Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 201: All of the following are true about prozone phenomenon in syphilis EXCEPT:

A. Can occur in HIV co-infection
B. Can be overcome by serum dilution
C. Results in false positive VDRL (Correct Answer)
D. More common in secondary syphilis

Explanation: ***Results in false positive VDRL*** - The **prozone phenomenon** occurs due to an **excess of antibodies** which prevents the formation of antigen-antibody complexes, leading to a **false negative result** in non-treponemal tests like VDRL, not a false positive. - In a false positive, the test incorrectly indicates the presence of syphilis when the person does not have the infection, which is not the mechanism of the prozone phenomenon. *Can occur in HIV co-infection* - The prozone phenomenon is more common in individuals with **higher antibody titers**, which can be seen in patients with **HIV co-infection** due to altered immune responses. - This heightened immune response can lead to a significant overproduction of antibodies, contributing to the prozone effect. *Can be overcome by serum dilution* - Diluting the patient's serum reduces the concentration of antibodies, allowing for the optimal **antigen-antibody ratio** needed for agglutination to occur. - This process helps eliminate the excess antibody that interferes with visible flocculation, thereby revealing the true positive result. *More common in secondary syphilis* - Patients in the **secondary stage of syphilis** typically have very high levels of circulating antibodies against *Treponema pallidum*. - This high antibody concentration predisposes them to the prozone phenomenon in non-treponemal tests like VDRL and RPR.

Question 202: Which of the following is TRUE about biological false positive VDRL test?

A. Treponemal tests are negative in biological false positives. (Correct Answer)
B. More common in chronic conditions
C. Commonly seen in leprosy
D. Usually high titers (>1:16)

Explanation: ***Treponemal tests are negative in biological false positives.*** - A biological false positive VDRL (Venereal Disease Research Laboratory) test indicates reactivity to **cardiolipin antigens** in the absence of *Treponema pallidum* infection. - **Treponemal tests** (e.g., TP-PA, EIA, FTA-ABS) specifically detect antibodies against *Treponema pallidum* and will therefore be negative in true biological false positives, helping to differentiate them from actual syphilis. *More common in chronic conditions* - Biological false positives can be associated with both **acute** (e.g., viral infections, vaccination) and **chronic conditions** (e.g., autoimmune diseases, IV drug use, malignancy). - While chronic conditions can definitely cause them, the statement "more common" is not universally applicable as acute causes are also significant. *Commonly seen in leprosy* - While **leprosy** can cause false positive non-treponemal tests, it is not the *most common* or primary condition associated with them compared to other autoimmune diseases or acute viral infections. - The occurrence in leprosy is due to cross-reactivity with cardiolipin antigens. *Usually high titers (>1:16)* - Biological false positive VDRL tests typically result in **low titers**, often ≤ 1:8. High titers (>1:16) are more characteristic of active syphilis infection. - Higher titers in non-treponemal tests increase the probability of true syphilis infection, especially when accompanied by positive treponemal tests.

Question 203: Which of the following is the BEST method for diagnosis of C. difficile infection?

A. Toxin gene detection by polymerase chain reaction (PCR) (Correct Answer)
B. Enzyme-linked immunosorbent assay (ELISA)
C. Culture
D. Glutamate dehydrogenase (GDH) antigen detection

