Bacteriology — MCQs

A 10-year-old boy suffers head trauma and lies unconscious for 2 weeks. He is now intubated. His temperature rises to 38.7°C (103°F) and oxygenation becomes more difficult. A chest X-ray reveals a pleural effusion and multiple abscesses in the lung parenchyma. Which of the following microorganisms is the most likely cause of this pulmonary infection?

Bacteriology Indian Medical PG Practice Questions and MCQs

Question 1701: A false positive VDRL test is seen in which of the following conditions?

A. Leprosy
B. Malaria
C. Relapsing fever
D. All of the above (Correct Answer)

Explanation: **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 acute infections (Malaria, Infectious Mononucleosis, Mycoplasma, atypical pneumonia). * **Chronic (>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.

Question 1702: Staphylococcal scalded skin syndrome is caused by which of the following?

A. Hemolysin
B. Coagulase
C. Enterotoxin
D. Epidermolytic toxin (Correct Answer)

Explanation: **Explanation:** **Staphylococcal Scalded Skin Syndrome (SSSS)**, also known as Ritter’s disease, is a toxin-mediated dermatological condition primarily affecting neonates and young children. **1. Why the Correct Answer is Right:** The condition is caused by **Epidermolytic toxins** (also known as **Exfoliative toxins** A and B) produced by certain strains of *Staphylococcus aureus*. These toxins act as serine proteases that specifically target and cleave **Desmoglein-1**, a cell-adhesion molecule in the *stratum granulosum* of the epidermis. This leads to intraepidermal splitting, resulting in diffuse erythema and large, flaccid bullae that eventually slough off, giving the skin a "scalded" appearance. **2. Why the Other Options are Incorrect:** * **Hemolysin:** These are exotoxins (like Alpha-toxin) that cause lysis of red blood cells and damage to various host cells by forming pores in cell membranes, but they do not cause skin exfoliation. * **Coagulase:** An enzyme that converts fibrinogen to fibrin. It is a key diagnostic marker for *S. aureus* and helps the bacteria evade phagocytosis by forming a fibrin coat, but it is not a dermo-lytic toxin. * **Enterotoxin:** These are heat-stable toxins responsible for **Staphylococcal Food Poisoning**. They act as superantigens in the gut, leading to vomiting and diarrhea, not skin desquamation. **3. High-Yield Clinical Pearls for NEET-PG:** * **Nikolsky Sign:** Positive (gentle pressure on the skin causes the top layer to slip off). * **Site of Cleavage:** Occurs high in the epidermis (Granular layer), unlike Pemphigus Vulgaris (Suprabasal). * **Culture:** In SSSS, the bullae fluid is typically **sterile** because the damage is caused by circulating toxins from a distant focus (e.g., umbilical stump or nasopharynx). * **Bullous Impetigo:** A localized form of SSSS where the toxin remains at the site of infection; here, cultures from bullae are usually positive.

Question 1703: Which of the following is NOT a black pigmented anaerobe?

A. Tannerella
B. Porphyromonas
C. Buccae (Correct Answer)
D. Prevotella

Explanation: **Explanation:** The "black-pigmented anaerobes" are a clinically significant group of Gram-negative, non-spore-forming bacilli that produce dark brown to black colonies on blood agar due to the accumulation of **hemin** (iron-containing porphyrins). **Why Option C is correct:** **Buccae** (specifically *Prevotella buccae*) is a non-pigmented species. While it belongs to the genus *Prevotella*, it is categorized under the **non-pigmented Prevotella** group. It is commonly found in the oral cavity and is associated with periodontitis and pleuropulmonary infections, but it lacks the characteristic dark pigmentation on culture. **Why the other options are incorrect:** * **Porphyromonas (B):** This genus consists entirely of pigmented species (e.g., *P. gingivalis*). They are asaccharolytic and highly associated with periodontal disease. * **Prevotella (D):** Many species in this genus, such as *P. melaninogenica* and *P. intermedia*, are the classic "black-pigmented" organisms. They are saccharolytic and common in oral and vaginal flora. * **Tannerella (A):** *Tannerella forsythia* is a member of the "Red Complex" of periodontal pathogens and is recognized as a black-pigmented anaerobe. **NEET-PG High-Yield Pearls:** 1. **Pigment Production:** The pigment is typically protohemin, and its production is enhanced by Vitamin K and Hemin in the culture medium (e.g., KVLB agar). 2. **Clinical Association:** These organisms are major pathogens in **aspiration pneumonia**, lung abscesses, and chronic periodontitis. 3. **Fluorescence:** Many *Prevotella* species show **brick-red fluorescence** under UV light (365 nm) before the black pigment develops, aiding early identification. 4. **Key Species:** *Porphyromonas gingivalis* (periodontitis) and *Prevotella melaninogenica* (respiratory infections) are the most frequently tested.

