically the amount of PPD found in the standard dose used for the **Mantoux tuberculin skin test**. *0.1 mg BCG* - **BCG (Bacillus Calmette-Guérin)** is a vaccine used to prevent tuberculosis, not a component of the tuberculin test. - The amount of BCG is unrelated to the definition of a tuberculin unit, which measures immune response to **Mycobacterium tuberculosis** antigens. *None of the options* - This option is incorrect because **1 unit of PPD 3** correctly refers to a tuberculin unit, which is a standardized measure used in tuberculosis testing.

Q: In a patient suspected to have Rabies, a corneal smear sample was taken. Which of the following is the MOST SENSITIVE investigation for this specimen?

RT PCR. ***RT PCR*** - **Reverse transcriptase polymerase chain reaction (RT-PCR)** is the **most sensitive molecular method** for detecting **rabies virus RNA** in corneal smear samples. - It provides **rapid, highly sensitive, and specific** detection of rabies viral nucleic acid, making it the preferred method for antemortem diagnosis from this specimen. - RT-PCR has **higher sensitivity than immunofluorescence** for corneal samples. *Immunofluorescence test* - **Direct fluorescent antibody (DFA) test** can be performed on corneal impression smears and is an established antemortem diagnostic method. - However, its **sensitivity is lower than RT-PCR** for this specific sample type, with higher false-negative rates. - DFA remains the gold standard primarily for **post-mortem brain tissue examination**. *Virus isolation* - Virus isolation is **time-consuming, less sensitive**, and requires specialized biosafety level 3 facilities. - **Corneal smears** have lower viral loads, making isolation less reliable compared to molecular methods. - Not routinely used for rapid diagnosis. *Negri body visualization* - **Negri bodies** are pathognomonic cytoplasmic inclusion bodies found in neurons, particularly in the **hippocampus and cerebellum**. - These can **only be visualized in brain tissue** through histopathological examination (post-mortem). - **Cannot be detected in corneal smears** as they are neuronal inclusions.

Q: Hanging drop method is used for:

Trichomonas. ***Trichomonas*** - The **hanging drop method** is a highly effective technique for visualizing the characteristic **motility** of *Trichomonas vaginalis*. - This method allows for the observation of living, unstained organisms directly from clinical samples, making it valuable for rapid diagnosis. *Toxoplasma* - **Toxoplasma gondii** is an intracellular parasite best identified through serological tests for **antibodies** or molecular diagnostics like **PCR**. - It does not exhibit characteristic motility in a hanging drop preparation that would aid in its direct identification. *Cryptosporidium* - **Cryptosporidium** species are typically identified by detecting **oocysts** in stool samples, often using **acid-fast staining** or **immunofluorescence assays**. - Their small size and lack of distinctive motility under a hanging drop method make this technique unsuitable for their diagnosis. *Plasmodium* - **Plasmodium** species, the causative agents of malaria, are diagnosed by visualizing **parasites within red blood cells** on **Giemsa-stained blood smears**. - The hanging drop method would not effectively identify these intracellular parasites for malaria diagnosis.

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

Immunofluorescence of skin biopsy. ***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.

Q: All are true about culture media except:

The best medium for anaerobes is chocolate agar. ***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.

Q: All tests are used to detect live bacteria, except:

ELISA for antibodies. ***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 1

Congenital syphilis can be diagnosed by:

Accepted Answer

IgM FTA-ABS

Answer

IgG FTA-ABS

Answer

TPI

Answer

VDRL

Question 2

Which of the following is NOT a test for chickenpox?

Accepted Answer

Widal test

Answer

ELISA

Answer

Immunofluorescence

Answer

PCR

Question 3

Craigie tube is used to differentiate between:

Accepted Answer

Motile and non-motile strains

Answer

Capsulated and non-capsulated strains

Answer

Virulent and avirulent strains

Answer

Rough and smooth strains

Question 4

Test done for Mycobacterium tuberculosis based on CMI is

Accepted Answer

IGRA

Answer

GenXpert

Answer

BACTEC

Answer

Culture

Question 5

Tuberculin unit is:

Accepted Answer

1 unit of PPD 3

Answer

0.0001 mg

Answer

0.1 mg BCG

Answer

None of the options

Question 6

In a patient suspected to have Rabies, a corneal smear sample was taken. Which of the following is the MOST SENSITIVE investigation for this specimen?

Accepted Answer

RT PCR

Answer

Virus isolation

Answer

Immunofluorescence test

Answer

Negri body visualization

Question 7

Hanging drop method is used for:

Accepted Answer

Trichomonas

Answer

Toxoplasma

Answer

Cryptosporidium

Answer

Plasmodium

Question 8

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

Accepted Answer

Immunofluorescence of skin biopsy

Answer

Immunofluorescence of corneal impressions

Answer

Isolation of virus from saliva

Answer

Antirabies antibodies in blood

Question 9

All are true about culture media except:

Accepted Answer

The best medium for anaerobes is chocolate agar

Answer

LJ medium is used for tubercle bacilli

Answer

Loeffler's serum slope is used for Corynebacterium diphtheriae

Answer

Blood agar supports fastidious organisms

Question 10

All tests are used to detect live bacteria, except:

Accepted Answer

ELISA for antibodies

Answer

Gram staining

Answer

Blood culture

Answer

Direct microscopy with vital stains

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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