Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 151: Which of the following are acid-fast staining organisms? 1. Nocardia 2. Mycobacterium leprae 3. Actinomyces 4. Cryptosporidium parvum 5. Isospora belli

A. 1,2,3
B. 1,2,3,4,5
C. 1,2,4,5 (Correct Answer)
D. 3,4,5

Explanation: ***1,2,4,5*** - **Nocardia**, **Mycobacterium leprae**, **Cryptosporidium parvum**, and **Isospora belli** all exhibit acid-fast properties, meaning they retain carbolfuchsin stain even after decolorization with acid alcohol due to the presence of mycolic acid in their cell walls or unique cyst structures. - This characteristic is crucial for their identification in clinical microbiology and distinguishes them from many other microorganisms. *1,2,3* - This option incorrectly includes **Actinomyces** as an acid-fast organism. **Actinomyces** are Gram-positive, filamentous bacteria that are **not acid-fast**. - While Nocardia and Mycobacterium leprae are acid-fast, the inclusion of Actinomyces makes this choice incorrect. *1,2,3,4,5* - This option is incorrect because it includes **Actinomyces** as an acid-fast organism, which is not true. - **Actinomyces** are Gram-positive, non-acid-fast bacteria, differentiating them from the other listed organisms that do possess acid-fast properties. *3,4,5* - This option is incorrect because it excludes **Nocardia** and **Mycobacterium leprae**, both of which are prominent acid-fast organisms. - While Cryptosporidium parvum and Isospora belli are acid-fast, the omission of Nocardia and Mycobacterium leprae makes this answer incomplete and incorrect.

Question 152: A 32 year old laborer working at a construction site presented with fever and hemoptysis. The sputum sample collected for examination showed the following. The smear will be stained by which of the following sequences?

A. Methylene blue- malachite green-acetic acid - water
B. Gentian violet - iodine - alcohol saffranin
C. Methanol - methylene blue-acid - water
D. Carbol fuchsin - acid - alcohol- methylene blue (Correct Answer)

Explanation: ***Carbol fuchsin - acid - alcohol- methylene blue*** - The image displays thin, red, rod-shaped bacteria against a blue background, characteristic of **acid-fast bacilli** stained using the **Ziehl-Neelsen (ZN) method**. This staining sequence identifies *Mycobacterium tuberculosis*. - The ZN stain involves **carbol fuchsin** as the primary stain, followed by **acid-alcohol** as a decolorizer, and then **methylene blue** as a counterstain. *Methylene blue- malachite green-acetic acid - water* - This sequence is not a standard microbiological staining procedure for identifying common pathogens or acid-fast bacteria. - It does not contain the necessary components to achieve **acid-fast staining**, which is crucial for identifying mycobacteria. *Gentian violet - iodine - alcohol saffranin* - This sequence describes the reagents used in a **Gram stain**, which differentiates bacteria based on their cell wall composition. - Gram staining would show either purple (Gram-positive) or pink (Gram-negative) bacteria, not the red acid-fast bacilli seen in the image. *Methanol - methylene blue-acid - water* - While methylene blue is a counterstain in ZN, this sequence is incomplete and incorrect for standard acid-fast staining or other common bacterial stains. - It lacks **carbol fuchsin** as the primary stain, which is essential for acid-fast bacteria to retain the stain after destaining.

Question 153: A sexually active adolescent presents with cervicitis. Which specimen collection method has the highest sensitivity for detecting N. gonorrhoeae?

A. First-catch urine
B. Cervical gram stain
C. Clinician-collected endocervical swab (Correct Answer)
D. Self-collected vaginal swab

Explanation: ***Clinician-collected endocervical swab*** - A **clinician-collected endocervical swab** is the gold standard for detecting *N. gonorrhoeae* in cervicitis due to direct sampling and high cellularity. - This method ensures proper collection from the **likely site of infection**, maximizing the yield of bacterial DNA for NAATs. *First-catch urine* - While useful for screening, **first-catch urine** has lower sensitivity than direct cervical sampling for cervicitis. - It primarily detects urethral infections and may miss organisms localized in the endocervix. *Cervical gram stain* - **Cervical Gram stain** can identify gram-negative intracellular diplococci but has lower sensitivity and specificity compared to NAATs. - It is often reserved for initial rapid assessment but is not definitive for detecting *N. gonorrhoeae*. *Self-collected vaginal swab* - **Self-collected vaginal swabs** are less invasive and can be a good screening tool, but may not be as sensitive as a clinician-collected endocervical swab. - The patient may not properly access the **endocervical canal**, reducing the quality of the sample.

Question 154: Which of the following statements about nucleic acid amplification tests (NAATs) for STIs is FALSE?