Question 1704: Which of the following statements about Clostridium perfringens is INCORRECT?

A. Gas gangrene alpha-toxin is produced by type A strains and is responsible for profound toxemia.
B. Clostridium perfringens is not a major cause of gas gangrene. (Correct Answer)
C. It is present in normal feces.
D. C. perfringens type C strains are responsible for necrotizing enteritis.

Explanation: ### Explanation **Clostridium perfringens** is a Gram-positive, anaerobic, spore-forming bacillus. The statement in **Option B** is incorrect because *C. perfringens* is, in fact, the **most common cause of gas gangrene** (clostridial myonecrosis), accounting for approximately 80–90% of cases. #### Analysis of Options: * **Option A (Correct Statement):** Type A strains are the most common human pathogens. They produce the **Alpha-toxin** (a lecithinase/phospholipase C), which splits lecithin into diglyceride and phosphorylcholine, leading to cell membrane destruction, hemolysis, and profound systemic toxemia. * **Option C (Correct Statement):** *C. perfringens* is a commensal organism. It is part of the **normal flora of the female genital tract and the lower gastrointestinal tract** (feces), and its spores are ubiquitous in soil. * **Option D (Correct Statement):** **Type C strains** produce the Beta-toxin, which causes **Necrotizing Enteritis** (also known as *Pigbel* in Papua New Guinea or *Darmbrand*). #### NEET-PG High-Yield Pearls: * **Nagler’s Reaction:** A rapid biochemical test used to identify *C. perfringens* based on its alpha-toxin (lecithinase) activity on egg yolk agar (inhibited by antitoxin). * **Stormy Fermentation:** Produced in litmus milk due to acid and heavy gas production. * **Morphology:** It is a "box-car" shaped bacillus. Notably, it is **non-motile** (unlike most Clostridia) and rarely shows spores in clinical samples. * **Target Hemolysis:** On blood agar, it shows a double zone of hemolysis (inner complete zone due to theta-toxin, outer incomplete zone due to alpha-toxin).

Question 1705: Buruli Ulcer is caused by which bacterium?

A. Streptococcus
B. Spirillum minus
C. Mycobacterium ulcerans (Correct Answer)
D. Brucella

Explanation: **Explanation:** **Mycobacterium ulcerans (Option C)** is the correct answer. It is a slow-growing environmental acid-fast bacillus (AFB) that causes **Buruli Ulcer**, a chronic, necrotizing skin and soft tissue infection. The hallmark of this disease is the production of a potent polyketide lipid toxin called **Mycolactone**. This toxin has cytotoxic and immunosuppressive properties, leading to extensive tissue destruction and painless, undermined ulcers. It is the third most common mycobacterial disease in immunocompetent humans, after tuberculosis and leprosy, and is most prevalent in West Africa. **Why other options are incorrect:** * **Streptococcus (Option A):** Primarily causes pyogenic infections like cellulitis, impetigo, and necrotizing fasciitis. While *S. pyogenes* causes skin ulcers, they lack the specific "undermined edges" and mycolactone-driven pathology of Buruli ulcer. * **Spirillum minus (Option B):** This is one of the causative agents of **Rat-bite fever** (specifically the sodoku form), characterized by fever, lymphadenopathy, and a rash, rather than chronic necrotizing ulcers. * **Brucella (Option D):** Causes **Brucellosis** (Undulant fever), a systemic zoonotic infection. It typically presents with fever, arthralgia, and hepatosplenomegaly, not primary cutaneous ulcers. **High-Yield Clinical Pearls for NEET-PG:** * **Toxin:** Mycolactone (essential for virulence; causes painless necrosis). * **Clinical Feature:** The ulcer is typically **painless** (due to the nerve-damaging effect of the toxin) and has characteristic **undermined edges**. * **Epidemiology:** Often associated with slow-moving or stagnant water bodies. * **Diagnosis:** Ziehl-Neelsen (ZN) stain from the ulcer base shows AFB; PCR is the gold standard for rapid diagnosis. * **Treatment:** WHO recommends a combination of **Rifampicin and Streptomycin** (or Clarithromycin) for 8 weeks.

Question 1706: What is the transport medium for Vibrio cholerae?

A. Alkaline peptone water
B. Cary-Blair medium (Correct Answer)
C. TC BS medium
D. None of the above

Explanation: **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).

