Diagnostic Microbiology Practice Questions

Q: In Pontiac fever, which antigen is seen in urine?

Legionella pneumophila serogroup 1 antigen. ***Legionella pneumophila serogroup 1 antigen*** - **Pontiac fever** is a self-limited, non-pneumonic form of illness caused by *Legionella* species, primarily **_Legionella pneumophila_ serogroup 1**. - The **urinary antigen test** for *Legionella pneumophila* serogroup 1 is a rapid diagnostic test primarily used for **Legionnaires' disease** (the pneumonic form of legionellosis). - While the same organism causes both Pontiac fever and Legionnaires' disease, urinary antigen testing is **more clinically relevant for Legionnaires' disease** as Pontiac fever is typically diagnosed clinically due to its self-limited nature. - The urinary antigen test detects **_Legionella pneumophila_ serogroup 1** specifically, which accounts for most cases of legionellosis. *Streptococcus pneumoniae antigen* - **_Streptococcus pneumoniae_ antigen** in urine is indicative of pneumococcal infection, which typically causes bacterial pneumonia. - This organism is not associated with Pontiac fever or any form of legionellosis. *Escherichia coli O157:H7 antigen* - **_Escherichia coli_ O157:H7 antigen** is associated with gastrointestinal infections, leading to hemorrhagic colitis and hemolytic uremic syndrome. - It is not a cause of respiratory illness and is unrelated to Pontiac fever. *Klebsiella pneumoniae antigen* - **_Klebsiella pneumoniae_** typically causes urinary tract infections or pneumonia, particularly in immunocompromised individuals. - It is not associated with Pontiac fever, which is caused by *Legionella* species.

Q: Not used in leptospirosis -

Weil felix reaction. ***Weil Felix reaction*** - The **Weil-Felix reaction** is a test for rickettsial infections, not leptospirosis, detecting antibodies to *Proteus* antigens that cross-react with *Rickettsia* species. It has largely been replaced by more specific serological methods. - This test is based on the principle of detecting **agglutination** of certain *Proteus* bacteria strains (OX-19, OX-2, OX-K) by antibodies present in the serum of patients with rickettsial infections. *Macroscopic agglutination test* - This test, often performed using a variety of formats (e.g., slide agglutination or tube agglutination), can be adapted for detecting antibodies to *Leptospira* antigens. However, it is generally considered less sensitive and specific than the **Microscopic Agglutination Test (MAT)** for leptospirosis. - While it can be used for initial screening or in resource-limited settings, its utility in definitively diagnosing **leptospirosis** is limited due to potential false positives and negatives. *Microscopic agglutination test* - The **Microscopic Agglutination Test (MAT)** is considered the **gold standard** for diagnosing leptospirosis, as it directly detects antibodies against *Leptospira* serovars. - It involves mixing patient serum with live or formaldehyde-fixed *Leptospira* cultures and observing agglutination under a **dark-field microscope**. *Dark field illumination* - **Dark field microscopy** is a technique used to directly visualize **live *Leptospira* spirochetes** in blood, urine, or cerebrospinal fluid samples, especially in the early acute phase of the infection. - While helpful for direct observation, it requires specialized equipment and expertise, and its sensitivity can be low, especially if the bacterial load is not high.

Q: Best method to diagnose HIV in an infant?

PCR. ***PCR*** - **Polymerase Chain Reaction (PCR)** detects **HIV nucleic acids** (DNA or RNA) directly, which is crucial for infants because maternal antibodies can persist for up to 18 months, interfering with antibody-based tests. - PCR allows for early diagnosis, often within the first few weeks or months of life, facilitating timely intervention. *ELISA* - **Enzyme-linked immunosorbent assay (ELISA)** detects HIV antibodies. - In infants, ELISA can be misleading due to the presence of **maternal HIV antibodies** transferred across the placenta, making it unreliable for diagnosing active infection. *Western blot* - **Western blot** is used to confirm positive ELISA results in adults by detecting specific HIV proteins. - Like ELISA, it relies on the detection of **antibodies** and is therefore not reliable in infants due to maternally transmitted antibodies. *All of the options* - This option is incorrect because **ELISA** and **Western blot** are antibody-based tests that are unreliable in infants due to the presence of **maternal antibodies**. - Only **PCR** directly detects the virus itself, making it the preferred diagnostic method in this age group.

