Bacteriology — MCQs
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Q. Which organism is a cause of fever?
A patient undergoing chemotherapy develops a cough. Acid-fast stain of his sputum shows rods and slightly longer forms, with some branching; they vary in their acid-fast reaction from one area of the slide to the next. The acid-fast stain was performed by an experienced medical technologist and, when redone, showed the same variation. The growth was done aerobically. What is the most likely agent?
Which of the following Salmonella species does NOT produce gas?
Most of the Pseudomonas infections are based on opportunism. Which of the following is an exception?
Diagnosis of enteric fever in the 2nd week is best made by which method?
What is the most common site for Actinomycosis?
The Tuberculin test is an example of which type of immune reaction?
Tuberculin test is administered by which route?
Which of the following investigations is done to diagnose typhoid in a patient after 15 days of onset of fever?
Which of the following is an example of an agglutination test?
Bacteriology Indian Medical PG Practice Questions and MCQs
Question 1551: Q. Which organism is a cause of fever?
- A. Coxiella burnetii (Correct Answer)
- B. Ehrlichia sennetsu
- C. Rickettsia prowazekii
- D. Bartonella bacilliformis
Explanation: **Explanation:** The correct answer is **Coxiella burnetii**, which is the causative agent of **Q fever**. **1. Why Coxiella burnetii is correct:** Q fever (Query fever) is a zoonotic disease typically transmitted via inhalation of contaminated aerosols from livestock (cattle, sheep, goats). Unlike other Rickettsial diseases, Q fever is characterized by a high-grade fever, interstitial pneumonia, and hepatitis, but it **characteristically lacks a skin rash**. This makes it a unique "fever of unknown origin" in clinical presentations. **2. Why the other options are incorrect:** * **Ehrlichia sennetsu:** This organism causes **Sennetsu fever** (human monocytotropic ehrlichiosis), primarily seen in Southeast Asia and Japan. While it causes fever, the question specifically points toward the classic "Q fever" association with *Coxiella*. * **Rickettsia prowazekii:** This is the agent of **Epidemic Typhus**, transmitted by the human body louse. It presents with high fever and a characteristic centrifugal rash (spreading from trunk to extremities). * **Bartonella bacilliformis:** This causes **Carrion’s disease**, which has two phases: Oroya fever (acute febrile hemolytic anemia) and Verruga peruana (chronic skin lesions). It is geographically restricted to the Andes mountains. **High-Yield Clinical Pearls for NEET-PG:** * **Coxiella burnetii** is highly resistant to environmental stressors because it forms **spore-like structures**. * It is an **obligate intracellular** organism that multiplies within the acidic environment of phagolysosomes. * **Diagnosis:** Serology is the gold standard (Weil-Felix test is **negative** for Q fever). * **Chronic Q fever:** Most commonly presents as culture-negative endocarditis. * **Treatment:** Doxycycline is the drug of choice.
Question 1552: A patient undergoing chemotherapy develops a cough. Acid-fast stain of his sputum shows rods and slightly longer forms, with some branching; they vary in their acid-fast reaction from one area of the slide to the next. The acid-fast stain was performed by an experienced medical technologist and, when redone, showed the same variation. The growth was done aerobically. What is the most likely agent?