Question 1707: 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?

A. ASLO titre (Correct Answer)
B. Antistreptolysin S
C. PCR for M protein
D. Anti-hyaluronidase titre

Explanation: **Explanation:** The diagnosis of Acute Rheumatic Fever (ARF) is based on the **Revised Jones Criteria**, which requires evidence of a preceding Group A Streptococcal (GAS) infection. In cases of **Sydenham’s chorea**, the latent period between the initial pharyngeal infection and the onset of symptoms is often long (1–6 months). By this time, throat cultures are typically negative, making serological markers essential for diagnosis. **1. Why ASLO titre is correct:** The **Antistreptolysin O (ASLO)** titre is the most widely used and standardized test to demonstrate a recent GAS infection. ASLO antibodies appear 1–3 weeks after infection and peak at 3–6 weeks. Since Sydenham’s chorea is a delayed manifestation, a high or rising ASLO titre provides the necessary evidence of a prior streptococcal trigger. **2. Why other options are incorrect:** * **Antistreptolysin S:** Unlike Streptolysin O, Streptolysin S is non-antigenic. Therefore, no antibodies are produced against it, making it useless for serological diagnosis. * **PCR for M protein:** PCR detects current bacterial presence (DNA). In ARF, the infection has usually cleared by the time immunological complications arise; thus, PCR would likely be negative. * **Anti-hyaluronidase titre:** While this is a valid streptococcal antibody test, it is typically used as a secondary test (often in cases of skin infections/pyoderma where ASLO may be low). ASLO remains the primary screening tool for post-streptococcal sequelae like ARF. **Clinical Pearls for NEET-PG:** * **Jones Criteria:** Chorea is a "Major" criterion. It can be the *only* manifestation of ARF. * **Anti-DNase B:** This is the most sensitive antibody test for documenting preceding streptococcal **skin infections** (Impetigo/Pyoderma) leading to Glomerulonephritis. * **Streptozyme Test:** A rapid slide agglutination test that detects a mixture of five streptococcal antibodies (ASLO, Anti-DNase B, Anti-hyaluronidase, Anti-streptokinase, and Anti-NADase).

Question 1708: What is the most common cause of acute pyogenic meningitis in adolescents and young adults?

A. Haemophilus influenzae
B. Neisseria meningitidis (Correct Answer)
C. Staphylococcus aureus
D. Group B streptococci

Explanation: **Explanation:** Acute pyogenic meningitis is a medical emergency where the causative agent varies significantly based on the patient's age group. **Why Neisseria meningitidis is correct:** *Neisseria meningitidis* (Meningococcus) is the leading cause of bacterial meningitis in **adolescents and young adults (ages 10–29)**. It often occurs in clusters, such as in college dormitories or military barracks, due to respiratory droplet transmission. It is characterized by a rapid onset and may be associated with a pathognomonic petechial or purpuric rash. **Analysis of Incorrect Options:** * **A. Haemophilus influenzae:** Previously the most common cause in children (6 months to 5 years), its incidence has drastically declined due to the widespread use of the **Hib vaccine**. * **C. Staphylococcus aureus:** This is an uncommon cause of primary meningitis; it is typically associated with post-neurosurgical procedures, head trauma, or secondary to infective endocarditis. * **D. Group B streptococci (S. agalactiae):** This is the leading cause of neonatal meningitis (0–3 months), transmitted from the maternal vaginal tract during birth. **NEET-PG High-Yield Pearls:** * **Most common cause overall (Adults):** *Streptococcus pneumoniae*. * **Newborns (0-3 months):** Group B Streptococcus (1st), *E. coli* (2nd), *Listeria monocytogenes*. * **Elderly (>65 years):** *Streptococcus pneumoniae* and *Listeria monocytogenes*. * **CSF Findings in Pyogenic Meningitis:** Elevated neutrophils (pleocytosis), markedly decreased glucose (<40 mg/dL), and elevated protein. * **Waterhouse-Friderichsen Syndrome:** Adrenal hemorrhage associated with fulminant meningococcemia.

Question 1709: What is the mechanism of action in the pathogenesis of pseudomembranous colitis caused by Clostridium difficile?