Explanation: ***Toxin gene detection by polymerase chain reaction (PCR)*** - **Nucleic acid amplification tests (NAAT/PCR)** for toxin genes (tcdA and tcdB) have the **highest sensitivity and specificity** among single-test methods, making them the preferred standalone diagnostic test. - Provides **rapid results** (2-4 hours), allowing for timely diagnosis and management of **Clostridioides difficile infection** (CDI). - **Clinical note:** While NAAT is highly sensitive, guidelines recommend **two-step algorithms** (GDH or NAAT + toxin EIA) to distinguish colonization from active infection in certain clinical settings. *Enzyme-linked immunosorbent assay (ELISA)* - **ELISA** for toxins A and B has **moderate specificity** but **lower sensitivity** (70-85%) compared to NAAT, potentially missing cases with lower toxin levels. - While it detects actual toxin production, the sensitivity limitation makes it suboptimal as a standalone test. *Culture* - **Culture** can detect the presence of *C. difficile* organism but **does not confirm toxin production**, meaning colonization cannot be distinguished from active disease without additional testing. - It is **time-consuming** (2-5 days), which delays diagnosis and treatment. - Useful for **epidemiological studies and strain typing** but not for routine diagnosis. *Glutamate dehydrogenase (GDH) antigen detection* - **GDH detection** is highly sensitive (>95%) for the presence of *C. difficile* organism, but has **low specificity** as it detects both toxigenic and non-toxigenic strains. - Best used as a **screening test** in two-step algorithms; a **positive GDH test must be confirmed** with toxin detection (EIA or NAAT).

Question 204: Lipoarabinomannan (LAM) assay in urine is used for screening of:

A. Histoplasma capsulatum
B. Pneumocystis jirovecii
C. Mycobacterium tuberculosis (Correct Answer)
D. Cryptococcus neoformans

Explanation: ***Mycobacterium tuberculosis*** - The **lipoarabinomannan (LAM) assay** detects a specific cell wall component of **Mycobacterium tuberculosis** in urine. - It is particularly useful for rapid diagnosis of **active tuberculosis**, especially in immunocompromised patients like those with HIV. *Histoplasma capsulatum* - **Histoplasma capsulatum** is a fungus, and its detection typically involves **antigen assays** in urine or serum, or fungal cultures. - LAM assay is not used for the diagnosis of **histoplasmosis**. *Pneumocystis jirovecii* - **Pneumocystis jirovecii** causes **Pneumocystis pneumonia (PCP)**, primarily in immunocompromised individuals. - Diagnosis is typically made by **microscopic examination** of respiratory secretions for the organism or **PCR**. *Cryptococcus neoformans* - **Cryptococcus neoformans** is a fungus that causes **cryptococcosis**, often manifesting as meningoencephalitis. - Diagnosis involves detection of **cryptococcal capsular antigen** in serum or cerebrospinal fluid, or culture.

Question 205: In which of the following conditions is viral load testing done by Real Time PCR of no role in investigative procedures?

A. Person with hepatitis B on Tenofovir therapy
B. HSV causing temporal encephalitis (Correct Answer)
C. CMV PCR in blood of patient of liver transplant
D. BK virus in patient of allograft renal transplant

Explanation: ***HSV causing temporal encephalitis*** - While **HSV PCR is crucial for diagnosing HSV encephalitis** from **cerebrospinal fluid (CSF)**, **quantitative viral load testing** does not guide clinical management or predict outcomes. - The key distinction: **qualitative PCR (detecting presence of HSV DNA)** is essential for diagnosis, but **viral load quantification** has no established role in treatment monitoring or prognostication. - The focus is on confirming HSV DNA presence to initiate **antiviral therapy (Acyclovir)**, not on serial viral load measurements for treatment efficacy. *Person with hepatitis B on Tenofovir therapy* - **HBV viral load testing** via **Real-Time PCR** is **absolutely essential** for monitoring the effectiveness of antiviral therapy like Tenofovir and detecting **drug resistance**. - **Quantitative monitoring** is crucial: decreasing HBV DNA levels indicate treatment response, while persistently high levels suggest resistance or non-adherence. - Viral load determines treatment duration and endpoints. *CMV PCR in blood of patient of liver transplant* - **CMV viral load monitoring** by **Real-Time PCR** in blood is **critical** in transplant recipients to detect **CMV reactivation** and guide pre-emptive antiviral therapy. - **Quantitative thresholds** are used to decide when to initiate treatment and assess treatment response. - Rising viral loads indicate need for intervention to prevent severe CMV disease. *BK virus in patient of allograft renal transplant* - **BK virus (BKV) PCR** in plasma or urine is **vital** for detecting **BKV reactivation** and monitoring **BKV nephropathy** in renal transplant recipients. - **Serial viral load measurements** prompt reduction in immunosuppression when levels rise, preventing allograft dysfunction or loss. - Quantitative monitoring is the standard of care for BKV management.

