Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 131: CSF gram stain of a child suffering with meningitis is shown below. What is the causative agent? (NEET Pattern 2019)

A. Streptococcus pneumoniae
B. H. influenzae (Correct Answer)
C. Klebsiella pneumoniae
D. Staphylococcus aureus

Explanation: ***H. Influenzae*** - The image shows **Gram-negative rods or coccobacillary forms**, which are characteristic of *Haemophilus influenzae*. - *H. influenzae* is a common cause of **bacterial meningitis** in children, particularly before the widespread use of vaccines. *Streptococcus pneumoniae* - This bacterium would appear as **Gram-positive cocci** in pairs or short chains, which is not consistent with the image. - *Streptococcus pneumoniae* is also a common cause of meningitis but has a distinct morphological appearance. *Klebsiella pneumonia* - While *Klebsiella pneumoniae* is a Gram-negative rod, it is typically a larger, more plump rod, and less commonly a cause of **pediatric meningitis** compared to *H. influenzae*. - The morphology in the image, particularly the coccobacillary appearance, is more typical of *Haemophilus*. *Staphylococcus aureus* - This bacterium would appear as **Gram-positive cocci** in grape-like clusters, which clearly differs from the Gram-negative rods/coccobacillary forms seen in the image. - *Staphylococcus aureus* can cause meningitis, especially in cases of head trauma or neurosurgery, but its morphology is distinct.

Question 132: The following culture medium is used for the isolation of: (NEET Pattern 2019)

A. Listeria
B. Legionella (Correct Answer)
C. Escherichia
D. Campylobacter

Explanation: ***Legionella*** - The image displays growth on **buffered charcoal yeast extract (BCYE) agar**, which is the selective medium for the isolation of *Legionella* species. - *Legionella* requires **L-cysteine** and **iron salts** for growth, which are provided in BCYE agar, making it distinctively well-suited for its isolation. *Listeria* - *Listeria* species are typically isolated on selective media such as Oxford agar or PALCAM agar, which contain **antibiotics** and **chromogenic substrates** to differentiate them from other bacteria. - While *Listeria* can grow on general purpose media, BCYE agar is not the primary selective medium for its isolation. *Escherichia* - **Escherichia coli** is a common fecal coliform that grows readily on various non-selective media like blood agar and selective/differential media such as MacConkey agar (lactose fermenter with pink colonies) or eosin methylene blue (EMB) agar (green metallic sheen colonies). - BCYE agar is not a standard medium for *Escherichia* isolation, as it lacks the necessary components for its optimal differentiation and growth. *Campylobacter* - **Campylobacter** species are **microaerophilic** and require specialized selective media like **Campy-BAP (Campylobacter Blood Agar Plate)**, Skirrow's medium, or CCDA (cefoperazone charcoal deoxycholate agar). - These media contain antibiotics to inhibit normal flora and are incubated in a microaerophilic atmosphere; BCYE agar is not used for *Campylobacter* isolation.

Question 133: A wet mount preparation of vaginal discharge shows the following. Identify the organism responsible.

A. Trichomonas vaginalis (Correct Answer)
B. Neisseria gonorrhoeae
C. Chlamydia
D. Treponema pallidum

Explanation: ***Trichomonas vaginalis*** - The image shows numerous flagellated protozoa, characterized by their **pear-shaped appearance** with **jerky motility** on wet mount examination. - **Trichomonas vaginalis** is a flagellated protozoan parasite causing vaginitis, typically presenting with frothy, yellow-green vaginal discharge and strawberry cervix. - Wet mount microscopy showing motile trophozoites is the classic diagnostic method for trichomoniasis. *Neisseria gonorrhoeae* - This bacterium is a **gram-negative diplococcus**, typically seen intracellularly within neutrophils on Gram stain. - It does not present as flagellated protozoa on microscopy. *Chlamydia* - **Chlamydia trachomatis** is an obligate intracellular bacterium, not visible on routine wet mount microscopy. - Diagnosis requires specialized staining (Giemsa), immunofluorescence, or molecular tests (NAAT). *Treponema pallidum* - This is a **spirochete** responsible for syphilis, typically identified using dark-field microscopy from ulcer exudate or serological tests. - It does not present as the large, flagellated protozoa characteristic of Trichomonas.

Question 134: Two microscopic preparations of cerebrospinal fluid from a patient with meningitis are shown. Identify the stains used in images A and B.

