Bacteriology Practice Questions

Q: A 45-year-old patient presents with symptoms of septic arthritis. What is the most common cause in this patient?

Staphylococcus aureus. **Explanation:** Septic arthritis is a medical emergency characterized by the infection of a joint space. In adults, the most common route of infection is **hematogenous spread**. **1. Why Staphylococcus aureus is correct:** *Staphylococcus aureus* is the **most common overall cause** of septic arthritis across all age groups (except neonates, where Group B Streptococcus is significant). It is a highly virulent organism that produces proteases, which can lead to rapid joint destruction if not treated promptly. In a 45-year-old patient, unless specific risk factors (like IV drug use or STIs) are mentioned, *S. aureus* remains the statistical primary pathogen. **2. Analysis of Incorrect Options:** * **A. Gonococcus:** *Neisseria gonorrhoeae* is a common cause of septic arthritis in **young, sexually active adults**. It often presents with a triad of tenosynovitis, dermatitis, and polyarthralgia. However, *S. aureus* still holds a higher overall prevalence in the general adult population. * **C. Salmonella:** This is a high-yield association for patients with **Sickle Cell Anemia**. While *S. aureus* is still common in these patients, *Salmonella* is a characteristic and frequently tested pathogen for this specific demographic. * **D. Pneumococci:** *Streptococcus pneumoniae* can cause septic arthritis but is much less common than Staphylococci, usually occurring secondary to pneumonia or meningitis. **High-Yield Clinical Pearls for NEET-PG:** * **Most common joint involved:** Knee joint (>50% of cases). * **Gold Standard Diagnosis:** Arthrocentesis (Joint fluid analysis) showing WBC count >50,000/mm³ with >75% polymorphonuclear cells. * **IV Drug Users:** Increased incidence of infection in fibrocartilaginous joints (Sternoclavicular and Sacroiliac joints); common organisms include *Pseudomonas aeruginosa*. * **Prosthetic Joints:** *Staphylococcus epidermidis* is a frequent culprit.

Q: Which of the following is not a fungal infection?

Actinomycosis. **Explanation:** The correct answer is **Actinomycosis**. Despite its name ending in "-mycosis" (a suffix typically denoting fungal infections), Actinomycosis is caused by **Actinomyces species**, which are **Gram-positive, anaerobic, non-acid-fast bacteria**. They are characterized by filamentous branching growth, which historically led to their misclassification as fungi. However, they lack a chitinous cell wall, possess a prokaryotic nucleus, and are susceptible to antibacterial agents (like Penicillin) rather than antifungals. **Analysis of Options:** * **Blastomycosis (A):** A systemic infection caused by the dimorphic fungus *Blastomyces dermatitidis*. It typically presents with pulmonary symptoms or verrucous skin lesions. * **Cryptococcus (B):** An infection caused by the encapsulated yeast *Cryptococcus neoformans*. It is a major cause of opportunistic meningitis in HIV/AIDS patients. * **Histoplasmosis (C):** A systemic fungal infection caused by *Histoplasma capsulatum*. It is a dimorphic fungus often associated with bird or bat droppings and mimics tuberculosis clinically. **High-Yield Clinical Pearls for NEET-PG:** * **Actinomyces israelii** is the most common human pathogen. It is a normal commensal of the oral cavity and female genital tract. * **Clinical Hallmark:** Presence of **"Sulfur granules"** (yellowish clumps of organisms) in the pus draining from sinus tracts. * **Classic Presentation:** "Lumpy jaw" (cervicofacial actinomycosis) following dental trauma or poor oral hygiene. * **Differential Diagnosis:** Do not confuse *Actinomyces* with *Nocardia*. Both are filamentous bacteria, but **Nocardia is aerobic and weakly acid-fast**, whereas Actinomyces is anaerobic and non-acid-fast.

Q: A mother with a reactive VDRL test gave birth to an infant. Which of the following would help in determining the risk of transmission to the infant, EXCEPT?

