Diagnostic Microbiology — MCQs

Diagnostic Microbiology Indian Medical PG Practice Questions and MCQs

Question 141: Which is the most commonly used antigen detection test for Chlamydia trachomatis in genital specimens?

A. Enzyme immunoassay (EIA) (Correct Answer)
B. Direct fluorescent antibody (DFA)
C. Complement fixation test
D. Western blot

Explanation: ***Enzyme immunoassay (EIA)*** - **EIA** is the **most commonly used antigen detection method** for *Chlamydia trachomatis* due to its **relative ease of use, cost-effectiveness, and suitability for high-throughput screening** in clinical laboratories. - This test identifies specific **chlamydial antigens** (primarily the Major Outer Membrane Protein - MOMP) directly from genital specimens. - While **NAATs have replaced EIA as the gold standard** due to superior sensitivity and specificity, EIA remains widely used, particularly in resource-limited settings. *Direct fluorescent antibody (DFA)* - **DFA** is also an antigen detection test that uses fluorescently labeled antibodies to visualize chlamydial elementary bodies under microscopy. - However, it is **less commonly used than EIA** because it is **labor-intensive, requires experienced personnel** for microscopic interpretation, and is **operator-dependent**. - DFA has **lower sensitivity** compared to EIA and NAATs, particularly in specimens with low organism burden. *Complement fixation test* - The **complement fixation test** is a serological method used to detect **antibodies** against *Chlamydia*, **not antigens**, making it unsuitable for direct detection of the organism in genital specimens. - Its utility is primarily for **detecting past exposure** or in diagnosing systemic chlamydial infections like psittacosis, not for routine diagnosis of genital infections. *Western blot* - **Western blot** is a laboratory technique used to detect **specific proteins**, but it is **not a routine diagnostic test** for *Chlamydia trachomatis* in genital specimens due to its **complexity, cost, and time requirements**. - It is typically reserved for **research purposes** or confirmatory testing rather than primary clinical diagnosis.

Question 142: Which test is used to directly visualize Treponema pallidum in lesion exudate?

A. Dark field microscopy (Correct Answer)
B. Gram stain
C. VDRL
D. TPHA

Explanation: ***Dark field microscopy*** - **Dark field microscopy** allows direct visualization of the **spiral morphology** and characteristic motility of *Treponema pallidum* in fresh exudate from a syphilitic lesion. - This method is highly specific and sensitive for primary syphilis, especially when serological tests may still be negative. *Gram stain* - **Gram staining** is not effective for visualizing *Treponema pallidum* because spirochetes are **extremely thin (0.1-0.2 μm)** and have a unique cell wall structure that **cannot be adequately stained** by conventional Gram staining methods. - Spirochetes appear only faintly or not at all on Gram stain slides, making it an unreliable diagnostic tool for syphilis. *VDRL* - **VDRL (Venereal Disease Research Laboratory)** is a **nontreponemal serological test** that detects antibodies against cardiolipin, a lipid released from damaged cells during infection, not the bacterium itself. - It is used for screening and monitoring treatment response but does not directly visualize the organism. *TPHA* - **TPHA (Treponema pallidum hemagglutination assay)** is a **treponemal serological test** that detects specific antibodies against *Treponema pallidum*. - While highly specific for confirming syphilis infection, it is an indirect test based on antibody detection and does not visualize the spirochete directly.

Question 143: A man presents with dysuria and urethral discharge after a history of unprotected sex. The Gram stain of his discharge is shown. What is the best culture medium for isolating the organism responsible?