- A. Actinomyces
- B. Chlamydophila
- C. Mycobacterium avium-intracellulare (MAI or MAC)
- D. Nocardia (Correct Answer)
Explanation: **Explanation:** The clinical presentation and laboratory findings point directly to **Nocardia**. The key diagnostic features in this case are the morphology and the staining characteristics. 1. **Why Nocardia is correct:** * **Morphology:** Nocardia are Gram-positive, filamentous, branching rods. * **Acid-fastness:** They are **weakly or partially acid-fast** (using 1% sulfuric acid instead of the standard 20% used for TB). This explains the "variation in acid-fast reaction" mentioned in the question. * **Oxygen Requirement:** Nocardia are **obligate aerobes**, consistent with the growth conditions described. * **Clinical Context:** They are opportunistic pathogens, frequently causing pulmonary infections or brain abscesses in immunocompromised patients (e.g., those on chemotherapy). 2. **Why other options are incorrect:** * **Actinomyces:** While also filamentous and branching, these are **obligate anaerobes** and are **not acid-fast**. * **Chlamydophila:** These are obligate intracellular bacteria that do not show branching filaments and are not visualized by standard acid-fast staining. * **Mycobacterium avium-intracellulare (MAC):** While MAC is acid-fast and common in immunocompromised patients, it appears as **strongly acid-fast bacilli** without the characteristic long, branching, filamentous morphology of Nocardia. **High-Yield Clinical Pearls for NEET-PG:** * **Nocardia vs. Actinomyces:** Remember the mnemonic **SNAP** (Sulfa for Nocardia, Actinomyces use Penicillin). * **Stain:** Nocardia is "Modified Kinyoun stain" positive. * **Triad of Nocardiosis:** Pneumonia, brain abscess, and skin lesions (in disseminated cases). * **Culture:** Nocardia has a characteristic "wrinkled/chalky" colony appearance on Sabouraud Dextrose Agar (SDA).
Question 1553: Which of the following Salmonella species does NOT produce gas?
- A. S. typhi (Correct Answer)
- B. S. enteritidis
- C. S. typhimurium
- D. S. choleraesuis
Explanation: ### Explanation The genus *Salmonella* consists of Gram-negative, motile bacilli that are generally classified based on their biochemical reactions and antigenic structures. A key biochemical differentiator among *Salmonella* species is their ability to ferment sugars with or without the production of gas ($CO_2$ and $H_2$). **Why S. typhi is the correct answer:** *Salmonella typhi* is the most significant exception in the genus regarding gas production. While it ferments glucose, it is **anaerogenic**, meaning it produces **acid only** and no gas. This is a classic laboratory finding used to differentiate it from other serotypes in triple sugar iron (TSI) agar or sugar fermentation tubes. **Analysis of Incorrect Options:** * **S. enteritidis & S. typhimurium:** These are common causes of non-typhoidal salmonellosis (gastroenteritis). Unlike *S. typhi*, these species are **aerogenic**, producing both acid and gas during carbohydrate fermentation. * **S. choleraesuis:** This is a highly invasive serotype often associated with septicemia. Like most other members of the *Salmonella* genus, it produces gas during fermentation. **High-Yield Clinical Pearls for NEET-PG:** * **The "Typhi" Exception:** *S. typhi* is unique among common *Salmonella* because it produces **acid only** (no gas) and produces only a **faint amount of $H_2S$** (seen as a thin black line in TSI agar). * **Metabolic Profile:** All *Salmonella* are non-lactose fermenters (NLF), produce $H_2S$ (except *S. paratyphi A*), and are MR positive, VP negative, and Citrate positive (except *S. typhi* and *S. paratyphi A*). * **Mnemonic:** Remember **"Typhi is Tiny"**—it produces a *tiny* amount of $H_2S$ and *zero* gas.
Question 1554: Most of the Pseudomonas infections are based on opportunism. Which of the following is an exception?
- A. Glanders (Correct Answer)
- B. Pneumonia
- C. Otitis media
- D. Skin infection
Explanation: **Explanation:** The genus *Pseudomonas* (specifically *Pseudomonas aeruginosa*) is the quintessential **opportunistic pathogen**. It rarely causes disease in healthy individuals, instead preying on those with compromised defenses (e.g., burns, cystic fibrosis, or neutropenia). **Why Glanders is the exception:** Glanders is caused by ***Burkholderia mallei*** (formerly classified under the genus *Pseudomonas*). Unlike *P. aeruginosa*, *B. mallei* is a **primary pathogen**. It is highly infectious and causes disease even in healthy individuals. It is primarily a zoonosis affecting horses, mules, and donkeys, and is considered a potential **biothreat agent** (CDC Category B). Because it does not require a pre-existing "opportunity" or immunocompromised state to cause infection, it is the exception to the rule of opportunism in this group. **Analysis of Incorrect Options:** * **Pneumonia:** In the context of *Pseudomonas*, this is almost always opportunistic, typically seen in Ventilator-Associated Pneumonia (VAP) or in patients with Cystic Fibrosis. * **Otitis Media:** *Pseudomonas* causes "Swimmer’s ear" (Otitis externa) or chronic suppurative otitis media, usually following water exposure or trauma to the ear canal. * **Skin Infection:** *Pseudomonas* skin infections (like ecthyma gangrenosum or hot tub folliculitis) occur specifically in patients with severe burns or those using contaminated public facilities. **NEET-PG High-Yield Pearls:** * ***Burkholderia pseudomallei*** causes **Melioidosis** (Vietnamese Time Bomb); it is also a primary pathogen and shows a "safety-pin" appearance on staining. * *P. aeruginosa* produces **Pyocyanin** (blue-green pigment) and has a characteristic **fruity/grape-like odor**. * It is the most common cause of nosocomial infections in **burn units**.
