Bacteriology — MCQs

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1601: A patient presents with fever for 3 weeks. On examination, splenomegaly is observed. Ultrasonography reveals a hypoechoic shadow in the spleen near the hilum. Gram-positive bacilli are isolated on blood culture. Which of the following is the most likely causative organism?

A. Cytomegalovirus
B. Toxoplasmosis
C. Staphylococcus (Correct Answer)
D. Lymphoma virus

Explanation: **Explanation:** The clinical presentation of prolonged fever (3 weeks) and splenomegaly, combined with a hypoechoic shadow on ultrasonography, is highly suggestive of a **splenic abscess**. The definitive clue in this question is the microbiological finding: **Gram-positive bacilli** (though *Staphylococcus* are technically cocci, in the context of this specific NEET-PG recall, it is the only bacterial pathogen listed that fits the clinical profile of a pyogenic abscess). 1. **Why Staphylococcus is correct:** *Staphylococcus aureus* is the most common cause of pyogenic splenic abscesses worldwide. These abscesses usually occur via hematogenous spread from a distant focus (like endocarditis or skin infections). On imaging (USG/CT), they typically appear as hypoechoic or low-attenuation lesions. 2. **Why other options are wrong:** * **Cytomegalovirus (CMV) & Toxoplasmosis:** While both can cause fever and splenomegaly (Infectious Mononucleosis-like syndrome), they do not typically cause focal hypoechoic "shadows" or abscesses in the spleen. Furthermore, they would not grow as Gram-positive organisms on standard blood culture. * **Lymphoma virus (HTLV/EBV):** Lymphoma can cause splenic masses, but it is a neoplastic process, not an acute bacterial infection isolated on blood culture. **NEET-PG High-Yield Pearls:** * **Most common cause of Splenic Abscess:** *Staphylococcus aureus*, followed by *Streptococcus* and *Salmonella* (especially in Sickle Cell Anemia patients). * **Imaging of choice:** CT scan is more sensitive than USG for diagnosing splenic abscesses. * **Triad of Splenic Abscess:** Fever, left upper quadrant pain, and tender splenomegaly (present in only ~30% of cases). * **Note on Gram Stain:** Always pay attention to the morphology. If the question states "Gram-positive cocci in clusters," it is definitively *S. aureus*. If "Gram-positive bacilli" is used in a recall, look for the most likely bacterial pathogen among the choices.

Question 1602: Which of the following statements is true about Helicobacter pylori?

A. It is flagellated.
B. It is involved in the causation of peptic ulcer disease.
C. It causes hypergastrinemia.
D. All of the above. (Correct Answer)

Explanation: **Explanation:** *Helicobacter pylori* is a microaerophilic, Gram-negative spiral bacterium that colonizes the gastric mucosa. It is a high-yield topic for NEET-PG due to its unique survival mechanisms and strong association with gastrointestinal pathologies. 1. **Flagellated (Option A):** *H. pylori* possesses 4–6 unipolar, sheathed **lophotrichous flagella**. These provide the high motility required to penetrate the thick gastric mucus layer and reach the neutral pH environment near the epithelial surface. 2. **Peptic Ulcer Disease (Option B):** It is the most common cause of peptic ulcers. By producing toxins like **CagA** (Cytotoxin-associated gene A) and **VacA** (Vacuolating cytotoxin), it induces chronic inflammation, leading to mucosal erosion. Approximately 70-90% of duodenal ulcers and 70% of gastric ulcers are linked to *H. pylori*. 3. **Hypergastrinemia (Option C):** Infection (especially antral-predominant) leads to a decrease in **Somatostatin**-producing D-cells. Since somatostatin normally inhibits gastrin release, its absence results in increased gastrin secretion (hypergastrinemia), which subsequently increases acid production. **Clinical Pearls for NEET-PG:** * **Urease Production:** The hallmark of *H. pylori* is its potent urease enzyme, which converts urea into ammonia and $CO_2$, neutralizing gastric acid to create a "protective cloud." This is the basis for the **Urea Breath Test**. * **Oncogenic Potential:** It is classified as a **Class I Carcinogen** and is associated with Gastric Adenocarcinoma and MALToma (Mucosa-Associated Lymphoid Tissue lymphoma). * **Diagnosis:** The **Endoscopic Biopsy (Rapid Urease Test/CLO test)** is the gold standard for invasive diagnosis, while the Stool Antigen test is preferred for monitoring cure.

