Diagnostic Microbiology Practice Questions

Q: Which selective medium is used for Corynebacterium diphtheriae?

Tellurite blood agar. **Explanation:** *Corynebacterium diphtheriae* requires specific media for isolation and identification. The correct answer is **Tellurite Blood Agar (TBA)**. **1. Why Tellurite Blood Agar is correct:** Tellurite blood agar (e.g., McLeod’s medium) is a **selective medium**. Potassium tellurite inhibits the growth of most normal upper respiratory flora (like Streptococci and Staphylococci) while allowing *C. diphtheriae* to grow. Furthermore, *C. diphtheriae* reduces tellurite to metallic tellurium, which results in the characteristic **grey-to-black colored colonies**, aiding in presumptive identification. **2. Analysis of other options:** * **Loffler’s Serum Slope (LSS):** This is an **enriched medium**, not selective. It is used for the rapid growth of *C. diphtheriae* (6-8 hours) and is excellent for demonstrating the characteristic **metachromatic granules** (Volutin/Babes-Ernst granules) via Albert’s stain. * **Chocolate Agar:** This is a non-selective enriched medium used primarily for fastidious organisms like *Haemophilus influenzae* and *Neisseria*. It does not inhibit commensal flora. * **Tinsdale Medium:** While Tinsdale is also a selective medium for *C. diphtheriae* (producing black colonies with a brown halo), **Tellurite Blood Agar** is the classic, more frequently tested primary selective medium in the context of this standard question. **High-Yield Clinical Pearls for NEET-PG:** * **Morphology:** Gram-positive, non-motile bacilli showing "Chinese letter" or cuneiform arrangement. * **Staining:** Albert’s stain is used to visualize metachromatic granules (granules appear bluish-black, body appears green). * **Toxin Detection:** The **Elek’s Gel Precipitation Test** is the gold standard for detecting toxigenicity (distinguishing *C. diphtheriae* from diphtheroids). * **Culture sequence:** Clinical samples are usually inoculated onto LSS (for speed) and TBA (for selectivity) simultaneously.

Q: Superoxol test is positive in which organism?

Neisseria gonorrhoeae. **Explanation:** The **Superoxol test** is a rapid biochemical test used primarily for the presumptive identification of *Neisseria gonorrhoeae*. It is essentially a modified catalase test that utilizes a **30% hydrogen peroxide ($H_2O_2$)** solution instead of the standard 3% used for most bacteria. 1. **Why Option A is Correct:** * *Neisseria gonorrhoeae* possesses a very high concentration of the catalase enzyme. When 30% $H_2O_2$ is applied to a colony, it results in **immediate, vigorous bubbling** (effervescence). This strong positive reaction is diagnostic for *N. gonorrhoeae*. 2. **Why Options B and C are Incorrect:** * **Neisseria meningitidis:** Most strains show a weak or delayed reaction, or are completely negative with 30% $H_2O_2$. * **Neisseria lactamica:** This commensal organism typically gives a negative or very weak reaction. * While all *Neisseria* species are catalase-positive (using 3% $H_2O_2$), only *N. gonorrhoeae* consistently produces the brisk reaction required for a positive **Superoxol** test. **High-Yield Clinical Pearls for NEET-PG:** * **Specimen of Choice:** In males, urethral discharge; in females, the endocervix is the preferred site for *N. gonorrhoeae* culture. * **Culture Media:** Thayer-Martin Medium (Selective) or Chocolate Agar (Non-selective). * **Gram Stain:** Gram-negative "kidney-bean" shaped diplococci, often found within polymorphonuclear leukocytes (intracellular). * **Biochemical Profile:** *N. gonorrhoeae* ferments **Glucose only**, whereas *N. meningitidis* ferments both **Glucose and Maltose**. * **Drug of Choice:** Ceftriaxone (due to widespread penicillin resistance/PPNG strains).

Q: What is the most commonly performed test for serodiagnosis of syphilis?