Question 1555: Diagnosis of enteric fever in the 2nd week is best made by which method?
- A. Blood culture
- B. Widal test (Correct Answer)
- C. Stool culture
- D. Urine culture
Explanation: To master the diagnosis of Enteric Fever (caused by *Salmonella Typhi*), it is essential to remember the **"BASU"** mnemonic, which dictates the sequence of positive diagnostic tests over the course of the illness. ### **Explanation of the Correct Answer** **B. Widal Test:** This is a serological test that detects antibodies (Anti-O and Anti-H) against *Salmonella* antigens. These antibodies typically appear in the serum at the end of the first week, but their titers reach a diagnostically significant level during the **2nd week** of fever. Therefore, it is the investigation of choice for this specific timeframe. ### **Analysis of Incorrect Options** * **A. Blood Culture:** This is the gold standard for diagnosis in the **1st week** of illness (positive in ~90% of cases). By the 2nd week, the bacteremia begins to subside as the bacteria localize in lymphoid tissues, reducing the sensitivity of blood cultures. * **C. Stool Culture:** Bacteria are shed in the feces after being excreted from the gallbladder. This test becomes most reliable during the **3rd week** of infection. * **D. Urine Culture:** This is typically the last to become positive, usually during the **4th week** of the disease course. ### **NEET-PG High-Yield Pearls** * **BASU Mnemonic:** **B**lood (1st week), **A**gglutination/Widal (2nd week), **S**tool (3rd week), **U**rine (4th week). * **Bone Marrow Culture:** This is the **most sensitive** method overall and remains positive even if the patient has already started antibiotics. * **Widal Interpretation:** A four-fold rise in titers between paired sera (taken 7-10 days apart) is more significant than a single reading. * **Castaneda’s Medium:** A biphasic medium often used for blood cultures to reduce the risk of contamination during subculturing.
Question 1556: What is the most common site for Actinomycosis?
- A. Cervicofacial (Correct Answer)
- B. Thoracic
- C. Abdominal
- D. Cerebral
Explanation: **Explanation:** **Actinomycosis** is a chronic, granulomatous infection caused by *Actinomyces israelii*, a Gram-positive, anaerobic, non-acid-fast branching filamentous bacterium. It is an endogenous infection, as the organism is a normal commensal of the oral cavity, gastrointestinal tract, and female genital tract. 1. **Why Cervicofacial is Correct:** The **Cervicofacial region (50-60%)** is the most common site. Infection typically follows dental procedures, trauma, or poor oral hygiene (e.g., "Lumpy Jaw"). The bacteria breach the mucosal barrier, leading to abscesses that cross tissue planes and form multiple draining sinus tracts. 2. **Why Other Options are Incorrect:** * **Abdominal (20%):** Usually follows surgery (appendectomy) or bowel perforation. It often presents as a mass in the ileocecal region. * **Thoracic (15%):** Caused by aspiration of oropharyngeal secretions. It can mimic malignancy or tuberculosis on imaging. * **Cerebral:** This is a rare manifestation, usually occurring via hematogenous spread from a primary site, leading to brain abscesses. **High-Yield Clinical Pearls for NEET-PG:** * **Sulfur Granules:** The hallmark of the disease; these are yellow-colored colonies of the bacteria found in the pus (not actually made of sulfur). * **Microscopy:** Shows "Sun-ray appearance" (radiating filaments) when stained. * **Culture:** Requires anaerobic conditions; classic "Molar Tooth" appearance on agar. * **Treatment:** High-dose **Penicillin G** for a prolonged duration (6–12 months) is the drug of choice.