Question 206: Diagnosis of C. difficile infection is made by which of the following methods?

A. Stool microscopy for pseudomembranes
B. Culture
C. Toxin gene detection by polymerase chain reaction (PCR) (Correct Answer)
D. Enzyme-linked immunosorbent assay (ELISA)

Explanation: ***Toxin gene detection by polymerase chain reaction (PCR)*** - **PCR for toxin genes (tcdA and tcdB)** is the most sensitive and specific method for diagnosing **Clostridioides difficile infection (CDI)**, directly detecting the genetic material responsible for the pathology. - This method is superior because it identifies the presence of toxigenic C. difficile, which is crucial for determining clinical significance and guiding treatment. *Stool microscopy for pseudomembranes* - While **pseudomembranes** are a hallmark of severe CDI, their detection requires **endoscopy** and is not a direct diagnostic test for the pathogen itself. - Furthermore, their absence does not rule out CDI, as pseudomembranes may not form in all cases, especially milder ones. *Culture* - **Culture for C. difficile** can identify the presence of the organism, but it does not differentiate between toxigenic and non-toxigenic strains. - Many individuals can be **colonized with non-toxigenic C. difficile** without having an active infection, leading to false positives if culture alone is used for diagnosis. *Enzyme - linked immunosorbent assay (ELISA)* - ELISA tests primarily detect **C. difficile toxins A and B** or **glutamate dehydrogenase (GDH)** antigen in stool. - While rapid, ELISA for toxins A/B has **lower sensitivity** than PCR, potentially missing cases, and GDH detection alone only indicates the presence of C. difficile (toxigenic or non-toxigenic), requiring further toxin testing for confirmation.

Question 207: 1-3 beta-d-glucan assay is done for which infection?

A. Invasive candidiasis (Correct Answer)
B. Penicillium
C. Cryptococcus
D. Rhinocerebral mucormycosis

Explanation: ***Invasive candidiasis*** - The **1-3 beta-D-glucan assay** detects a component of the cell wall of many fungi, including **Candida** species, making it useful for diagnosing invasive candidiasis. - Elevated levels in a patient with risk factors for fungal infection can indicate an active **candidal infection**. *Penicillium* - While **Penicillium** is a fungus, specific antigens or metabolic products differentiate its infection from others. - Diagnosis of **penicilliosis** often relies on culture or molecular methods rather than the 1-3 beta-D-glucan assay, which has broader utility. *Cryptococcus* - **Cryptococcus neoformans** has a polysaccharide capsule that is the target of specific antigen tests, particularly the **cryptococcal antigen test (CrAg)**. - The cell wall of Cryptococcus contains very little **beta-D-glucan**, making the assay less sensitive for cryptococcal infections. *Rhinocerebral mucormycosis* - **Mucormycosis** is caused by fungi belonging to the order Mucorales, whose cell walls contain very little **beta-D-glucan**. - The 1-3 beta-D-glucan assay is generally **not positive** in cases of mucormycosis, making it a poor diagnostic tool for this infection.

Question 208: Giemsa stained smear cannot detect:

A. Coxiella burnetii (Correct Answer)
B. Bartonella
C. E. chaffeensis
D. Toxoplasmosis

Explanation: ***Coxiella burnetii*** - **Coxiella burnetii** is an **obligate intracellular bacterium** that **does not stain well with Giemsa stain**. - It is typically detected using specific immunofluorescence assays or molecular methods like PCR. *Bartonella* - **Bartonella species** are **Gram-negative bacteria** that can be visualized using Giemsa stain, especially in tissue sections or smears from infected patients. - They are known to cause diseases like **cat scratch disease** and **bacillary angiomatosis**. *E. chaffeensis* - **Ehlichia chaffeensis** is an **obligate intracellular bacterium** that infects monocytes and can form characteristic intracellular inclusions called **morulae**, which are visible with Giemsa stain. - This bacterium causes **human monocytic ehrlichiosis**. *Toxoplasmosis* - **Toxoplasma gondii** is an **obligate intracellular protozoan parasite** whose **tachyzoites** and **cyst forms** can be identified in tissue and fluid smears stained with Giemsa stain. - Detection with Giemsa staining is a common method for diagnosing **toxoplasmosis**.