Question 1603: Chlamydia trachomatis serovar D to K causes which of the following conditions?

A. Aeriosclerosis
B. Trachoma
C. Lymphogranuloma venereum (Correct Answer)
D. Urethritis

Explanation: ### Explanation *Chlamydia trachomatis* is an obligate intracellular bacterium classified into different serovars based on major outer membrane protein (MOMP) variations. Understanding the clinical manifestations associated with specific serovars is high-yield for NEET-PG. **Correct Answer: C. Lymphogranuloma venereum (LGV)** Serovars **L1, L2, and L3** are responsible for Lymphogranuloma venereum, a systemic sexually transmitted infection characterized by painless genital ulcers followed by painful inguinal lymphadenopathy (buboes) and potential proctocolitis. **Analysis of Options:** * **A. Atherosclerosis:** This is associated with *Chlamydophila pneumoniae*, not *Chlamydia trachomatis*. Chronic inflammation from *C. pneumoniae* is implicated in the development of arterial plaques. * **B. Trachoma:** Endemic trachoma (leading to blindness) is caused by **Serovars A, B, Ba, and C**. (Mnemonic: **A-C** causes **A**frican **C**hronic blindness). * **D. Urethritis:** Non-gonococcal urethritis (NGU), cervicitis, and inclusion conjunctivitis are caused by **Serovars D through K**. These are the most common causes of bacterial STIs worldwide. **Wait! Correction Note:** There appears to be a discrepancy in the provided question key. **Serovars D-K actually cause Urethritis (Option D)**, while **Serovars L1-L3 cause LGV (Option C)**. In the context of the provided "Correct Answer" being LGV, it is vital to remember for the exam: * **A, B, C:** Trachoma * **D-K:** Genital infections (Urethritis/Cervicitis), Neonatal pneumonia, and Inclusion conjunctivitis. * **L1, L2, L3:** Lymphogranuloma venereum. **High-Yield Clinical Pearls:** * **Diagnosis:** Nucleic Acid Amplification Test (NAAT) is the gold standard. * **Cytology:** Look for **Halberstaedter-Prowazek inclusion bodies** (intracytoplasmic) on Giemsa stain. * **Treatment:** Azithromycin (single dose) or Doxycycline (7 days). For LGV, Doxycycline for 21 days is preferred.

Question 1604: Which bacteria is catalase positive and novobiocin resistant?

A. Staphylococcus aureus
B. Staphylococcus epidermidis
C. Staphylococcus saprophyticus (Correct Answer)
D. None of the above

Explanation: ### Explanation **Staphylococcus saprophyticus** is the correct answer based on its biochemical profile and antibiotic susceptibility pattern. #### 1. Why the Correct Answer is Right: * **Catalase Positive:** All members of the genus *Staphylococcus* produce the enzyme catalase, which differentiates them from *Streptococcus* species. * **Novobiocin Resistance:** Among the coagulase-negative staphylococci (CoNS), *S. saprophyticus* is uniquely identified by its natural resistance to the antibiotic Novobiocin. In a laboratory setting, a zone of inhibition <16 mm around a 5 µg novobiocin disk confirms its identity. #### 2. Why the Incorrect Options are Wrong: * **Staphylococcus aureus (Option A):** While it is catalase-positive, it is **Coagulase-positive**. Furthermore, it is typically sensitive to novobiocin. It is primarily identified by its ability to ferment mannitol and produce golden-yellow colonies. * **Staphylococcus epidermidis (Option B):** Like *S. saprophyticus*, it is a CoNS and catalase-positive. However, it is **Novobiocin-sensitive**. It is the most common contaminant in blood cultures and is associated with prosthetic valve endocarditis and catheter infections. #### 3. NEET-PG High-Yield Clinical Pearls: * **"Honeymoon Cystitis":** *S. saprophyticus* is the second most common cause of uncomplicated Urinary Tract Infections (UTIs) in young, sexually active females (after *E. coli*). * **Identification Flowchart:** 1. Gram-positive cocci in clusters → *Staphylococcus* 2. Catalase positive → *Staphylococcus* 3. Coagulase negative → CoNS 4. Novobiocin resistant → ***S. saprophyticus*** * **Mnemonic:** "On the **Staph** retreat, **NO** **S**aprophyticus **P**arty" (**NO**vobiocin: **S**aprophyticus is **R**esistant, **E**pidermidis is **S**ensitive).

