Diagnostic Microbiology Practice Questions

Q: Which test is considered the gold standard for diagnosing genital herpes?

PCR for HSV. ***PCR for HSV*** - **Polymerase Chain Reaction (PCR)** is the gold standard due to its high **sensitivity** and **specificity** in detecting HSV DNA directly from lesion fluid or tissue samples. - It can differentiate between **HSV-1** and **HSV-2**, which is crucial for prognosis and counseling. *Tzanck smear* - This test identifies **multinucleated giant cells** and **intranuclear inclusions**, suggesting herpes but lacks specificity to differentiate HSV from other herpesviruses. - Its **sensitivity is low**, especially in healed or atypical lesions, making it unreliable for definitive diagnosis. *Viral culture* - While it can detect viable virus, **viral culture** has a lower sensitivity than PCR, especially in later stages of lesions or recurrent outbreaks. - It is **time-consuming**, with results often taking several days, which can delay diagnosis and treatment. *Serology for HSV antibodies* - **Serology** detects antibodies to HSV-1 or HSV-2, indicating past exposure but not necessarily active infection. - It is useful for **epidemiological studies** and for diagnosing asymptomatic individuals or those with atypical symptoms to determine prior exposure.

Q: Urea breath test is used for diagnosis of:

H. pylori. ***H. pylori*** - The **urea breath test** is a common and accurate non-invasive method to detect current **Helicobacter pylori** infection. - H. pylori produces the enzyme **urease**, which breaks down ingested urea into ammonia and carbon dioxide, which is then exhaled and measured. *Campylobacter jejuni* - **Campylobacter jejuni** is a common cause of **bacterial gastroenteritis** and typically diagnosed via **stool culture**. - It does not possess the **urease** enzyme, so a urea breath test would not be positive. *E.coli* - **E. coli** is a diverse group of bacteria, some strains causing gastrointestinal illness, but it is not associated with **urease production** for diagnostic purposes. - Diagnosis for diarrheagenic E. coli strains usually involves **stool culture** and specific toxin assays. *Lactobacillus* - **Lactobacillus** are beneficial bacteria commonly found in the gut and are not typically associated with pathogenic infections requiring a **urea breath test**. - They do not produce the **urease** enzyme in quantities relevant for this diagnostic test.

Q: Whole blood is used as a sample for which of the following tests?

Interferon Gamma Release Assay (IGRA). ***Interferon Gamma Release Assay (IGRA)*** - IGRAs, such as QuantiFERON-TB Gold, directly measure **interferon-gamma release** from T-lymphocytes stimulated by *Mycobacterium tuberculosis* antigens. - This test requires **fresh whole blood** as the living lymphocytes are essential for the immune response measured. *Blood Culture for Bacteria* - While blood is cultured, the primary goal is to **isolate and identify viable bacteria** from the bloodstream, not to detect an immune response within the whole blood itself. - Blood cultures typically involve placing blood into specific **culture media** to promote bacterial growth. *Genexpert* - The GeneXpert MTB/RIF assay is a **molecular test** used to detect *Mycobacterium tuberculosis* DNA and rifampicin resistance. - This assay is typically performed on **sputum samples** or other bodily fluids, not whole blood, to diagnose active tuberculosis. *Serological Test for Viruses* - Serological tests for viruses detect **antibodies or antigens** in the blood, which circulate in the **plasma or serum** component of blood. - These tests typically use **separated serum or plasma**, rather than whole blood, as the cellular components are not required for antibody or antigen detection.

Q: Which of the following is NOT included in the Nugent score?

Gonococcus. ***Gonococcus*** - The **Nugent score** is a **microscopic grading system** used to diagnose **bacterial vaginosis**, not sexually transmitted infections caused by specific bacteria like *Gonococcus*. - *Gonococcus* (specifically *Neisseria gonorrhoeae*) is indicative of **gonorrhea**, a separate clinical entity requiring different diagnostic methods like NAAT or culture. *Lactobacillus* - The Nugent score assesses the **morphotypes of bacteria** present on a **Gram-stained vaginal smear**, and a high number of *Lactobacillus* species is associated with a **healthy vaginal flora**. - A score of **0-3** indicates normal flora, characterized by abundant lactobacilli and absence of other morphotypes. *Gardnerella* - The Nugent score assigns points based on the presence of **small, curved Gram-variable rods** (typically *Gardnerella* species or similar). - An increase in these **coccobacillary morphotypes** contributes to a higher score, indicating a shift towards bacterial vaginosis. *Mobiluncus* - **Curved Gram-variable rods**, specifically *Mobiluncus* species, are also assessed in the Nugent score. - Their presence contributes points towards a higher score, as they are commonly associated with the pathogenesis of **bacterial vaginosis**.