A. Thayer-Martin agar (Correct Answer)
B. MacConkey agar
C. Chocolate agar
D. TCBS agar

Explanation: ***Thayer-Martin agar*** - The image shows numerous **polymorphonuclear leukocytes (neutrophils)** with intracellular, gram-negative diplococci, which is characteristic of **Neisseria gonorrhoeae**. - **Thayer-Martin agar** is a selective medium specifically formulated for the isolation of *Neisseria gonorrhoeae* from specimens containing flora. *MacConkey agar* - **MacConkey agar** is a selective and differential medium used primarily for the isolation of Gram-negative **enteric bacilli** and differentiation based on lactose fermentation. - It is not suitable for *Neisseria* species, which are fastidious organisms requiring enriched media. *Chocolate agar* - **Chocolate agar** is an enriched, non-selective medium that supports the growth of fastidious organisms like *Neisseria* species and *Haemophilus influenzae*. - While *Neisseria gonorrhoeae* grows on chocolate agar, **Thayer-Martin agar** is preferred for specimens from sites with normal flora as it inhibits contaminants. *TCBS agar* - **Thiosulfate Citrate Bile Salts Sucrose (TCBS) agar** is a selective medium used for the isolation of *Vibrio* species, particularly *Vibrio cholerae*. - This medium is completely unsuitable for the growth of *Neisseria gonorrhoeae*, which has entirely different nutritional and environmental requirements.

Question 144: A female patient presents with dysuria and frequency. A coagulase-negative, novobiocin-resistant Staphylococcus species (>10^4 CFU/mL) was grown in urine culture. What does this indicate?

A. UTI (Correct Answer)
B. Commensal
C. Contamination
D. Repeat culture needed

Explanation: ***UTI*** - The isolation of a **coagulase-negative, novobiocin-resistant Staphylococcus** in a patient with UTI symptoms suggests **_Staphylococcus saprophyticus_**, a common cause of UTIs in young women. - A bacterial count of **>10^4 CFU/mL** is generally considered significant for diagnosing a UTI, indicating active infection rather than contamination. - _S. saprophyticus_ accounts for 10-20% of UTIs in sexually active young women and is the second most common cause after _E. coli_. *Commensal* - While some coagulase-negative staphylococci can be commensals, **_S. saprophyticus_** is an important pathogen, especially in UTIs. - The combination of **novobiocin resistance** and a significant bacterial count in a symptomatic patient strongly points away from a commensal role. *Contamination* - **Contamination** usually involves lower bacterial counts (<10^4 CFU/mL) or the isolation of multiple different organisms. - The presence of **>10^4 CFU/mL** of a pure culture of a known urinary pathogen (_S. saprophyticus_) in a symptomatic patient makes contamination unlikely. *Repeat culture needed* - Repeat cultures are indicated when initial results are equivocal (e.g., low counts, mixed flora, or asymptomatic bacteriuria). - For symptomatic UTI with **>10^4 CFU/mL** of a known pathogen, a single culture is sufficient for diagnosis and treatment initiation. - Multiple consecutive samples are primarily used for diagnosing **bacteremia** or **endocarditis**, not routine UTI.

Question 145: A frequent traveler presented with 4 days of continuous fever, abdominal pain, and bradycardia. What is the best diagnostic test to confirm the pathogen?

A. Widal test
B. Blood culture (Correct Answer)
C. Urine culture
D. Stool culture

Explanation: ***Blood culture*** - **Blood culture** is the most sensitive and specific test for confirming **typhoid fever** in the first week of illness. - The presence of **continuous fever** (step-ladder pattern), **abdominal pain**, and **relative bradycardia** in a traveler strongly suggests typhoid fever caused by *Salmonella Typhi*. *Widal test* - The **Widal test** detects antibodies against *Salmonella Typhi* antigens and is often positive later in the disease course. - It has **limited sensitivity and specificity**, especially in endemic areas or with prior vaccination, leading to false positives and negatives. *Urine culture* - **Urine culture** has a low yield for *Salmonella Typhi*, as bacteria are intermittently shed in urine, usually later in the disease. - It's primarily useful for diagnosing **urinary tract infections** or in chronic carriers of typhoid. *Stool culture* - **Stool culture** yield is higher in the later stages of typhoid fever, as *Salmonella Typhi* is shed in feces. - Its sensitivity is lower than blood culture in the early acute phase when bacteremia is most prominent.