Q: Antibiotic sensitivity and resistance of microorganisms are determined by

Culture. ***Culture*** - **Culture** allows for the isolation and growth of microorganisms, which is essential for subsequent testing of their susceptibility to various antibiotics. - Standardized methods like the **Kirby-Bauer disk diffusion method** or **broth microdilution** are performed on cultured organisms to determine antibiotic sensitivity and resistance. *DNA probe* - **DNA probes** are primarily used for identifying specific genes or sequences within a microorganism, often for rapid identification or detection of resistance genes, but not for direct determination of phenotypic susceptibility. - While they can detect genetic markers associated with resistance, they don't directly measure how an antibiotic affects the *growth* of the organism. *Direct microscopy* - **Direct microscopy** is used to visualize microorganisms, determine their morphology, and estimate their quantity in a sample. - It does not provide information about a microorganism's ability to grow in the presence of antibiotics. *ELISA* - **ELISA (Enzyme-Linked Immunosorbent Assay)** is an immunological test used to detect antigens or antibodies in a sample. - It is used for diagnosis of infections or detection of toxins, but not for determining the susceptibility of microorganisms to antibiotics.

Q: Which of the following is not a test for syphilis?

Frei test. ***Frei test*** - The Frei test is a historical skin test used to diagnose **lymphogranuloma venereum (LGV)**, not syphilis. - It detects antibodies to *Chlamydia trachomatis* serovars L1, L2, and L3, which cause LGV. *TPHA (Treponema pallidum Hemagglutination Assay)* - TPHA is a specific **treponemal test** used to confirm the presence of antibodies to *Treponema pallidum*, the causative agent of syphilis. - It is used for confirmatory diagnosis and remains positive for life, indicating past or present infection. *FTA-Abs (Fluorescent Treponemal Antibody Absorption)* - FTA-Abs is another **specific treponemal test** for syphilis that detects antibodies against *Treponema pallidum* antigens. - It is highly sensitive and specific, often used to confirm positive non-treponemal tests or in cases of suspected latent syphilis. *VDRL (Venereal Disease Research Laboratory)* - VDRL is a **non-treponemal test** that detects antibodies to cardiolipin, a lipid released from damaged host cells during syphilis infection. - It is commonly used for screening and monitoring treatment response, as titers decrease with successful treatment.

Q: Which of the following organizations is recognized for establishing guidelines for antimicrobial susceptibility testing?

NCCLS. ***NCCLS*** - The **National Committee for Clinical Laboratory Standards (NCCLS)**, now known as the **Clinical and Laboratory Standards Institute (CLSI)**, is the primary organization that develops and publishes **standards and guidelines** for antimicrobial susceptibility testing. - These guidelines are crucial for ensuring the **accuracy, reproducibility, and standardization** of antimicrobial susceptibility test results in clinical microbiology laboratories worldwide. *NCTC* - The **National Collection of Type Cultures (NCTC)** is a UK-based collection that provides a wide range of **bacterial strains** for research, identification, and quality control purposes. - While it supplies important reference strains, it does not primarily establish the **methodological guidelines** for susceptibility testing. *ATCC* - The **American Type Culture Collection (ATCC)** is a global non-profit organization that meticulously collects, authenticates, preserves, and distributes diverse **biological materials**, including microbial strains, cell lines, and other essential biomaterials, which are indispensable for global research and development. - While **ATCC strains** are used to validate susceptibility tests, ATCC itself does not develop the **testing guidelines**. *EUCAST* - The **European Committee on Antimicrobial Susceptibility Testing (EUCAST)** is a European organization that develops and publishes guidelines for antimicrobial susceptibility testing used primarily in Europe. - While EUCAST is an important guideline-setting body, **NCCLS/CLSI** is more widely recognized internationally and is the standard reference for most clinical laboratories globally, especially in the context of Indian medical examinations.