A. A= Mucicarmine stain, B= India Ink stain (Correct Answer)
B. A= Wright stain, B= India Ink stain
C. A= Giemsa stain, B= India Ink stain
D. A= Fontana Masson stain, B= India Ink stain

Explanation: ***A= Mucicarmine stain, B= India Ink stain*** - Image A shows fungal elements (likely *Cryptococcus*) stained red/pink, which is characteristic of a **Mucicarmine stain** due to its ability to highlight the mucopolysaccharide capsule. - Image B depicts encapsulated yeast cells (like *Cryptococcus*) surrounded by a clear halo against a dark background, a classic appearance when visualization with an **India ink stain** due to the ink's inability to penetrate the capsule. *A= Wright stain, B= India Ink stain* - Wright stain is primarily used for blood smears to differentiate blood cells and parasites, not for fungal capsules, rendering its use in image A incorrect. - While image B correctly identifies an India Ink stain for showing fungal capsules, the first part of the option is incorrect. *A= Giemsa stain, B= India Ink stain* - Giemsa stain is generally used for bacterial and parasitic identification, as well as blood cell morphology, and does not specifically stain fungal capsules in the manner shown in image A. - As with other options, image B is accurately identified, but the pairing with Giemsa stain for image A is incorrect. *A= Fontana Masson stain, B= India Ink stain* - Fontana Masson stain is used to detect melanin and argentaffin granules, and not typically for staining fungal capsules, so it does not match the appearance in image A. - The correct identification of India Ink stain for image B makes part of this option correct, but the first part is inaccurate.

Question 135: What is the most sensitive diagnostic method for detecting Trichomonas vaginalis?

A. Pap smear
B. Culture
C. Nucleic acid amplification test (NAAT) (Correct Answer)
D. Wet mount microscopy

Explanation: ***Nucleic acid amplification test (NAAT)*** - **NAATs** detect **_Trichomonas vaginalis_** DNA or RNA, offering the **highest sensitivity and specificity** among available diagnostic methods. - This method is particularly useful for detecting low parasitic loads and in asymptomatic patients, improving diagnostic accuracy. *Pap smear* - While a **Pap smear** can sometimes incidentally detect **_Trichomonas vaginalis_**, it is not a dedicated or **sensitive diagnostic tool** for this infection. - Its primary purpose is cervical cancer screening, and its sensitivity for trichomoniasis is low, often leading to false negatives. *Culture* - **Culture** was previously considered the **gold standard** but is less sensitive and takes longer (up to 7 days) to yield results compared to NAATs. - Its sensitivity is significantly reduced when parasite loads are low or if samples are not processed promptly. *Wet mount microscopy* - **Wet mount microscopy** allows for the visualization of **motile trichomonads**, but its sensitivity is highly dependent on operator experience and parasitic load. - It has a **sensitivity of 50-70%**, meaning a significant number of infections can be missed.

Question 136: What is the significance of prozone phenomenon in syphilis testing?

A. False negative result in early primary syphilis
B. False positive result in treated syphilis
C. False negative result in high-titer secondary syphilis (Correct Answer)
D. False positive result in pregnancy

Explanation: ***False negative result in high-titer secondary syphilis*** - The **prozone phenomenon** occurs when extremely high concentrations of antibodies saturate all available antigen-binding sites, preventing effective lattice formation required for agglutination. - This leads to a **false negative result** because the reaction appears negative despite the presence of a target antigen in the sample, commonly observed in **secondary syphilis** due to high antibody titers. *False negative result in early primary syphilis* - In **early primary syphilis**, antibody titers are still low, not high enough to induce the prozone phenomenon. - **False negatives** in early primary syphilis are typically due to insufficient time for antibody production, rather than antibody excess. *False positive result in treated syphilis* - **Treated syphilis** generally results in declining antibody titers, which would not cause a false positive due to prozone. - A **false positive** result means the test is positive when the condition is not present or has been treated, which is not usually caused by prozone. *False positive result in pregnancy* - Pregnancy can sometimes cause **biological false positives** in non-treponemal tests due to autoimmune reactions or other factors. - However, this is not related to the **prozone phenomenon**, which is caused by antibody excess.

Question 137: Which of the following specimens is most appropriate for Chlamydia trachomatis NAAT testing in men?

A. Blood sample
B. Prostatic massage fluid
C. Mid-stream urine
D. First-void urine (Correct Answer)

Explanation: ***First-void urine*** - **First-void urine** (the initial 10-20 mL of urine) is the **most appropriate specimen** for NAAT testing for *Chlamydia trachomatis* in men because it contains the highest concentration of **urethral epithelial cells and organisms** from the urethra. - This method is also **non-invasive** and **cost-effective**, making it suitable for screening and diagnosis. *Blood sample* - **Blood samples** are not suitable for detecting *Chlamydia trachomatis* at the site of infection (genital tract). - While **serological tests** on blood can detect antibodies, they indicate past exposure rather than current infection and are not used for routine diagnostic screening. *Prostatic massage fluid* - **Prostatic massage fluid** is more typically used to diagnose **prostatitis** or other infections within the prostate gland itself. - It is not the preferred or most sensitive specimen for routine **urethral *Chlamydia trachomatis*** detection. *Mid-stream urine* - **Mid-stream urine** primarily reflects infections in the **bladder** or **kidneys**. - It is unlikely to contain sufficient numbers of **chlamydial organisms** from the urethra to provide an accurate NAAT result compared to first-void urine.