TPHA test on the serum sample of the infant. **Explanation:** The diagnosis of congenital syphilis relies on distinguishing between maternal antibodies transferred across the placenta and the infant’s own immune response. **Why Option B is the correct answer (Except):** TPHA (Treponema Pallidum Hemagglutination Assay) is a **treponemal test** that detects IgG antibodies. IgG antibodies are small enough to cross the placenta from the mother to the fetus. Therefore, a positive TPHA in an infant does not necessarily indicate an active infection; it likely reflects the passive transfer of maternal antibodies, which can persist for up to 12–15 months. Thus, it is **not helpful** in determining the risk of transmission or diagnosing active congenital syphilis. **Analysis of Incorrect Options:** * **Option A:** TPHA on the mother confirms her syphilis status. A reactive VDRL (non-treponemal) must be confirmed with a treponemal test (TPHA/FTA-ABS) to rule out Biological False Positives (BFP). * **Option C:** Comparing VDRL titers in paired samples is crucial. An infant’s VDRL titer **fourfold higher** than the mother’s is highly suggestive of congenital infection. * **Option D:** The timing of maternal treatment is a major risk factor. Treatment must be completed at least **30 days prior to delivery** to be considered effective in preventing transmission. **Clinical Pearls for NEET-PG:** * **Screening:** VDRL/RPR (Non-treponemal). * **Confirmation:** TPHA/FTA-ABS (Treponemal). * **Gold Standard for Infant:** Detection of **IgM antibodies** (e.g., FTA-ABS 19S IgM or IgM ELISA) because IgM does not cross the placenta; its presence confirms the infant is producing its own antibodies. * **VDRL** is the only test used for monitoring treatment response (titers should fall).

Q: Which of the following statements best describes Helicobacter pylori?

An obligate parasite of the gastric mucosa. **Explanation:** *Helicobacter pylori* is a unique human pathogen specifically adapted to the harsh acidic environment of the stomach. **1. Why Option B is Correct:** *H. pylori* is considered an **obligate parasite of the gastric mucosa**. It has a high tissue specificity for gastric-type epithelium. It survives the acidic pH of the stomach by producing large amounts of **urease**, which creates a protective "ammonia cloud" around the bacterium, neutralizing gastric acid. **2. Why the other options are Incorrect:** * **Option A:** While it is a Gram-negative spiral organism, it is **highly motile** due to multiple unipolar **flagella** (lophotrichous). This motility is crucial for burrowing through the gastric mucus layer. * **Option C:** *H. pylori* **can infect the duodenal mucosa**, but only if there is **gastric metaplasia** (where the duodenum develops gastric-type cells due to high acid exposure). This is a key step in the pathogenesis of duodenal ulcers. * **Option D:** *H. pylori* is **responsive to antibiotics**. Standard treatment involves "Triple Therapy" (Clarithromycin, Amoxicillin/Metronidazole, and a PPI) or "Quadruple Therapy" involving Bismuth. **High-Yield NEET-PG Pearls:** * **Microscopy:** Described as "seagull-wing" shaped. * **Virulence Factors:** **CagA** (associated with gastric cancer) and **VacA** (vacuolating cytotoxin). * **Diagnosis:** The **Urea Breath Test** (using C13 or C14) is the non-invasive gold standard for confirming eradication. * **Associations:** It is a Type 1 Carcinogen, strongly linked to **Gastric Adenocarcinoma** and **MALT Lymphoma**.

Q: Vi antigen is present in all of the following bacteria except?

S.paratyphi A. **Explanation:** The **Vi (Virulence) antigen** is a surface capsular polysaccharide (K antigen) found in certain strains of *Salmonella*. It acts as a protective layer that masks the underlying O-antigen, preventing phagocytosis and complement-mediated killing, thereby increasing the virulence of the organism. **Why S. paratyphi A is the correct answer:** Among the common causes of enteric fever, **Salmonella paratyphi A** is uniquely characterized by the **absence of the Vi antigen**. This is a high-yield diagnostic point: because it lacks the Vi antigen, patients with Paratyphi A infections will not show a rise in anti-Vi antibodies, and vaccines targeting the Vi antigen (like the Typhim Vi vaccine) do not provide protection against this specific serotype. **Analysis of Incorrect Options:** * **S. typhi:** This is the most well-known carrier of the Vi antigen. It is the primary target for Vi-based typhoid vaccines. * **S. paratyphi C:** Unlike Paratyphi A and B, Paratyphi C expresses the Vi antigen. * **S. dublin:** This is a primarily zoonotic (bovine) serotype that is known to express the Vi antigen, occasionally causing systemic infections in humans. **NEET-PG High-Yield Pearls:** 1. **Detection:** If the Vi antigen is present, it may mask the O-antigen during agglutination tests (Widal). To unmask the O-antigen, the bacterial suspension must be **boiled** to destroy the heat-labile Vi antigen. 2. **Carrier State:** Detection of Vi antibodies is a useful screening tool for identifying **chronic carriers** of *S. typhi*. 3. **Vaccinology:** The Vi polysaccharide vaccine is a subunit vaccine used for typhoid prophylaxis, but it is ineffective against *S. paratyphi A*.