Question 1605: Cat scratch disease is:

A. Associated with a positive Frei skin test
B. Caused by a DNA virus
C. Associated with a pathognomonic histological picture (Correct Answer)
D. All of the above

Explanation: **Explanation:** **Cat Scratch Disease (CSD)** is primarily caused by **_Bartonella henselae_**, a fastidious Gram-negative rod. 1. **Why Option C is correct:** CSD is characterized by a highly suggestive, often considered **pathognomonic histological triad** in the lymph nodes. This includes: * Follicular hyperplasia. * **Stellate (star-shaped) granulomas** with central necrotic debris and neutrophils (microabscesses). * Presence of pleomorphic bacilli visible on **Warthin-Starry silver stain**. 2. **Why other options are incorrect:** * **Option A:** The **Frei skin test** is specific for **Lymphogranuloma Venereum (LGV)** caused by *Chlamydia trachomatis* (serotypes L1-L3). It is not used for CSD. * **Option B:** CSD is caused by a **bacterium** (*Bartonella henselae*), not a DNA virus. **High-Yield Clinical Pearls for NEET-PG:** * **Transmission:** Usually follows a scratch or bite from a domestic cat (often kittens) or via the cat flea (*Ctenocephalides felis*). * **Clinical Presentation:** Regional lymphadenopathy (most common in axillary or cervical nodes) following a primary skin papule. * **Parinaud Oculoglandular Syndrome:** A specific presentation of CSD involving granulomatous conjunctivitis and preauricular lymphadenopathy. * **Bacillary Angiomatosis:** In immunocompromised (HIV) patients, *B. henselae* causes vascular proliferative lesions, which must be differentiated from Kaposi Sarcoma. * **Diagnosis:** Serology (IFA/ELISA) is the preferred initial test; Warthin-Starry stain is the classic histological gold standard.

Question 1606: An elderly male patient presented with fever, chest pain, and dry cough. Sputum culture on charcoal yeast extract medium identified an organism. Which of the following is the most likely organism?

A. Haemophilus influenzae
B. Moraxella catarrhalis
C. Legionella species (Correct Answer)
D. Burkholderia cepacia

Explanation: **Explanation:** The correct answer is **Legionella species**. This question tests your knowledge of specific culture requirements for respiratory pathogens. **Why Legionella is correct:** *Legionella pneumophila* is a fastidious Gram-negative bacillus that causes Legionnaires' disease (atypical pneumonia). It has a unique metabolic requirement for **L-cysteine** and **iron**. The gold standard selective medium for its isolation is **Buffered Charcoal Yeast Extract (BCYE) agar**. The charcoal in the medium serves to detoxify fatty acids and absorb metabolic byproducts (like hydrogen peroxide) that would otherwise inhibit the growth of *Legionella*. **Why other options are incorrect:** * **Haemophilus influenzae:** Requires **Chocolate Agar** supplemented with Factor V (NAD) and Factor X (Hemin) for growth. * **Moraxella catarrhalis:** Grows well on routine media like Blood Agar and Chocolate Agar; it does not require specialized charcoal media. * **Burkholderia cepacia:** Typically isolated using selective media like **BCSA (Burkholderia cepacia selective agar)** or **Ashdown’s medium**, especially in patients with Cystic Fibrosis. **High-Yield Clinical Pearls for NEET-PG:** * **Clinical Presentation:** Often presents as "atypical pneumonia" with high fever, dry cough, and extra-pulmonary symptoms like **hyponatremia**, diarrhea, and confusion. * **Transmission:** Associated with contaminated water systems, cooling towers, and air conditioners (no person-to-person spread). * **Diagnosis:** While BCYE is the gold standard for culture, the **Urinary Antigen Test** is the most common rapid diagnostic method used clinically. * **Staining:** Poorly visualized on Gram stain; **Silver stains (Dieterle stain)** or Direct Fluorescent Antibody (DFA) are preferred.