Question 138: Which of the following is TRUE about screening for Trichomonas vaginalis?

A. Urine samples are inadequate for testing
B. Culture is no longer used for diagnosis
C. NAAT is recommended for screening high-risk women (Correct Answer)
D. Wet mount microscopy has sensitivity >95%

Explanation: ***NAAT is recommended for screening high-risk women*** - **Nucleic Acid Amplification Tests (NAATs)** are highly sensitive and specific for detecting *Trichomonas vaginalis*, making them the preferred method for screening in high-risk populations due to their superior performance over traditional methods. - Screening high-risk women (e.g., those with multiple sexual partners, other STIs, or in areas with high prevalence) with NAATs helps in early detection and treatment, which is crucial for preventing further transmission and complications. *Urine samples are inadequate for testing* - **Urine samples** can indeed be used for *Trichomonas vaginalis* testing, particularly with NAATs, as they provide an alternative to vaginal swabs and are often preferred for their ease of collection and non-invasiveness. - While less sensitive than vaginal swabs for microscopy or culture, **NAATs performed on urine** have good sensitivity and specificity, making them a common option. *Culture is no longer used for diagnosis* - **Culture (e.g., InPouch TV system)** is still considered a **gold standard** for *Trichomonas vaginalis* diagnosis due to its high sensitivity and ability to detect viable organisms, especially when NAATs are not available or for confirming ambiguous results. - It is particularly useful in cases where organisms are present in low numbers or in settings where resources for advanced molecular testing are limited, though it is **less rapid** than NAATs. *Wet mount microscopy has sensitivity >95%* - **Wet mount microscopy** is an inexpensive and rapid diagnostic method, but its sensitivity for detecting *Trichomonas vaginalis* is **relatively low**, typically ranging from **50-70%**, not >95%. - The sensitivity of wet mounts is highly dependent on the **operator's skill**, the concentration of organisms, and the time elapsed since sample collection, making it prone to false negatives.

Question 139: Which serologic test for syphilis relies on flocculation reaction using cardiolipin-lecithin-cholesterol antigen?

A. FTA-ABS
B. TPHA
C. PCR
D. VDRL (Correct Answer)

Explanation: ***VDRL*** - The **VDRL (Venereal Disease Research Laboratory)** test is a non-treponemal serologic test for syphilis that detects antibodies against **cardiolipin-lecithin-cholesterol antigen**. - It works based on a **flocculation reaction** where microscopic aggregates form when patient antibodies react with the antigen. *FTA-ABS* - The **FTA-ABS (Fluorescent Treponemal Antibody Absorption)** test is a treponemal test that detects antibodies specific to *Treponema pallidum*. - It uses a **fluorescent immunoassay** principle, not flocculation. *TPHA* - The **TPHA (Treponema Pallidum Hemagglutination Assay)** is a treponemal test that detects antibodies to *Treponema pallidum* antigens. - It involves the **agglutination of sensitized sheep red blood cells**, not flocculation with cardiolipin antigen. *PCR* - **PCR (Polymerase Chain Reaction)** is a molecular method that detects the **genetic material (DNA)** of *Treponema pallidum*. - It is used for direct detection of the pathogen, not for serologic antibody testing via flocculation.

Question 140: Which of the following statements about non-treponemal tests for syphilis is NOT correct?

A. They become negative after successful treatment
B. They detect antibodies to cardiolipin-lecithin-cholesterol antigens
C. They can be used to monitor treatment response
D. They are highly specific for treponemal infection (Correct Answer)

Explanation: ***They are highly specific for treponemal infection*** - **Non-treponemal tests** detect antibodies against **cardiolipin**, a lipid released from damaged host cells and the spirochetes themselves. - While useful for screening, these tests are prone to **false positives** in various conditions such as autoimmune diseases, acute infections, and pregnancy, making them **non-specific** for treponemal infection alone. *They become negative after successful treatment* - This statement is correct. A significant **decrease in titers** (e.g., a four-fold drop) in non-treponemal tests like VDRL or RPR indicates a successful response to syphilis treatment. - While they may not always become entirely negative (some individuals may have a persistent low-level titer known as a **serofast state**), a negative conversion or substantial titer reduction is expected. *They detect antibodies to cardiolipin-lecithin-cholesterol antigens* - This statement is correct. Non-treponemal tests such as **VDRL (Venereal Disease Research Laboratory)** and **RPR (Rapid Plasma Reagin)** detect antibodies directed against a lipoidal antigen composed of **cardiolipin, lecithin, and cholesterol**. - These antigens are derived from host cells damaged by the syphilis infection and are also present on the surface of *Treponema pallidum*. *They can be used to monitor treatment response* - This statement is correct. The **quantitative titers** obtained from non-treponemal tests are crucial for monitoring the effectiveness of syphilis treatment. - A sustained **four-fold or greater decrease** in titer (e.g., from 1:32 to 1:8) over 6-12 months typically indicates an adequate response to therapy.

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