Diagnostic Microbiology Practice Questions

Q: A false-negative tuberculin test is seen in all the following conditions EXCEPT:

Atypical mycobacterial infection. The Tuberculin Skin Test (Mantoux test) is a classic example of a **Type IV (Delayed-Type) Hypersensitivity reaction**, mediated by T-lymphocytes. A false-negative result occurs when the body fails to mount an adequate cell-mediated immune (CMI) response despite being infected with *M. tuberculosis*. ### **Why "Atypical mycobacterial infection" is the correct answer:** Atypical mycobacteria (Non-Tuberculous Mycobacteria or NTM) share several common antigens with *M. tuberculosis*. Therefore, infection with NTM typically causes **cross-reactivity**, leading to a **false-positive** result (or a small induration), rather than a false-negative. ### **Explanation of Incorrect Options (Causes of False-Negatives):** * **After a 4-6 week history of measles attack:** Viral infections like measles, mumps, and chickenpox are known to cause transient **anergy** (suppression of CMI), leading to false-negative results. * **Immunodeficiency state:** Conditions such as HIV/AIDS, Hodgkin’s lymphoma, or treatment with immunosuppressants (steroids, anti-TNF agents) impair T-cell function, preventing the induration. * **Miliary tuberculosis:** In overwhelming or disseminated TB, the body’s immune system is "overloaded" or exhausted, leading to a lack of localized skin response (anergy). ### **High-Yield Clinical Pearls for NEET-PG:** * **Reading the test:** The result is read after **48–72 hours**. Only the **induration** (palpable hardness) is measured, not the erythema (redness). * **The "Window Period":** It takes 2–10 weeks after initial infection for the Mantoux test to become positive. Testing during this window yields a false-negative. * **Storage:** PPD (Purified Protein Derivative) should be stored at 2–8°C and protected from light to prevent degradation. * **BCG Vaccine:** Can cause a false-positive, though the induration is usually smaller and wanes over time.

Q: A false positive VDRL test is seen in which of the following conditions?

All of the above. **Explanation:** The **VDRL (Venereal Disease Research Laboratory)** test is a non-specific, non-treponemal screening test for Syphilis. It detects **reagin antibodies** (IgM and IgG) directed against a **cardiolipin-cholesterol-lecithin antigen**. Because cardiolipin is a normal component of mitochondrial membranes, any condition causing significant tissue damage or autoimmune stimulation can lead to the production of anti-cardiolipin antibodies, resulting in a **Biological False Positive (BFP)** reaction. **Analysis of Options:** * **Leprosy:** Specifically lepromatous leprosy, is a classic cause of chronic BFP due to altered immune responses and tissue destruction. * **Malaria:** Acute febrile illnesses like malaria often cause transient (acute) BFP reactions. * **Relapsing Fever:** Caused by *Borrelia recurrentis*, this spirochetal infection shares antigenic similarities or causes enough systemic inflammation to trigger a positive VDRL. Since all three conditions are well-documented causes of BFP, **Option D** is the correct answer. **High-Yield Clinical Pearls for NEET-PG:** * **Classification of BFP:** * **Acute ( 6 months):** Seen in Leprosy, SLE (Systemic Lupus Erythematosus), Rheumatoid Arthritis, and IV drug use. * **Confirmatory Test:** To rule out a false positive, a specific treponemal test like **FTA-ABS** or **TPHA** must be performed. * **Prozone Phenomenon:** A false-negative VDRL can occur due to very high antibody titers (seen in secondary syphilis); this is corrected by diluting the serum.

Q: What is the transport medium for Vibrio cholerae?

Cary-Blair medium. **Explanation:** The correct answer is **Cary-Blair medium**. In microbiology, choosing the right medium depends on whether you intend to transport, enrich, or selectively grow the pathogen. **1. Why Cary-Blair is correct:** Cary-Blair is the **preferred transport medium** for *Vibrio cholerae* and other enteric pathogens (like *Salmonella* and *Shigella*). It is a non-nutritive semi-solid medium with a high pH (8.4), which is essential because *Vibrio* species are highly sensitive to acidic environments but thrive in alkaline conditions. It maintains the viability of the organism without allowing overgrowth of commensal flora during transit to the lab. **2. Why other options are incorrect:** * **Alkaline Peptone Water (APW):** This is an **enrichment medium**, not primarily a transport medium. It is used to increase the concentration of *Vibrio* in a stool sample before subculturing onto solid agar. * **TCBS (Thiosulfate Citrate Bile Salts Sucrose) Agar:** This is a **selective and differential medium** used for the primary isolation of *V. cholerae*. It is not used for transport. On TCBS, *V. cholerae* produces large yellow colonies due to sucrose fermentation. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Diagnosis:** Stool culture on TCBS agar. * **Other Transport Media:** Venkatraman-Ramakrishnan (VR) medium and Monsur’s anaerobic medium are also used for *Vibrio*. * **Microscopy:** *Vibrio* exhibits characteristic **"darting motility"** (like a swarm of gnats), which can be inhibited by specific antisera (Pfeiffer’s phenomenon). * **Staining:** Gram-negative, comma-shaped bacilli (curved rods).