A. They can be used for test of cure after 3 weeks
B. They can detect dead organisms after treatment
C. They can be used for pharyngeal gonorrhea screening
D. They are less sensitive than culture for rectal chlamydia (Correct Answer)

Explanation: ***They are less sensitive than culture for rectal chlamydia*** - This statement is **FALSE**. NAATs are generally **more sensitive** than culture methods for detecting *Chlamydia trachomatis* in all anatomical sites, including the rectum. - The high sensitivity of NAATs allows for the detection of very low bacterial loads, making them the preferred diagnostic method for many STIs. *They can be used for test of cure after 3 weeks* - This statement is generally **true**. While a "test of cure" (TOC) is not routinely recommended for uncomplicated *Chlamydia* or *Gonorrhea* infections due to high treatment efficacy, it can be considered in specific circumstances (e.g., persistent symptoms, pregnancy, or use of alternative regimens). - If a TOC is performed, it should ideally be done **no sooner than 3 weeks post-treatment** to minimize potential false positives from detecting residual nucleic acids from dead organisms. *They can detect dead organisms after treatment* - This statement is **true**. NAATs detect the **nucleic acids (DNA or RNA)** of the target organism. - These nucleic acids can persist in the body for a period even after the organism has been killed by treatment, leading to a positive NAAT result despite successful eradication of the infection. *They can be used for pharyngeal gonorrhea screening* - This statement is **true**. NAATs are the **recommended method** for detecting *Neisseria gonorrhoeae* in extragenital sites, including the pharynx. - Pharyngeal gonorrhea is often **asymptomatic**, making screening of at-risk individuals important for public health.

Question 155: What is the principle behind the FTA-ABS test?

A. Indirect immunofluorescence (Correct Answer)
B. Enzyme immunoassay
C. Agglutination
D. Complement fixation

Explanation: ***Indirect immunofluorescence*** - The **FTA-ABS test** (fluorescent treponemal antibody-absorption) detects antibodies against *Treponema pallidum* using an **indirect immunofluorescence** method. - Patient serum antibodies bind to fixed *Treponema pallidum* organisms, and then a **fluorescently labeled anti-human immunoglobulin** binds to the patient's antibodies, allowing for visualization under UV light. *Enzyme immunoassay* - **Enzyme immunoassays** (EIAs) use an enzyme-linked antibody to produce a color change for detection, which is different from the fluorescent detection in FTA-ABS. - While EIAs are also used for antibody detection, they employ a different signaling mechanism and often involve chromogenic substrates. *Agglutination* - **Agglutination tests** involve the clumping of particles (e.g., red blood cells, latex beads) when antibodies bind to multiple antigens. - This principle is used in tests like RPR or VDRL, but not the FTA-ABS test, which relies on direct visualization of bound fluorescent antibodies. *Complement fixation* - **Complement fixation tests** detect specific antibodies or antigens by observing whether complement components are consumed in an antigen-antibody reaction. - This method is older and involves a complex series of steps, not the direct observation of fluorescently labeled antibodies as in FTA-ABS.

Question 156: A Gram stain from a pharyngeal membrane shows bacteria in 'Chinese letter' pattern with metachromatic granules. Which growth characteristic would confirm Corynebacterium diphtheriae?

A. Gray-white colonies with black center (Correct Answer)
B. Beta hemolysis on blood agar
C. Yellow pigment on tellurite
D. Green colonies on chocolate agar

Explanation: ***Gray-white colonies with black center*** - *Corynebacterium diphtheriae* grows as **gray-white colonies** that develop a **black center** on **Tinsdale agar** or tellurite medium due to the reduction of tellurite. - This distinctive colonial morphology, along with the **"Chinese letter" arrangement** and **metachromatic granules** (Babes-Ernst bodies) on Gram stain, is characteristic of *C. diphtheriae*. *Beta hemolysis on blood agar* - While some strains of *C. diphtheriae* can be hemolytic, **beta-hemolysis** is not the most definitive or unique growth characteristic for its identification, as many other bacteria exhibit this. - The distinctive black colonies on tellurite medium are far more specific for *C. diphtheriae*. *Yellow pigment on tellurite* - *Corynebacterium diphtheriae* typically reduces tellurite to produce **black or dark gray colonies**, not a yellow pigment. - A yellow pigment on tellurite might be seen with other non-diphtherial corynebacteria but is not characteristic of the pathogenic *C. diphtheriae*. *Green colonies on chocolate agar* - **Green colonies on chocolate agar** are not a specific characteristic for *Corynebacterium diphtheriae* and are more commonly associated with other bacteria, such as some species of *Neisseria* or *Haemophilus*. - Chocolate agar is a non-selective medium, and while *C. diphtheriae* can grow on it, the appearance is not distinctive enough for identification.

Question 157: A patient presents with suspected diphtheria. What media will be used to diagnose this condition?