Q: Atypical pneumonia is caused by all of the following pathogens except:

Haemophilus. **Explanation:** **Atypical pneumonia** refers to pneumonia that presents with a subacute onset, non-productive cough, and systemic symptoms (headache, malaise) rather than the classic signs of consolidation. Radiologically, it often shows diffuse interstitial infiltrates ("patchy" appearance) that look worse than the clinical presentation. **Why Haemophilus is the correct answer:** *Haemophilus influenzae* is a classic cause of **Typical Pneumonia**. It is a Gram-negative coccobacillus that typically causes lobar consolidation, productive cough with purulent sputum, and high fever. It possesses a polysaccharide capsule (Type b being the most virulent) and is a common cause of community-acquired pneumonia (CAP), especially in patients with underlying COPD. **Why the other options are incorrect:** * **Mycoplasma pneumoniae:** The most common cause of atypical pneumonia ("Walking Pneumonia"). It lacks a cell wall (resistant to beta-lactams) and is associated with cold agglutinins. * **Adenovirus:** A common viral cause of atypical pneumonia, particularly in military recruits and children. * **Chlamydia:** Both *C. pneumoniae* and *C. psittaci* are well-known causes of atypical pneumonia, often presenting with a persistent dry cough and pharyngitis. **High-Yield Clinical Pearls for NEET-PG:** * **Mycoplasma:** Look for "Bullous Myringitis" or "Erythema Multiforme" in the clinical vignette. * **Legionella:** Another atypical pathogen; look for hyponatremia, diarrhea, and history of exposure to air conditioners/water towers. * **Treatment:** Atypical pathogens lack a cell wall or are intracellular; therefore, they are **intrinsically resistant to Beta-lactams**. The drugs of choice are **Macrolides** (Azithromycin), Tetracyclines (Doxycycline), or Fluoroquinolones.

Q: What is the recommended approach for BCG vaccination in an HIV-positive newborn?

Contraindicated. **Explanation:** The correct answer is **A. Contraindicated**. **1. Why it is correct:** BCG (Bacillus Calmette-Guérin) is a **live-attenuated vaccine** derived from *Mycobacterium bovis*. In individuals with compromised immune systems, such as HIV-positive newborns, the body cannot effectively contain the attenuated bacteria. This poses a significant risk of **Disseminated BCG infection**, which is often fatal. According to WHO and National AIDS Control Organization (NACO) guidelines, BCG should be withheld in infants known to be HIV-infected. If the infant's status is unknown but they show clinical signs/symptoms suggestive of HIV, the vaccine is also contraindicated. **2. Why incorrect options are wrong:** * **B, C, & D (Dilution/Dose adjustments):** Modifying the concentration or volume of the vaccine does not mitigate the risk of dissemination in an immunocompromised host. The issue is not the quantity of the bacteria, but the host's inability to mount a cell-mediated immune response against a live pathogen. There is no clinical protocol involving "double" or "half" dilutions for BCG in the context of HIV. **3. High-Yield Clinical Pearls for NEET-PG:** * **General Rule:** All live vaccines (BCG, OPV, MMR, Yellow Fever, Varicella) are generally contraindicated in symptomatic HIV/immunocompromised states. * **Exception:** In asymptomatic HIV-infected children, WHO suggests MMR and Measles vaccines can be given, but BCG remains strictly contraindicated if the HIV status is confirmed positive. * **Normal BCG Dose:** 0.1 ml (0.05 ml for neonates <1 month) given intradermally over the left deltoid. * **Diagnostic Tip:** If a child presents with hepatosplenomegaly and lymphadenopathy after BCG vaccination, suspect **SCID** (Severe Combined Immunodeficiency) or **HIV**.