A. Invasiveness
B. Endotoxin
C. Exotoxin (Correct Answer)
D. Neuromuscular blockade

Explanation: **Explanation:** *Clostridium difficile* (now *Clostridioides difficile*) is a Gram-positive, anaerobic, spore-forming bacillus. Its pathogenesis is primarily mediated by the release of two potent **exotoxins**: **Toxin A (Enterotoxin)** and **Toxin B (Cytotoxin)**. 1. **Mechanism of Correct Option (C):** Both toxins act by inactivating Rho GTPases through glucosylation. This leads to the disruption of the actin cytoskeleton, loss of tight junctions, and cell death. Toxin A causes fluid secretion and inflammation, while Toxin B is more potent and causes direct mucosal damage, leading to the formation of "pseudomembranes" (composed of fibrin, mucus, and inflammatory cells). 2. **Why other options are incorrect:** * **Invasiveness (A):** *C. difficile* does not typically invade the intestinal wall; the damage is localized to the mucosal surface via secreted toxins. * **Endotoxin (B):** Endotoxins (LPS) are characteristic of Gram-negative bacteria. As a Gram-positive organism, *C. difficile* lacks LPS. * **Neuromuscular blockade (D):** This is the mechanism of other Clostridia, such as *C. botulinum* (blocks ACh release) and *C. tetani* (blocks GABA/Glycine release). **High-Yield Clinical Pearls for NEET-PG:** * **Risk Factor:** Most commonly associated with the use of broad-spectrum antibiotics (e.g., **Clindamycin**, Fluoroquinolones, Cephalosporins). * **Diagnosis:** The gold standard is the **Cell Cytotoxicity Assay**, but the most common initial test is the **ELISA for Toxins A and B** in stool. * **Treatment:** Oral **Fidaxomicin** or **Vancomycin** are first-line agents. Metronidazole is used for mild cases if others are unavailable. * **Hypervirulent Strain:** The **NAP1/BI/027** strain produces higher levels of toxins and an additional "Binary Toxin."

Question 1710: A 10-year-old boy suffers head trauma and lies unconscious for 2 weeks. He is now intubated. His temperature rises to 38.7°C (103°F) and oxygenation becomes more difficult. A chest X-ray reveals a pleural effusion and multiple abscesses in the lung parenchyma. Which of the following microorganisms is the most likely cause of this pulmonary infection?

A. Legionella pneumophila
B. Mycoplasma pneumoniae
C. Pneumocystis carinii
D. Staphylococcus aureus (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Staphylococcus aureus** **1. Why it is correct:** The clinical presentation describes a classic case of **Hospital-Acquired Pneumonia (HAP)** or **Ventilator-Associated Pneumonia (VAP)**. The patient has been intubated and unconscious for two weeks, which are major risk factors for aspiration and colonization by virulent hospital flora. * **Staphylococcus aureus** (especially MRSA) is a leading cause of HAP. * The hallmark of *S. aureus* pneumonia is its **necrotizing nature**, leading to tissue destruction. This manifests radiologically as **multiple lung abscesses**, cavitation, and **pleural effusion** (often progressing to empyema). In children, it is also the most common cause of pneumatoceles. **2. Why the other options are incorrect:** * **A. Legionella pneumophila:** Typically associated with contaminated water systems (AC ducts) and presents with high fever, diarrhea, and hyponatremia. While it can cause severe pneumonia, it rarely causes multiple abscesses compared to *S. aureus*. * **B. Mycoplasma pneumoniae:** Causes "Atypical" or "Walking" Pneumonia. It usually presents in young adults with mild symptoms and "patchy" infiltrates on X-ray. It does not cause abscesses or pleural effusions. * **C. Pneumocystis carinii (jirovecii):** An opportunistic fungal infection seen in immunocompromised patients (e.g., HIV/AIDS). It typically presents with bilateral ground-glass opacities and interstitial infiltrates, not abscesses. **3. High-Yield NEET-PG Pearls:** * **Post-viral Pneumonia:** *S. aureus* is the most common cause of secondary bacterial pneumonia following an **Influenza** infection. * **Radiology:** Look for the triad of **Pneumatoceles, Pyopneumothorax, and Abscesses** to identify *S. aureus* in pediatric/ventilated cases. * **Virulence Factor:** The **Panton-Valentine Leukocidin (PVL)** toxin is specifically associated with necrotic lung lesions and severe skin infections in MRSA strains.

Practice by Chapter

Staphylococci

Practice Questions

Streptococci and Enterococci

Practice Questions

Neisseria and Moraxella

Practice Questions

Corynebacterium and Listeria

Practice Questions

Bacillus and Clostridium

Practice Questions

Enterobacteriaceae

Practice Questions

Vibrio, Aeromonas, and Plesiomonas

Practice Questions

Pseudomonas and Related Bacteria

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Haemophilus and HACEK Group

Practice Questions

Bordetella and Brucella

Practice Questions

Mycobacteria

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Spirochetes

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