A. Chocolate agar
B. Cary-Blair
C. Loffler's serum slope (Correct Answer)
D. Lowenstein-Jensen

Explanation: ***Loffler's serum slope*** - **Löffler's serum slope** is a specific enrichment medium used for the isolation and identification of *Corynebacterium diphtheriae*, the causative agent of diphtheria. - It enhances the characteristic **metachromatic granules** (Babes-Ernst bodies) within the bacteria, aiding in microscopic identification. *Chocolate agar* - **Chocolate agar** is a non-selective enrichment medium often used for fastidious organisms like *Haemophilus influenzae* and *Neisseria* species. - While it supports the growth of many bacteria, it is not specifically optimized for the isolation or enhanced identification of *Corynebacterium diphtheriae*. *Cary-Blair* - **Cary-Blair transport medium** is designed to preserve enteric pathogens like *Salmonella* and *Shigella* in fecal samples during transport. - It is not a primary culture medium for *Corynebacterium diphtheriae* and would not be used for diagnosis of diphtheria. *Lowenstein-Jensen* - **Lowenstein-Jensen (LJ) medium** is a specialized egg-based medium primarily used for the isolation and culture of *Mycobacterium tuberculosis*. - It contains malachite green to inhibit the growth of other bacteria and is not suitable for the growth of *Corynebacterium diphtheriae*.

Question 158: Antemortem diagnosis of rabies is made with:

A. Inoculation in culture media
B. Negri bodies in hippocampus
C. Corneal impression smear (Correct Answer)
D. Rabies virus specific antibodies

Explanation: ***Corneal impression smear*** - A **corneal impression smear** can detect viral antigens in the cornea using fluorescent antibody staining, a method that can be performed on living patients. - This technique provides a relatively rapid and non-invasive way to diagnose rabies **antemortem**. *Inoculation in culture media* - Rabies virus is notoriously difficult to culture in standard cell culture media, making this method impractical and unreliable for **antemortem diagnosis**. - While viral isolation is possible in specialized research settings, it is not a routine diagnostic tool for rabies in living patients. *Negri bodies in hippocampus* - **Negri bodies** are eosinophilic inclusions found in the cytoplasm of neurons, particularly in the hippocampus, which are pathognomonic for rabies. - However, their detection requires **postmortem brain tissue biopsy**, making this a **postmortem diagnostic** method, not antemortem. *Rabies virus specific antibodies* - While the presence of **rabies virus-specific antibodies** (particularly in CSF) can indicate exposure and infection, they often appear late in the disease course. - The detection of antibodies may not be reliable for early **antemortem diagnosis**, especially in naive individuals whose immune response has not yet fully developed.

Question 159: Auramine and rhodamine staining are used for:

A. Nocardia
B. Rapid diagnosis of TB (Correct Answer)
C. Spirochetes
D. All of the options

Explanation: ***Rapid diagnosis of TB*** - **Auramine and rhodamine** are fluorescent dyes that bind to the **mycolic acid** in the cell walls of mycobacteria. - This method allows for a quicker screening of sputum samples for **Mycobacterium tuberculosis** compared to traditional Ziehl-Neelsen staining, as the fluorescent bacilli are easily identifiable under a fluorescent microscope. *Nocardia* - While *Nocardia* species are **partially acid-fast**, they are typically identified using modified Ziehl-Neelsen or Kinyoun stains, not auramine-rhodamine. - The primary clinical relevance of auramine-rhodamine staining is for **mycobacterial detection**, not *Nocardia*. *Spirochetes* - **Spirochetes** are too thin to be visualized effectively by direct light microscopy with auramine-rhodamine or even Gram stain. - They are typically detected using **darkfield microscopy**, immunofluorescence, or silver stains like **Warthin-Starry stain**. *All of the options.* - This option is incorrect because while *Nocardia* shares some staining characteristics with *Mycobacteria*, auramine-rhodamine is specifically optimized and primarily used for the detection of **acid-fast bacilli** like *Mycobacterium tuberculosis*, not *Nocardia* or spirochetes.

Question 160: Rapid examination of tubercle bacilli is possible with:

A. Ziehl–Neelsen method for AFB detection.
B. Auramine-Rhodamine fluorescent stain. (Correct Answer)
C. Giemsa stain for blood smears.
D. Kinyoun cold acid-fast stain.

Explanation: ***Auramine-Rhodamine fluorescent stain*** - This method provides a **more rapid** means of examining tubercle bacilli because it allows for the use of lower magnifications (e.g., 20x or 40x), enabling the viewing of a larger field and faster scanning of smears. - The **fluorescent bacilli** appear as bright yellow-green rods against a dark background, making them easier and quicker to detect compared to conventional light microscopy. *Ziehl–Neelsen method for AFB detection* - While it is a standard method for identifying **acid-fast bacilli (AFB)**, it requires higher magnification (100x oil immersion) and more extensive scanning to detect the red bacilli, making it **slower** than fluorescent methods. - It uses a basic fuchsin stain and heat to drive the stain into the waxy mycobacterial cell wall, followed by decolorization with acid-alcohol. *Giemsa stain for blood smears* - This stain is primarily used for identifying **blood parasites** (e.g., malaria) and cellular morphology in **hematological disorders**, and is not suitable for detecting mycobacteria. - It stains nuclear and cytoplasmic components differently, providing morphological details of blood cells and pathogens. *Kinyoun cold acid-fast stain* - Similar to the Ziehl-Neelsen stain, the Kinyoun method is also a conventional acid-fast stain that does not involve heat, but it still requires **high magnification** and **slow scanning** for detection. - It uses a higher concentration of basic fuchsin and a wetting agent to penetrate the cell wall without heating, but it is not considered rapid.

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