Question 146: A child presented with bloody stools and abdominal pain. Which enrichment medium should be used for processing the fecal sample?

A. Blood agar
B. Selenite F broth (Correct Answer)
C. Alkaline peptone water
D. Muller Hinton Broth

Explanation: ***Selenite F broth*** - This **enrichment medium** is specifically designed to isolate **Salmonella** and some species of **Shigella**, which are common causes of bloody stools and abdominal pain in children. - It inhibits the growth of commensal gut flora, allowing pathogenic bacteria to proliferate and be subsequently identified on selective media. *Blood agar* - Blood agar is a **general-purpose enrichment medium** that supports the growth of a wide range of bacteria but does not selectively enrich for specific pathogens. - It would be ineffective in outcompeting the normal fecal flora to isolate rarer enteric pathogens causing the symptoms. *Alkaline peptone water* - This medium is primarily used for the enrichment of **Vibrio cholerae** species, which typically cause watery diarrhea, not bloody stools. - While it helps in the isolation of *Vibrio* species, it is not suitable for the suspected pathogens in this clinical scenario. *Muller Hinton Broth* - Muller-Hinton media are primarily used for **antimicrobial susceptibility testing** (antibiotic sensitivity testing) and are not designed for the primary isolation or enrichment of specific pathogens from clinical samples. - It would not provide a selective advantage for the recovery of organisms causing bloody diarrhea from a fecal sample.

Question 147: A patient on steroids presented with nocturnal cough and chronic urticaria. Bronchoalveolar lavage (BAL) staining was done, and the organism shown in the image was identified. What is the most likely organism?

A. Enterobius vermicularis
B. Strongyloides stercoralis (Correct Answer)
C. Capillaria philippinensis
D. Ancylostoma duodenale

Explanation: ***Strongyloides stercoralis*** - The image shows a nematode larva in a **bronchoalveolar lavage (BAL) sample**, consistent with **Strongyloides stercoralis**, which is known for its ability to **autoinfect** and cause chronic infections. - The patient's **steroid use** can lead to **Strongyloides hyperinfection syndrome**, explaining the pulmonary symptoms (**nocturnal cough**) due to larval migration into the lungs and the chronic **urticaria** often associated with strongyloidiasis. *Enterobius vermicularis* - This organism primarily causes **pinworm infection**, characterized by **perianal itching** and is typically identified by the **scotch tape test** for eggs, not usually found in BAL or associated with chronic urticaria. - It does not commonly cause significant pulmonary symptoms or hyperinfection syndrome, especially not triggered by steroid use. *Capillaria philippinensis* - This nematode causes **intestinal capillariasis**, characterized by **chronic diarrhea**, **abdominal pain**, and **malabsorption**. - It is not typically associated with pulmonary symptoms like nocturnal cough or chronic urticaria, and is transmitted through fish consumption. *Ancylostoma duodenale* - This is a **hookworm** that causes **iron deficiency anemia**. While it does have a lung migration phase that can cause transient cough, it is not typically associated with chronic urticaria or severe pulmonary disease due to steroid-induced hyperinfection. - The image shows larvae, but the clinical context strongly points to *Strongyloides* due to the steroid use and specific symptoms.

Question 148: A patient was suspected of having brucellosis. A serum sample was sent for a standard agglutination test, which was initially negative but became positive after dilution of the sample. What is the most likely reason for the initial negative test?