VDRL. **Explanation:** The **VDRL (Venereal Disease Research Laboratory)** test is the most commonly performed screening test for the serodiagnosis of syphilis. It is a **non-specific (non-treponemal)** test that detects **reagin antibodies** (IgM and IgG) produced against cardiolipin-lecithin-cholesterol antigen. Its popularity in clinical practice stems from its low cost, rapid results, and high sensitivity during the secondary stage of syphilis. **Analysis of Options:** * **VDRL (Correct):** It is the standard screening tool. Because it is a flocculation test, it can be used for both qualitative and quantitative (titre) assessment, making it ideal for monitoring treatment response. * **TPHA (Treponema Pallidum Hemagglutination Assay):** This is a **specific (treponemal)** test. While highly specific, it is generally used as a confirmatory test after a reactive screening result, rather than the initial common screening tool. * **Wassermann Test:** This was the first complement fixation test for syphilis. It is now obsolete and has been replaced by modern flocculation tests like VDRL and RPR. * **TPT:** This is not a standard acronym for a primary syphilis diagnostic test in the context of common serology. **High-Yield Clinical Pearls for NEET-PG:** 1. **CSF Examination:** VDRL is the only serological test recommended for the diagnosis of **Neurosyphilis** using CSF. 2. **Prozone Phenomenon:** False negatives can occur in secondary syphilis due to very high antibody titers; this is corrected by diluting the serum. 3. **Biological False Positives (BFP):** Conditions like SLE, leprosy, malaria, and pregnancy can cause false-positive VDRL results. 4. **Treatment Monitoring:** A four-fold drop in VDRL titer indicates successful treatment.

Q: A patient was suffering from hepatorenal syndrome. A urine sample was obtained from the patient and examined under microscopy. Which of the following techniques is being used to make the organism visible?

Dark field microscopy. ***Dark field microscopy*** - **Leptospira** organisms are extremely thin (~0.1 µm) and cannot be visualized under routine light microscopy due to their **spiral morphology** and **low refractive index**. - **Dark field illumination** creates a bright organism against a dark background by scattering light, making these thin spirochetes visible in urine samples from **leptospirosis** patients. *Routine microscopy with negatively stained background* - **Negative staining** uses electron-dense stains like **nigrosin** or **Congo red** for electron microscopy, not for live spirochete visualization. - This technique is primarily used for **structural studies** and **viral particles**, not for diagnosing leptospirosis in clinical samples. *Phase contrast microscopy* - **Phase contrast** enhances contrast in **transparent specimens** by converting phase differences into amplitude differences, but **Leptospira** are too thin to be effectively visualized. - This technique is better suited for **larger microorganisms** like **yeasts** and **parasites**, not for thin spirochetes. *Compound light microscopy* - **Conventional light microscopy** cannot visualize **Leptospira** due to their extremely small diameter and **lack of sufficient contrast** against the background. - **Leptospira** organisms have a **refractive index** similar to water, making them essentially invisible under standard bright-field illumination.

Q: Reverse transcriptase polymerase chain reactions can aid in the diagnosis of all of the following viral infections except?

Adenovirus. **Explanation:** The fundamental principle behind this question lies in the type of nucleic acid present in the virus. **Reverse Transcriptase Polymerase Chain Reaction (RT-PCR)** is a laboratory technique used to amplify **RNA** sequences. It involves two steps: first, the enzyme reverse transcriptase converts RNA into complementary DNA (cDNA), which is then amplified using standard PCR. 1. **Why Adenovirus is the correct answer:** **Adenovirus** is a **double-stranded DNA (dsDNA) virus**. Since its genome is already DNA, it does not require the reverse transcription step. Diagnosis of Adenovirus is typically performed using standard **PCR**, viral culture, or antigen detection. 2. **Why the other options are incorrect:** * **Astrovirus:** These are positive-sense, single-stranded RNA (+ssRNA) viruses. * **Rotavirus:** These are double-stranded RNA (dsRNA) viruses (Reoviridae family). * **Poliovirus:** These are +ssRNA viruses (Picornaviridae family). Because Astrovirus, Rotavirus, and Poliovirus all possess **RNA genomes**, RT-PCR is the gold-standard molecular method for their detection and quantification. **High-Yield Clinical Pearls for NEET-PG:** * **DNA Viruses:** Most are dsDNA (except Parvoviridae, which is ssDNA). Remember the mnemonic "HHAPPPy" (Herpes, Hepadna, Adeno, Papova, Pox, Parvo). * **RNA Viruses:** Most are ssRNA (except Reoviridae/Rotavirus, which is dsRNA). * **RT-PCR vs. PCR:** Use RT-PCR for RNA viruses (HIV, HCV, SARS-CoV-2, Influenza) and standard PCR for DNA viruses (HBV, HSV, CMV). * **Adenovirus:** Common cause of pharyngoconjunctival fever, hemorrhagic cystitis, and epidemic keratoconjunctivitis (Pink eye).