Q: Dark ground microscopy is used for which of the following tests?

TPI (Treponema pallidum immobilization test). **Explanation:** **1. Why TPI is the correct answer:** The **Treponema pallidum immobilization (TPI) test** is a specific treponemal test used to detect antibodies in a patient's serum. In this test, live *Treponema pallidum* (Nichols strain) are mixed with the patient's serum and complement. If specific antibodies are present, they cause the live spirochetes to lose their motility (immobilize). Because *T. pallidum* is extremely thin and cannot be seen under a standard light microscope, **Dark Ground Microscopy (DGM)** is essential to visualize the live, moving spirochetes and observe the immobilization effect. **2. Why other options are incorrect:** * **FTA-ABS:** This is an indirect immunofluorescence test. It requires a **Fluorescence Microscope** to detect the presence of fluorescent-tagged antibodies bound to *T. pallidum* on a slide. * **VDRL & Kahn’s Test:** These are non-treponemal (reaginic) screening tests based on the principle of **flocculation**. VDRL results are typically read using a standard **light microscope** (low power) to look for clumps, while Kahn’s test is a tube flocculation test read macroscopically. **3. NEET-PG High-Yield Pearls:** * **DGM Principle:** It uses a special condenser that prevents direct light from entering the objective; only light reflected/scattered by the specimen enters, making the organism appear bright against a dark background. * **Primary Syphilis:** DGM is the method of choice for diagnosing primary syphilis by demonstrating *T. pallidum* from chancre fluid. * **TPI Status:** Although the "Gold Standard" for specificity, TPI is no longer used routinely in clinical practice because it requires maintaining live treponemes in rabbits. * **Other uses of DGM:** Apart from *Treponema*, it is used to visualize *Leptospira* and *Borrelia*.

Q: The Tuberculin test is administered via which route of administration?

Intradermal. The Tuberculin Skin Test (TST), also known as the **Mantoux test**, is a classic example of a **Type IV (Delayed-type) Hypersensitivity reaction**. ### Why Intradermal is Correct The test involves the injection of 0.1 mL of **Purified Protein Derivative (PPD)** containing 5 Tuberculin Units (TU). The **intradermal route** is essential because the dermis contains a high concentration of antigen-presenting cells (Langerhans cells). These cells process the PPD and present it to sensitized T-lymphocytes. If a person has been previously exposed to *M. tuberculosis*, a localized inflammatory response occurs, resulting in **induration** (palpable hardness) within 48–72 hours. ### Why Other Options are Incorrect * **Subcutaneous:** Injecting PPD into the fatty tissue below the dermis leads to rapid absorption into the systemic circulation. This prevents the localized cellular interaction required for a visible skin reaction, leading to a **false-negative** result. * **Intramuscular:** Similar to the subcutaneous route, the vascularity of muscle tissue causes the antigen to be carried away too quickly to trigger a local delayed-type hypersensitivity response. * **Subdermal:** This is a non-standard medical term often confused with intradermal; however, the specific technique required for Mantoux is strictly intradermal (forming a 6–10 mm wheal). ### NEET-PG High-Yield Pearls * **Reading the Test:** Measure the diameter of **induration** (not erythema) transverse to the long axis of the forearm. * **False Positives:** Common in individuals vaccinated with **BCG** or those with Non-Tuberculous Mycobacteria (NTM) infections. * **False Negatives (Anergy):** Can occur in miliary TB, HIV/AIDS (low CD4 count), malnutrition, or recent viral infections (e.g., Measles). * **Alternative:** The **IGRA (Interferon-Gamma Release Assay)** is preferred for BCG-vaccinated individuals as it is more specific.

Q: Which compound is used for the fixation of protozoa found in stool?