Question 1607: All of the following are acid-fast except:

A. Mycobacterium tuberculosis
B. Mycobacterium leprae
C. Isospora
D. Mycobacterium avium complex (Correct Answer)

Explanation: The question appears to be based on a technicality or a potential error in the provided key, as **all four options are technically acid-fast**. However, in the context of NEET-PG, this question often tests the **degree of acid-fastness** or specific staining requirements. ### **Explanation of Options** 1. **Mycobacterium avium complex (MAC):** While MAC is acid-fast, it is often described as "weakly" or "variably" acid-fast compared to *M. tuberculosis*. In certain laboratory settings or specific staining protocols, it may show less consistency. However, if this is a "least likely" question, MAC is often the intended answer in older question banks despite being biologically acid-fast. 2. **Mycobacterium tuberculosis:** This is the classic **strongly acid-fast** organism. It resists decolorization by 20% sulfuric acid due to the high concentration of mycolic acids in its cell wall. 3. **Mycobacterium leprae:** This is **weakly acid-fast**. It requires a modified Ziehl-Neelsen stain (using 5% sulfuric acid instead of 20%) because its cell wall is more permeable. 4. **Isospora (Cystoisospora belli):** This is a protozoan parasite that is **acid-fast**. Its oocysts stain bright red against a blue background using a modified acid-fast stain (1% sulfuric acid). ### **High-Yield Clinical Pearls for NEET-PG** * **Acid-fastness** is due to **Mycolic Acid** in the cell wall. * **Modified ZN Stain Concentrations:** * *M. tuberculosis*: 20% $H_2SO_4$ * *M. leprae*: 5% $H_2SO_4$ * *Nocardia*: 1% $H_2SO_4$ * *Oocysts (Cryptosporidium, Isospora, Cyclospora)*: 1% $H_2SO_4$ * *Bacterial Spores*: 0.25-0.5% $H_2SO_4$ * **Non-bacterial acid-fast structures:** Hooklets of *Echinococcus granulosus* and the head of the sperm.

Question 1608: Which of the following is most resistant to gonococcal infection?

A. Prostate
B. Epididymis
C. Testis (Correct Answer)
D. Urethra

Explanation: **Explanation:** The correct answer is **Testis (Option C)**. **Why the Testis is resistant:** In the male reproductive system, *Neisseria gonorrhoeae* primarily causes ascending infections. While the infection frequently involves the urethra, prostate, and epididymis, the **testis is remarkably resistant** to gonococcal invasion. This resistance is attributed to the **blood-testis barrier** and the organ's inherent local immunity. Even when a patient develops severe gonococcal epididymitis, the infection rarely crosses into the testicular parenchyma. If the testis does become involved, it is usually a secondary result of inflammation spreading from the epididymis (epididymo-orchitis), but isolated gonococcal orchitis is almost non-existent. **Analysis of Incorrect Options:** * **Urethra (Option D):** This is the most common site of infection in men, presenting as acute purulent urethritis. * **Prostate (Option A):** Chronic or acute prostatitis is a well-documented complication of untreated ascending gonococcal infection. * **Epididymis (Option B):** Acute epididymitis is a common complication of gonorrhea in young men, typically presenting with unilateral scrotal pain and swelling. **High-Yield Clinical Pearls for NEET-PG:** * **Most common site in females:** The **Endocervix** (the squamous epithelium of the vagina is resistant in adults). * **Vulvovaginitis:** Occurs in prepubertal girls due to the thin, alkaline vaginal mucosa (unlike the acidic, stratified squamous epithelium in adults). * **Disseminated Gonococcal Infection (DGI):** Characterized by the triad of polyarthralgia, tenosynovitis, and dermatitis. * **Gold Standard Diagnosis:** Culture on **Thayer-Martin medium** (selective medium). * **Treatment of choice:** Ceftriaxone (IM) + Azithromycin (Oral) to cover co-infection with *Chlamydia trachomatis*.

Question 1609: In a patient with syphilis, which site is least helpful for the isolation of the causative organism?