Q: Which culture medium contains potassium tellurite?

Monsur medium. ***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.

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

Bronchoalveolar lavage (BAL) with microscopic examination. **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.

Q: Sabouraud's dextrose agar is primarily used for the isolation of which of the following?

Candida albicans. ***Candida albicans*** - **Sabouraud's dextrose agar (SDA)** is the gold standard medium for **isolation of all fungi** (both yeasts and molds), formulated with low pH (5.6) and high glucose concentration to favor fungal growth while inhibiting most bacteria. - Among the fungal options listed, ***Candida albicans*** is the **best answer** because Candida species are the **most commonly encountered fungal pathogens** in clinical microbiology, making SDA most frequently used for their isolation in routine clinical practice. - While SDA supports growth of all fungi including Aspergillus and Rhizopus, **Candida species** (especially *C. albicans*) represent the majority of clinical fungal infections requiring laboratory diagnosis. *Staphylococcus aureus* - This is a **bacterium**, not a fungus. - The **low pH (5.6)** and high sugar content of SDA are specifically designed to **inhibit bacterial growth** while promoting fungal growth. - *Staphylococcus aureus* is isolated on bacterial media like **blood agar** or **mannitol salt agar (MSA)**, not fungal media. *Aspergillus niger* - This is a **filamentous fungus (mold)** that grows very well on SDA. - However, while *Aspergillus* species are important opportunistic pathogens, they are **less commonly isolated** in routine clinical microbiology compared to *Candida* species. - *Aspergillus* infections occur primarily in immunocompromised patients, whereas *Candida* infections are more frequent across all patient populations. *Rhizopus stolonifer* - This is a **zygomycete fungus (bread mold)** that can grow on SDA. - While *Rhizopus* species cause mucormycosis in susceptible hosts, these infections are **relatively uncommon** compared to candidiasis. - SDA supports *Rhizopus* growth, but this organism is not the primary target in routine clinical fungal isolation.

Q: Test used for screening for urinary tract infection is

Nitrite test. ***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.

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

Enteroinvasive E. coli (EIEC). ***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 1

Which test is considered the gold standard for diagnosing genital herpes?

Accepted Answer

PCR for HSV

Answer

Tzanck smear

Answer

Viral culture

Answer

Serology for HSV antibodies

Question 2

Urea breath test is used for diagnosis of:

Accepted Answer

H. pylori

Answer

E. coli

Answer

Lactobacillus

Answer

Campylobacter jejuni

Question 3

Whole blood is used as a sample for which of the following tests?

Accepted Answer

Interferon Gamma Release Assay (IGRA)

Answer

Genexpert

Answer

Blood Culture for Bacteria

Answer

Serological Test for Viruses

Question 4

Which of the following is NOT included in the Nugent score?

Accepted Answer

Gonococcus

Answer

Lactobacillus

Answer

Gardnerella

Answer

Mobiluncus

Question 5

Which culture medium contains potassium tellurite?

Accepted Answer

Monsur medium

Answer

TCBS medium

Answer

BYCE medium

Answer

Muller Hinton agar

Question 6

What was the Complement Fixation test historically used for?

Accepted Answer

A historical test for syphilis (Wassermann test) - now obsolete

Answer

A historical test for typhoid fever diagnosis

Answer

A historical test for viral infections (e.g., influenza)

Answer

A historical test for rickettsial diseases

Question 7

What is the primary method for diagnosing Pneumocystis jirovecii pneumonia?

Accepted Answer

Bronchoalveolar lavage (BAL) with microscopic examination

Answer

Positive serology

Answer

Growth on artificial media

Answer

Sputum examination for trophozoites and cysts under microscope

Question 8

Sabouraud's dextrose agar is primarily used for the isolation of which of the following?

Accepted Answer

Candida albicans

Answer

Staphylococcus aureus

Answer

Aspergillus niger

Answer

Rhizopus stolonifer

Question 9

Test used for screening for urinary tract infection is

Accepted Answer

Nitrite test

Answer

Paul-Bunnell test

Answer

Fenton's test

Answer

Sodium nitroprusside test

Question 10

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

Accepted Answer

Enteroinvasive E. coli (EIEC)

Answer

Enterohemorrhagic E. coli (EHEC)

Answer

Enterotoxigenic E. coli (ETEC)

Answer

Enteroaggregative E. coli (EAEC)

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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