Q: 1 mL of expectoration contains what number of TB bacilli in an active case of TB?

10,000. ***10,000*** - In an active case of tuberculosis, approximately **10,000 TB bacilli** per milliliter of sputum is the **minimum concentration required for detection** by conventional **acid-fast bacillus (AFB) smear microscopy**. - This is the established threshold in diagnostic microbiology and represents the typical bacterial load needed for reliable microscopic visualization. - The sensitivity of Ziehl-Neelsen staining (AFB smear) requires this concentration for consistent detection. *1,00,000* - A concentration of **1,00,000 (100,000) bacilli** per mL represents a very high bacterial load seen in severe, highly transmissible cases of active tuberculosis. - While this can occur in cavitary TB with heavy bacillary burden, it exceeds the typical diagnostic threshold and is not the standard reference value. *1,000* - A concentration of **1,000 bacilli** per mL is below the detection limit of conventional **AFB microscopy** (which requires 5,000-10,000 bacilli/mL). - Such low concentrations would require more sensitive methods like **culture** or **molecular techniques (PCR/CBNAAT)** for detection. *100* - A concentration of **100 bacilli** per mL is far below the microscopic detection threshold and would be considered a very low bacterial count. - Detection at this level would necessitate highly sensitive methods like **liquid culture (MGIT)** or **nucleic acid amplification tests**.

Q: Acute Hepatitis B can be earliest diagnosed by -

Hbs Ag. ***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.

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.

Question 1

In Pontiac fever, which antigen is seen in urine?

Accepted Answer

Legionella pneumophila serogroup 1 antigen

Answer

Streptococcus pneumoniae antigen

Answer

Escherichia coli O157:H7 antigen

Answer

Klebsiella pneumoniae antigen

Question 2

Not used in leptospirosis -

Accepted Answer

Weil felix reaction

Answer

Macroscopic agglutination test

Answer

Microscopic agglutination test

Answer

Dark field illumination

Question 3

Best method to diagnose HIV in an infant?

Accepted Answer

PCR

Answer

ELISA

Answer

Western blot

Answer

All of the options

Question 4

Antibiotic sensitivity and resistance of microorganisms are determined by

Accepted Answer

Culture

Answer

DNA probe

Answer

Direct microscopy

Answer

ELISA

Question 5

Which of the following is not a test for syphilis?

Accepted Answer

Frei test

Answer

TPHA

Answer

FTA-Abs

Answer

VDRL

Question 6

Which of the following organizations is recognized for establishing guidelines for antimicrobial susceptibility testing?

Accepted Answer

NCCLS

Answer

NCTC

Answer

ATCC

Answer

EUCAST

Question 7

A patient presented with cough and evening rise of temperature since one month. Which among the following is the definitive diagnostic test for tuberculosis?

Accepted Answer

Culture

Answer

Gram's staining

Answer

Tuberculin testing

Answer

Guinea-pig inoculation

Question 8

1 mL of expectoration contains what number of TB bacilli in an active case of TB?

Accepted Answer

10,000

Answer

1,00,000

Answer

1,000

Answer

100

Question 9

Acute Hepatitis B can be earliest diagnosed by -

Accepted Answer

Hbs Ag

Answer

IgM anti HBc ab

Answer

Anti HBs ab

Answer

IgG anti HBc abs

Question 10

Congenital syphilis can be diagnosed by:

Accepted Answer

IgM FTA-ABS

Answer

IgG FTA-ABS

Answer

TPI

Answer

VDRL

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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