A. Gumma (Correct Answer)
B. Primary chancre
C. Mucosal patch
D. Maculopapular rash

Explanation: In syphilis, the ability to isolate or visualize *Treponema pallidum* depends on the bacterial load present at the site of the lesion. **Explanation of the Correct Answer:** **A. Gumma:** This is the hallmark of **Tertiary Syphilis**. Gummas are chronic granulomatous lesions resulting from a delayed-type hypersensitivity reaction rather than active bacterial proliferation. Because the host's immune response has largely cleared the spirochetes by this stage, gummas contain **very few to no organisms**. Therefore, they are the least helpful site for isolation or microscopic detection (Dark Ground Microscopy). **Explanation of Incorrect Options:** * **B. Primary Chancre:** This is the site of initial inoculation in **Primary Syphilis**. It is teeming with live spirochetes, making it the ideal site for Dark Ground Microscopy (DGM). * **C. Mucosal Patch:** These occur during **Secondary Syphilis** (the most infectious stage). Mucosal patches are highly infectious and contain a high density of *T. pallidum*. * **D. Maculopapular Rash:** Also a feature of **Secondary Syphilis**, these skin lesions contain viable organisms that can be identified via biopsy or DGM (though fluid from moist lesions like condyloma lata is preferred). **High-Yield NEET-PG Pearls:** * **Gold Standard for Diagnosis:** Serology (Non-treponemal like VDRL/RPR for screening; Treponemal like FTA-ABS/TPHA for confirmation). * **Dark Ground Microscopy (DGM):** Best for primary and secondary stages; however, it is **not** used for oral lesions due to the presence of commensal treponemes (*T. denticola*). * **Cultivability:** *T. pallidum* cannot be grown on artificial culture media; it is maintained via serial passage in rabbit testes (Nichol’s strain).

Question 1610: What is the most common form of Nocardial respiratory tract infection?

A. Laryngitis
B. Pharyngitis
C. Tonsillitis
D. Pneumonia (Correct Answer)

Explanation: **Explanation:** **Nocardia** species (most commonly *N. asteroides* complex) are Gram-positive, aerobic, branching filamentous bacteria that are weakly acid-fast. They are primarily exogenous saprophytes found in soil and water. **Why Pneumonia is the correct answer:** Inhalation is the primary route of exposure for systemic Nocardiosis. Once inhaled, the bacteria settle in the lower respiratory tract. In immunocompromised individuals (and occasionally immunocompetent hosts), this leads to **Pneumonia**, which is the most common clinical manifestation of Nocardial respiratory infection. It typically presents as a subacute or chronic necrotizing pneumonia, often characterized by cavitation, nodules, or abscess formation. From the lungs, the organism frequently hematogenously disseminates, showing a high predilection for the Central Nervous System (Brain Abscess). **Why other options are incorrect:** * **Laryngitis, Pharyngitis, and Tonsillitis (Options A, B, C):** These are infections of the upper respiratory tract. *Nocardia* is not a commensal of the human oropharynx and does not typically colonize these areas. While rare cases of localized upper airway infection can occur in severely debilitated patients, they are not the standard or most common presentation. **High-Yield Clinical Pearls for NEET-PG:** * **Staining:** Use **Modified Ziehl-Neelsen stain** (using 1% sulfuric acid as a decolorizer instead of 20%) to demonstrate its weak acid-fast nature. * **Morphology:** Described as "beaded, branching filaments." * **Triad of Nocardiosis:** Pulmonary infection, brain abscess, and cutaneous lesions. * **Drug of Choice:** **Trimethoprim-Sulfamethoxazole (TMP-SMX)** is the mainstay of treatment. * **Culture:** Grows slowly on routine media (like Blood Agar or Sabouraud Dextrose Agar); colonies often have a "waxy" or "wrinkled" appearance with a characteristic earthy odor.

Practice by Chapter

Staphylococci

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Streptococci and Enterococci

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Neisseria and Moraxella

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Corynebacterium and Listeria

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Bacillus and Clostridium

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Enterobacteriaceae

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Vibrio, Aeromonas, and Plesiomonas

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Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

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Bordetella and Brucella

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Mycobacteria

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Spirochetes

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