Formalin. **Explanation:** The preservation of stool samples is critical for the accurate diagnosis of parasitic infections, as trophozoites and cysts can degenerate rapidly after collection. **1. Why Formalin is the Correct Answer:** **Formalin (10% aqueous solution)** is the most widely used preservative for stool specimens. It is highly effective at preserving the morphology of **protozoan cysts** and helminth eggs/larvae for long periods. It is the gold standard for concentration techniques (like the Formalin-Ether sedimentation method) and is compatible with direct wet mounts and automated immunoassay kits. **2. Analysis of Incorrect Options:** * **Phenol:** While phenol has disinfectant properties, it is not used as a primary fixative for stool parasites because it can distort delicate cellular structures. * **Hypochlorite:** Sodium hypochlorite (bleach) is a powerful disinfectant used for surface decontamination. It is **contraindicated** for parasite preservation as it destroys the morphology of cysts and trophozoites, making microscopic identification impossible. * **Alcohol:** While 70% ethanol is used for preserving adult worms (helminths), it is not the primary choice for protozoa in stool. However, Polyvinyl Alcohol (PVA) is often used as a "glue" to attach stool to slides for permanent staining (like Trichrome), but the active fixative in that mixture is usually Schaudinn’s fluid (mercuric chloride). **Clinical Pearls for NEET-PG:** * **Two-Vial System:** Standard protocol often involves one vial of **10% Formalin** (for concentration/wet mounts) and one vial of **PVA** (for permanent staining). * **Trophozoite Preservation:** Formalin preserves cysts well, but for the fragile **trophozoite** stage, Schaudinn’s fixative or SAF (Sodium acetate-acetic acid-formalin) is preferred. * **MIF (Merthiolate-Iodine-Formalin):** A common field stain/fixative that both preserves and stains the specimen simultaneously.

Q: Reverse CAMP test is positive for which bacterium?

Clostridium perfringens. ### Explanation The **Reverse CAMP test** is a diagnostic laboratory technique used primarily for the presumptive identification of **Clostridium perfringens**. #### Why Clostridium perfringens is correct: In a standard CAMP test, *Streptococcus agalactiae* produces a diffusible protein (CAMP factor) that enhances the beta-hemolysis of *Staphylococcus aureus*. In the **Reverse CAMP test**, the roles are flipped: a known strain of **Streptococcus agalactiae** (Group B Strep) is streaked perpendicularly to the test organism (**C. perfringens**). * *C. perfringens* produces **alpha-toxin (lecithinase)**. * When this toxin interacts with the CAMP factor produced by *S. agalactiae*, it results in synergistic hemolysis. * **Positive Result:** An **arrowhead-shaped zone of hemolysis** pointing toward the *S. agalactiae* streak. #### Why other options are incorrect: * **Streptococcus agalactiae:** It is the classic organism used to perform a **Standard CAMP test** (positive when streaked against *S. aureus*). * **Listeria monocytogenes:** It also gives a **positive Standard CAMP test** (producing a rectangular or "block" shape of hemolysis), not a Reverse CAMP test. * **Vibrio parahaemolyticus:** This organism is associated with the **Kanagawa phenomenon** (heat-stable hemolysin on Wagatsuma agar), not the CAMP reaction. #### High-Yield Clinical Pearls for NEET-PG: * **C. perfringens:** Most common cause of gas gangrene (myonecrosis) and food poisoning. * **Nagler’s Reaction:** Another biochemical test for *C. perfringens* that detects lecithinase activity on egg yolk agar (inhibited by antitoxin). * **Stormy fermentation:** Characteristically seen in litmus milk culture due to acid and gas production by *C. perfringens*. * **Double zone of hemolysis:** A key morphological feature of *C. perfringens* on blood agar (inner zone of complete hemolysis due to theta-toxin; outer zone of incomplete hemolysis due to alpha-toxin).

Q: Phage typing is widely used for the intraspecies classification of which of the following bacteria?

Staphylococci. ### Explanation **Phage typing** is a phenotypic method used for the epidemiological surveillance and intraspecies differentiation of bacteria based on their susceptibility to specific bacteriophages (viruses that infect bacteria). **1. Why Staphylococci is Correct:** Phage typing is the "Gold Standard" classical method for the epidemiological subtyping of **_Staphylococcus aureus_**. It utilizes a standardized set of phages (The International Basic Set) to identify specific strains. This is crucial in clinical settings to trace the source of hospital-acquired outbreaks (nosocomial infections), such as identifying a specific "nursery strain" or a colonized healthcare worker. **2. Analysis of Incorrect Options:** * **E. coli:** While phage typing exists for *E. coli*, it is not the primary method for intraspecies classification. Instead, **Serotyping** (based on O, H, and K antigens) is the standard method used for *E. coli* classification. * **Klebsiella pneumoniae:** The primary methods for subtyping *Klebsiella* are **Bacteriocin (Pyocin) typing** and **Capsular (K) typing** via the Quellung reaction. * **Pseudomonas aeruginosa:** Although phage typing can be used, **Pyocin typing** (bacteriocin typing) is more commonly associated with *Pseudomonas* in classical microbiology textbooks and exams. **High-Yield Clinical Pearls for NEET-PG:** * **Bacteriocin Typing:** Most commonly used for *Shigella sonnei* and *Pseudomonas aeruginosa*. * **Dienes Phenomenon:** Used for the intraspecies differentiation of *Proteus mirabilis*. * **Modern Trend:** While phage typing is high-yield for exams, it is being replaced in modern labs by molecular methods like **PFGE** (Pulsed-Field Gel Electrophoresis) and **WGS** (Whole Genome Sequencing). * **Vi-phage typing:** Specifically used for the epidemiological typing of *Salmonella Typhi*.