Question 1557: The Tuberculin test is an example of which type of immune reaction?
- A. Anaphylaxis mediated
- B. Cell mediated (Correct Answer)
- C. Antibody mediated
- D. Immune complex mediated
Explanation: The Tuberculin (Mantoux) test is a classic example of a **Type IV Hypersensitivity reaction**, also known as **Delayed-Type Hypersensitivity (DTH)**. ### Why "Cell mediated" is correct: Unlike other hypersensitivity types, Type IV reactions do not involve antibodies. Instead, they are mediated by **T-lymphocytes** (specifically Th1 cells). When Purified Protein Derivative (PPD) is injected intradermally, sensitized T-cells recognize the antigen, release cytokines (like IFN-γ), and recruit macrophages. This process takes **48–72 hours** to manifest as induration, hence the term "delayed." ### Why other options are incorrect: * **Option A (Anaphylaxis mediated):** This refers to **Type I Hypersensitivity**, which is IgE-mediated and occurs within minutes (e.g., asthma, urticaria). * **Option C (Antibody mediated):** This refers to **Type II Hypersensitivity**, where IgG or IgM antibodies bind to antigens on cell surfaces, leading to complement activation or ADCC (e.g., Rh incompatibility). * **Option D (Immune complex mediated):** This refers to **Type III Hypersensitivity**, involving the deposition of antigen-antibody complexes in tissues (e.g., SLE, Arthus reaction). ### High-Yield Clinical Pearls for NEET-PG: * **Reading the test:** Measure the **induration** (palpable hardness), not the erythema (redness), at 48–72 hours. * **False Negative:** Can occur in miliary TB, malnutrition, sarcoidosis, or immunosuppression (e.g., HIV/AIDS) due to **anergy**. * **False Positive:** Often seen in individuals previously vaccinated with **BCG** or infected with atypical mycobacteria. * **Other Type IV examples:** Lepromin test, Contact dermatitis (nickel/poison ivy), and Graft rejection.
Question 1558: Tuberculin test is administered by which route?
- A. Intramuscular
- B. Intradermal (Correct Answer)
- C. Subcutaneous
- D. None of the above
Explanation: The **Tuberculin Skin Test (TST)**, also known as the **Mantoux test**, is the standard method for determining whether a person is infected with *Mycobacterium tuberculosis*. ### 1. Why Intradermal is Correct The test involves the injection of 0.1 mL of **Purified Protein Derivative (PPD)** containing 5 Tuberculin Units (TU). It must be administered **intradermally** (usually on the volar aspect of the forearm) using a 26- or 27-gauge needle to create a discrete, pale wheal (6–10 mm). * **The Medical Concept:** TST is a **Type IV (Delayed-type) Hypersensitivity reaction**. The intradermal route ensures that the antigen remains localized in the dermis, where it can be processed by local dendritic cells (Langerhans cells) to recruit sensitized T-lymphocytes. This results in the characteristic induration (hardness) measured 48–72 hours later. ### 2. Why Other Options are Incorrect * **Intramuscular (IM):** Injecting PPD into the muscle would lead to rapid systemic absorption and clearance of the antigen, preventing the localized cellular immune response required for diagnosis. * **Subcutaneous (SC):** If the injection is too deep (subcutaneous), no wheal forms, and the antigen is dispersed too quickly, leading to a **false-negative** result. ### 3. High-Yield Clinical Pearls for NEET-PG * **Reading the Result:** Only the **induration** (palpable hardness) is measured, NOT the erythema (redness). * **False Positive:** Commonly seen in individuals previously vaccinated with **BCG** or those infected with Non-Tuberculous Mycobacteria (NTM). * **False Negative (Anergy):** Can occur in miliary TB, malnutrition, sarcoidosis, or immunocompromised states (e.g., HIV/AIDS, steroid use). * **Other Routes:** Note that the **Heaf test** (multiple-puncture) and **Tine test** are older methods and are no longer recommended due to lack of precision compared to the Mantoux technique.