A. Antigen antibody complexes
B. Postzone phenomenon
C. Complement inactivation
D. Prozone phenomenon (Correct Answer)

Explanation: ***Correct: Prozone phenomenon*** - The **prozone phenomenon** occurs when there is a very high concentration of antibodies in the patient's serum, leading to the formation of small antigen-antibody complexes that do not agglutinate or precipitate. - Diluting the sample reduces the antibody concentration, allowing for optimal antigen-antibody lattice formation and visible agglutination. - This is the classic explanation for a **negative test becoming positive after dilution** in brucellosis serology. *Incorrect: Antigen antibody complexes* - While agglutination tests rely on the formation of **antigen-antibody complexes**, the initial negative result despite a positive finding after dilution indicates a specific issue with complex *visibility* or *stability* rather than the general presence of complexes. - This option is too general and doesn't explain why dilution would change the result from negative to positive. *Incorrect: Postzone phenomenon* - The **postzone phenomenon** occurs when there is an *excess of antigen* relative to antibody, leading to no visible agglutination. - In such a case, diluting the sample (which would reduce antigen concentration or keep antibody concentration too low) would typically *not* lead to a positive result; in fact, further dilution of antibodies would worsen the outcome. - Postzone is the opposite mechanism and would not be corrected by dilution. *Incorrect: Complement inactivation* - **Complement inactivation** is not directly relevant to the mechanism of agglutination tests, which primarily depend on direct antibody-antigen binding for visible clumping. - These tests do not typically require complement activity for their primary reaction, nor are they inhibited by complement inactivation.

Question 149: A truck driver presented with a painless, demarcated ulcer on the penis and inguinal lymphadenopathy. What is the best method to visualize the motility of the most likely causative agent?

A. Fluorescent microscopy
B. Light microscopy
C. Dark field microscopy (Correct Answer)
D. Electron microscopy

Explanation: ***Dark field microscopy*** - The symptoms (painless, demarcated penile ulcer and inguinal lymphadenopathy) are highly suggestive of **primary syphilis**, caused by *Treponema pallidum*. - **Dark field microscopy** is the gold standard for visualizing the characteristic **corkscrew motility** of *T. pallidum* directly from lesion exudate. *Fluorescent microscopy* - This technique uses **fluorochromes** to stain structures and is often used in **immunofluorescence** assays to detect antibodies or antigens. - While useful for some microbial identification, it is not the primary method for visualizing the motility of *Treponema pallidum*. *Light microscopy* - Standard light microscopy has **insufficient resolution** to clearly visualize the thin, coiled spirochetes of *Treponema pallidum* or their motility. - The organisms are generally **too small and refractile** to be easily seen without specialized illumination. *Electron microscopy* - Provides extremely **high resolution** and is used for studying viral structures or detailed cellular ultrastructure. - It is **not practical** for routine clinical diagnosis, especially for live, motile bacteria, and is not used to observe motility.

Question 150: On Ziehl-Neelsen (ZN) staining oocysts of size 8-10 µm are visible. Identify the organism.

A. Microsporidium
B. Cryptosporidium
C. Isospora belli
D. Cyclospora (Correct Answer)

Explanation: ***Cyclospora*** - *Cyclospora cayetanensis* oocysts are **spherical** and measure **8-10 µm** in diameter, exactly matching the size described in the question. - They are **partially acid-fast** on ZN staining and may show **internal structures** or appear **wrinkled**, making them visible under microscopic examination. *Microsporidium* - *Microsporidium* are extremely small (**1-4 µm**) **intracellular obligate parasites**, much smaller than the 8-10 µm oocysts described. - They require **special staining methods** and are not typically visible with standard ZN staining due to their minute size. *Cryptosporidium* - *Cryptosporidium parvum* oocysts are **spherical** and measure **4-6 µm** in diameter, which is smaller than the 8-10 µm described. - While they are **acid-fast** on ZN staining, their smaller size distinguishes them from the oocysts in question. *Isospora belli* - *Isospora belli* oocysts are **elongated or oval** and measure **20-30 µm** in length, making them significantly larger than the 8-10 µm oocysts described. - Although they are **acid-fast** and visible on ZN staining, their larger size and oval morphology do not match the question parameters.

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