Q: What is the fastest method for the diagnosis of TB?

GeneXpert. **Explanation:** The correct answer is **GeneXpert (CBNAAT)** because it is a molecular diagnostic tool that utilizes real-time PCR technology. It can detect *Mycobacterium tuberculosis* (MTB) and rifampicin resistance simultaneously within **approximately 2 hours**. In the context of rapid diagnosis, molecular methods are significantly faster than any culture-based method. **Analysis of Options:** * **GeneXpert (CBNAAT):** The fastest method. It detects the *rpoB* gene of MTB. It is currently the initial diagnostic test of choice under the National TB Elimination Program (NTEP) in India. * **Liquid Medium (e.g., Middlebrook 7H12):** While faster than solid media (like LJ medium), it still requires bacterial growth, which typically takes **1 to 3 weeks**. * **MGIT (Mycobacteria Growth Indicator Tube):** This is a type of liquid culture system. While it is the "Gold Standard" for sensitivity, it is not the "fastest" as it relies on metabolic activity and growth. * **Bactec MGIT 960:** An automated version of the liquid culture. It is highly efficient for high-volume labs but still takes **days to weeks** to provide a result compared to the **hours** required for GeneXpert. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** Culture (specifically Liquid Culture/MGIT). * **Fastest Method:** GeneXpert (CBNAAT). * **Most Sensitive Method:** Liquid Culture (MGIT). * **Solid Medium:** Lowenstein-Jensen (LJ) medium (takes 6–8 weeks; "Rough, Tough, and Buff" colonies). * **True Speed:** While Microscopy (ZN Stain) is technically faster (minutes), it has very low sensitivity (requires >10,000 bacilli/ml) and cannot confirm viability or drug resistance, making GeneXpert the definitive "fastest diagnostic" answer in exams.

Q: Which of the following methods allows for rapid evaluation of fungal hyphae and spores?

Potassium hydroxide (KOH) wet mount. **Explanation:** The correct answer is **C. Potassium hydroxide (KOH) wet mount.** **1. Why KOH Wet Mount is Correct:** The KOH mount is the **gold standard for rapid bedside evaluation** of fungal elements. When a clinical sample (skin scrapings, hair, or nails) is treated with 10–20% KOH, the strong alkali digests the keratin and cellular debris without affecting the fungal cell walls (which contain chitin). This "clearing" effect makes the fungal hyphae, spores, and budding yeast cells highly refractile and easily visible under a light microscope within minutes. **2. Why Other Options are Incorrect:** * **A. Grocott Gomori Methenamine Silver (GMS):** While this is the best stain for visualizing fungal morphology (staining them black), it is a complex histological process that takes hours to perform. It is not a "rapid" evaluation tool. * **B. Hematoxylin and Eosin (H&E):** This is a routine tissue stain. Many fungi are poorly visualized or appear pale/negative on H&E, making it unreliable for primary fungal screening. * **D. Periodic Acid Schiff (PAS):** This stain highlights the polysaccharides in the fungal cell wall (staining them bright pink/magenta). Like GMS, it is a laboratory-based histological stain and not a rapid point-of-care test. **Clinical Pearls for NEET-PG:** * **Modified KOH:** Adding **Calcofluor White** (a fluorescent dye) to the KOH mount enhances sensitivity by binding to cellulose/chitin, making fungi fluoresce under UV light. * **Tinea Versicolor:** KOH mount typically shows the characteristic **"Spaghetti and Meatball"** appearance (short hyphae and yeast clusters). * **Safety:** KOH is used specifically for specimens rich in keratin; for mucoid specimens like sputum, it acts as a mucolytic agent.

Q: Which of the following is a rapid growth medium for Klebs-Loeffler Bacillus?