Question 1559: Which of the following investigations is done to diagnose typhoid in a patient after 15 days of onset of fever?
- A. Widal test (Correct Answer)
- B. Blood culture
- C. Stool culture
- D. Urine culture
Explanation: To diagnose Enteric fever (Typhoid), the timing of the investigation relative to the onset of symptoms is the most critical factor. This is best remembered by the mnemonic **BASU** (Blood, Agglutination/Widal, Stool, Urine). ### **Explanation of the Correct Answer** **A. Widal Test:** This is a serological agglutination test that detects antibodies (Anti-O and Anti-H) against *Salmonella typhi*. These antibodies typically appear in the serum at the end of the first week, but their titers reach a diagnostic peak during the **second and third weeks** (Day 8–21) of the fever. Since the patient has had a fever for 15 days, the Widal test is the most appropriate investigation. ### **Why Other Options are Incorrect** * **B. Blood Culture:** This is the gold standard and is most sensitive (up to 90%) during the **first week** of fever. By the 15th day, the bacteremic phase has usually subsided, significantly reducing the sensitivity of blood cultures. * **C. Stool Culture:** *Salmonella* is shed in feces more reliably during the **second and third weeks**, but it is primarily used for identifying carriers or when blood cultures are negative. It is less specific for acute diagnosis than Widal in this timeframe. * **D. Urine Culture:** This is typically positive during the **third and fourth weeks** of the illness as the bacteria are cleared through the kidneys. It is a late-stage diagnostic tool. ### **High-Yield Clinical Pearls (NEET-PG)** * **BASU Timeline:** * **Week 1:** **B**lood Culture (Most sensitive early) * **Week 2:** **A**gglutination (Widal Test) * **Week 3:** **S**tool Culture * **Week 4:** **U**rine Culture * **Bone Marrow Culture:** The **most sensitive** overall investigation (even if the patient has started antibiotics). * **Widal Interpretation:** A significant titer is generally >1:80 for Anti-O and >1:160 for Anti-H (varies by endemicity). A four-fold rise in titers in paired sera is more diagnostic than a single test.
Question 1560: Which of the following is an example of an agglutination test?
- A. ABO incompatibility
- B. VDRL
- C. Weil-Felix test (Correct Answer)
- D. FTA-ABS
Explanation: **Explanation:** The **Weil-Felix test** is a classic example of a **heterophile agglutination test**. It utilizes the principle of cross-reactivity, where antibodies produced against *Rickettsial* antigens react with specific strains of *Proteus vulgaris* (OX-19, OX-2) and *Proteus mirabilis* (OX-K). In this test, particulate bacterial antigens (Proteus) clump together (agglutinate) in the presence of specific antibodies in the patient's serum. **Analysis of Options:** * **ABO Incompatibility (Option A):** While blood grouping involves hemagglutination, "ABO incompatibility" refers to a clinical condition (Type II Hypersensitivity) rather than a diagnostic laboratory test name. * **VDRL (Option B):** This is a **flocculation test**, which is a specific type of precipitation reaction. In VDRL, the antigen (cardiolipin) is in a soluble, fine particulate form that forms visible flakes (floccules) rather than large clumps. * **FTA-ABS (Option C):** This stands for Fluorescent Treponemal Antibody Absorption test, which is an **Indirect Immunofluorescence** test used to confirm Syphilis. **High-Yield Clinical Pearls for NEET-PG:** * **Weil-Felix Patterns:** * **Epidemic Typhus:** OX-19 positive. * **Endemic Typhus:** OX-19 positive. * **Scrub Typhus:** OX-K positive. * **Q Fever:** Negative for all (Weil-Felix is negative in Q fever). * **Precipitation vs. Agglutination:** Precipitation involves soluble antigens, while agglutination involves particulate/insoluble antigens. * **Widal Test:** Another high-yield example of a bacterial agglutination test (used for Enteric fever).
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