Question 209: Which of the following is an advantage of using CLED media compared to MacConkey media?

A. Differentiates between Lactose fermenter and non-lactose fermenters
B. Inhibits swarming of proteus (Correct Answer)
C. Sodium taurocholate is used as selective agent
D. It stimulates growth of Staph and Candida as it is non selective

Explanation: ***Inhibits swarming of proteus*** - CLED (Cystine-Lactose-Electrolyte Deficient) media lacks electrolytes, specifically **sodium chloride**, which is essential for the swarming motility of *Proteus* species. - This characteristic makes CLED media particularly useful for isolating and enumerating **urinary pathogens** by preventing the overgrowth and spreading of motile bacteria like *Proteus*. - This is a **unique advantage** of CLED over MacConkey media, especially important in urine cultures where swarming can obscure other pathogens. *Differentiates between Lactose fermenter and non-lactose fermenters* - Both CLED and MacConkey media can differentiate between **lactose fermenters** (producing acid and changing indicator color) and **non-lactose fermenters**. - This feature is a shared characteristic with MacConkey agar, **not a unique advantage** of CLED over MacConkey. *Sodium taurocholate is used as selective agent* - **Sodium taurocholate** (a bile salt) is a selective agent primarily used in **MacConkey agar** to inhibit the growth of Gram-positive bacteria. - CLED media **does not contain bile salts**, making it less selective than MacConkey but suitable for a wider range of urinary tract pathogens, including some Gram-positives. *It stimulates growth of Staph and Candida as it is non selective* - While CLED media does support growth of *Staphylococcus* species and *Candida* because it lacks bile salts (making it less selective), this is **not necessarily an advantage** when comparing to MacConkey for specific purposes. - MacConkey is designed for **Gram-negative differentiation** and inhibits Gram-positives, while CLED's reduced selectivity can allow unwanted overgrowth in some contexts. - For urine cultures, CLED's ability to grow diverse pathogens can be useful, but the key advantage remains its **inhibition of Proteus swarming**, not simply being non-selective.

Question 210: All are true about Widal test EXCEPT:

A. Detects IgG antibodies
B. Cross-reacts with malaria
C. O titer rises earlier than H
D. Single high titer diagnostic (Correct Answer)

Explanation: ***Single high titer diagnostic*** - **FALSE/INCORRECT Statement** - This is the **EXCEPTION** - a single high Widal titer alone is **NOT diagnostic** for typhoid fever - Healthy individuals in endemic areas may have high background titers due to previous exposure - **Diagnostic criteria** require either: - **Four-fold rise** in antibody titers between acute and convalescent sera (2-3 weeks apart), OR - Single titer ≥1:160 (O antigen) or ≥1:160 (H antigen) **with supportive clinical features** in non-endemic areas - Blood culture remains the **gold standard** for diagnosis *Detects IgG antibodies* - TRUE - The Widal test detects both **IgM** and **IgG antibodies** against O and H antigens of *Salmonella Typhi* - **IgM antibodies** indicate recent/acute infection - **IgG antibodies** suggest past exposure, vaccination, or chronic carriage *Cross-reacts with malaria* - TRUE - The Widal test has **low specificity** and produces **false positives** due to cross-reactivity - Known cross-reactions with: **malaria**, dengue fever, non-typhoidal Salmonella infections, and other Gram-negative infections - This limitation can lead to misdiagnosis in endemic regions *O titer rises earlier than H* - TRUE - **Anti-O antibodies** (primarily IgM) appear earlier, typically within **6-8 days** after fever onset - **Anti-H antibodies** (primarily IgG) appear later but **persist longer** in serum - O antibodies indicate acute infection; H antibodies may persist for years after infection or vaccination

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