Loeffler’s serum slope. **Explanation:** **Klebs-Loeffler Bacillus (KLB)** is the eponym for *Corynebacterium diphtheriae*. The diagnosis of diphtheria relies heavily on specific culture media due to the fastidious nature of the organism. **1. Why Loeffler’s Serum Slope (LSS) is correct:** LSS is an **enriched medium** containing horse, ox, or sheep serum. It is specifically designed for the **rapid growth** of *C. diphtheriae*. The organism grows faster on LSS (within 6–8 hours) than on most other media. Furthermore, LSS enhances the development of the characteristic **metachromatic granules** (Volutin/Babes-Ernst granules) and preserves the typical morphology of the bacilli, which may be lost on more selective media. **2. Why other options are incorrect:** * **A. Cysteine-tellurite blood agar (CTBA):** This is a **selective medium** (e.g., McLeod’s or Hoyle’s medium). While it is essential for isolating *C. diphtheriae* from mixed flora (by inhibiting commensals), it is **slow-growing**, requiring 24–48 hours. Potassium tellurite is reduced to metallic tellurium, giving the colonies a characteristic grey-black color. * **C. Nutrient broth:** This is a basal medium. *C. diphtheriae* is fastidious and requires enrichment (serum or blood) for optimal growth; it will not show rapid or characteristic growth in simple nutrient broth. **NEET-PG High-Yield Pearls:** * **Staining:** Use **Albert’s stain** to visualize metachromatic granules (granules appear bluish-black, while the body appears green). * **Morphology:** Described as "Chinese letter" or "Cuneiform" arrangement due to incomplete separation during binary fission (snapping division). * **Toxigenicity Testing:** The **Elek’s gel precipitation test** is the gold standard for detecting toxin production. * **Culture sequence:** In clinical practice, a swab is usually inoculated onto both LSS (for rapid presumptive diagnosis) and a Tellurite medium (for selective isolation).

Question 1

Which selective medium is used for Corynebacterium diphtheriae?

Accepted Answer

Tellurite blood agar

Answer

Loffeler's serum slop

Answer

Chocolate agar

Answer

Tinsdale medium

Question 2

Superoxol test is positive in which organism?

Accepted Answer

Neisseria gonorrhoeae

Answer

Neisseria meningitides

Answer

Neisseria lactamica

Answer

All of the above

Question 3

What is the most commonly performed test for serodiagnosis of syphilis?

Accepted Answer

VDRL

Answer

TPHA

Answer

Wassermann test

Answer

TPT

Question 4

A patient was suffering from hepatorenal syndrome. A urine sample was obtained from the patient and examined under microscopy. Which of the following techniques is being used to make the organism visible?

Accepted Answer

Dark field microscopy

Answer

Routine microscopy with negatively stained background

Answer

Phase contrast microscopy

Answer

Compound light microscopy

Question 5

Reverse transcriptase polymerase chain reactions can aid in the diagnosis of all of the following viral infections except?

Accepted Answer

Adenovirus

Answer

Astrovirus

Answer

Rotavirus

Answer

Poliovirus

Question 6

What is the fastest method for the diagnosis of TB?

Accepted Answer

GeneXpert

Answer

Liquid medium

Answer

MGIT

Answer

Bactec MGIT 960

Question 7

Which gene is used for the diagnosis of TB?

Accepted Answer

1-2 hours

Answer

5 hours

Answer

10 hours

Answer

20 hours

Question 8

Which of the following methods allows for rapid evaluation of fungal hyphae and spores?

Accepted Answer

Potassium hydroxide (KOH) wet mount

Answer

Grocott Gomori Methenamine silver stain

Answer

Hematoxylin and Eosin stain

Answer

Periodic Acid Schiff stain

Question 9

Which of the following is a rapid growth medium for Klebs-Loeffler Bacillus?

Accepted Answer

Loeffler’s serum slope

Answer

Cysteine-tellurite blood agar

Answer

Nutrient broth

Answer

None of these

Question 10

Which of the following antibiotics is added to Sabouraud's dextrose agar to inhibit the growth of contaminating bacteria?

Accepted Answer

Chloramphenicol

Answer

Gentamicin

Answer

Penicillin

Answer

Doxycycline

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

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