Q: Which test is used to differentiate staphylococci from micrococci?

Hugh and Leifson's oxidation-fermentation test. **Explanation:** The differentiation between **Staphylococci** and **Micrococci** is a common high-yield topic in bacteriology. Both are Gram-positive, catalase-positive cocci, necessitating specialized biochemical tests for distinction. **Why Option B is correct:** The **Hugh and Leifson’s Oxidation-Fermentation (OF) test** is the gold standard for this differentiation. * **Staphylococci** are facultative anaerobes; they utilize glucose **fermentatively** (producing acid in both open and sealed tubes). * **Micrococci** are obligate aerobes; they utilize glucose **oxidatively** (producing acid only in the open tube) or are non-saccharolytic. **Why other options are incorrect:** * **A. Catalase test:** Both Staphylococci and Micrococci are catalase-positive. This test is used to differentiate them from Streptococci (catalase-negative). * **C. Gram staining:** Both appear as Gram-positive cocci. While Staphylococci typically form "grape-like clusters" and Micrococci often form tetrads or sarcina (packets of eight), this is not definitive for differentiation. * **D. Acid-fast staining (AFS):** This is used for Mycobacteria and Nocardia. Neither Staphylococci nor Micrococci are acid-fast. **NEET-PG High-Yield Pearls:** To differentiate these two genera, remember the **"MODS"** criteria for **Micrococci**: 1. **M**icrodase (Modified Oxidase) test: **Positive** (Staph is negative). 2. **O**xidation-Fermentation test: **Oxidative** (Staph is Fermentative). 3. **D**ecolourisation by Lysostaphin: **Resistant** (Staph is Sensitive). 4. **S**ensitivity to Bacitracin (0.04U): **Sensitive** (Staph is Resistant). 5. **Furazolidone sensitivity:** Micrococci are **Resistant**, while Staphylococci are **Sensitive**.

Question 1

A false-negative tuberculin test is seen in all the following conditions EXCEPT:

Accepted Answer

Atypical mycobacterial infection

Answer

After a 4-6 week history of measles attack

Answer

Immunodeficiency state

Answer

Miliary tuberculosis

Question 2

A false positive VDRL test is seen in which of the following conditions?

Accepted Answer

All of the above

Answer

Leprosy

Answer

Malaria

Answer

Relapsing fever

Question 3

What is the transport medium for Vibrio cholerae?

Accepted Answer

Cary-Blair medium

Answer

Alkaline peptone water

Answer

TC BS medium

Answer

None of the above

Question 4

A girl child with involuntary movements was diagnosed as a case of Sydenham's chorea, suggestive of acute rheumatic fever. There are no complaints of carditis or arthritis. Throat culture is negative. Which of the following will most likely suggest recent streptococcal infection?

Accepted Answer

ASLO titre

Answer

Antistreptolysin S

Answer

PCR for M protein

Answer

Anti-hyaluronidase titre

Question 5

Dark ground microscopy is used for which of the following tests?

Accepted Answer

TPI (Treponema pallidum immobilization test)

Answer

FTA-ABS (Fluorescent treponemal antibody absorption test)

Answer

Kahn's test

Answer

VDRL (Venereal Disease Research Laboratory test)

Question 6

The Tuberculin test is administered via which route of administration?

Accepted Answer

Intradermal

Answer

Subcutaneous

Answer

Intramuscular

Answer

Subdermal

Question 7

Which compound is used for the fixation of protozoa found in stool?

Accepted Answer

Formalin

Answer

Phenol

Answer

Hypochlorite

Answer

Alcohol

Question 8

Reverse CAMP test is positive for which bacterium?

Accepted Answer

Clostridium perfringens

Answer

Streptococcus agalactiae

Answer

Listeria monocytogenes

Answer

Vibrio parahemolyticus

Question 9

Phage typing is widely used for the intraspecies classification of which of the following bacteria?

Accepted Answer

Staphylococci

Answer

E. coli

Answer

Klebsiella pneumoniae

Answer

Pseudomonas aeruginosa

Question 10

Which test is used to differentiate staphylococci from micrococci?

Accepted Answer

Hugh and Leifson's oxidation-fermentation test

Answer

Catalase test

Answer

Gram staining

Answer

Acid-fast staining (AFS)

Diagnostic Microbiology — MCQs

Diagnostic Microbiology — MCQs

On this page

Practice by Chapter

Want unlimited practice?