Q: Which serovar of Chlamydia trachomatis causes Lymphogranuloma venereum (LGV)?

Lymphogranuloma venereum (LGV). **Explanation:** *Chlamydia trachomatis* is an obligate intracellular bacterium classified into different serovars based on major outer membrane protein (MOMP) antigens. These serovars exhibit distinct tissue tropisms and clinical manifestations. **1. Why the Correct Answer is Right:** **Lymphogranuloma venereum (LGV)** is caused by **serovars L1, L2, and L3**. Unlike the oculogenital strains, these serovars are more invasive and possess a predilection for lymphatic tissue. The disease typically presents in three stages: a painless primary lesion, followed by painful inguinal lymphadenopathy (the "bubo" stage), and potentially chronic proctocolitis or lymphatic obstruction (elephantiasis) if left untreated. **2. Analysis of Incorrect Options:** * **A. Trachoma:** Caused by **serovars A, B, Ba, and C**. It is a leading cause of preventable blindness worldwide, characterized by chronic follicular conjunctivitis and subsequent scarring (entropion/trichiasis). * **B. Inclusion Conjunctivitis:** Caused by **serovars D–K**. In adults, it is often associated with genital infections (paratrachoma), while in neonates, it occurs via birth canal transmission. * **C. Non-gonococcal Urethritis (NGU):** Also caused by **serovars D–K**. These are the most common cause of sexually transmitted bacterial urethritis and cervicitis, often leading to complications like Pelvic Inflammatory Disease (PID) or epididymitis. **High-Yield Clinical Pearls for NEET-PG:** * **Groove Sign of Greenblatt:** A characteristic clinical finding in LGV where the inguinal ligament creates a depression between inflamed superficial and deep inguinal lymph nodes. * **Frei Test:** A delayed hypersensitivity skin test previously used for LGV diagnosis (now largely replaced by NAAT). * **Drug of Choice:** Doxycycline (100 mg twice daily) is the preferred treatment for LGV, typically for a longer duration (21 days) compared to standard chlamydial urethritis.

Q: Which halophilic Vibrio species is implicated in gastroenteritis?

Vibrio Parahaemolyticus. **Explanation:** **Vibrio parahaemolyticus** is the correct answer because it is the most common halophilic (salt-loving) Vibrio species associated with **gastroenteritis** worldwide. It is typically contracted through the consumption of contaminated raw or undercooked seafood (especially shellfish/oysters). Its pathogenicity is primarily linked to the **Kanagawa phenomenon**, where the production of a thermostable direct hemolysin (TDH) results in beta-hemolysis on Wagatsuma agar. **Analysis of Incorrect Options:** * **Vibrio mimicus:** While it can cause gastroenteritis, it is unique because it is **non-halophilic** (it can grow in the absence of salt), mimicking the growth characteristics of *V. cholerae*. * **Vibrio vulnificus:** This is a highly virulent halophilic species, but it is primarily associated with **primary septicemia** and severe **necrotizing fasciitis** (wound infections) following exposure to seawater, rather than simple gastroenteritis. * **Vibrio alginolyticus:** This is the most salt-tolerant species (can grow in 10% NaCl). It is usually implicated in **extra-intestinal infections** such as otitis media and wound infections, but rarely causes diarrhea. **High-Yield Clinical Pearls for NEET-PG:** * **Halophilism:** All Vibrios are halophilic (require NaCl) **except** *V. cholerae* and *V. mimicus*. * **Culture:** Vibrios grow on **TCBS agar** (Thiosulfate Citrate Bile salts Sucrose). *V. parahaemolyticus* appears as **Green colonies** (sucrose non-fermenter), whereas *V. cholerae* appears as Yellow colonies. * **Morphology:** They exhibit **darting motility** (comma-shaped with a single polar flagellum). * **Salt Tolerance:** *V. parahaemolyticus* grows best at 3% NaCl but cannot grow at 10% NaCl (unlike *V. alginolyticus*).

Question 1

A 45-year-old patient presents with symptoms of septic arthritis. What is the most common cause in this patient?

Accepted Answer

Staphylococcus aureus

Answer

Gonococcus

Answer

Salmonella

Answer

Pneumococci

Question 2

What are the common causes of community-acquired native valve endocarditis?

Accepted Answer

Streptococcus viridians

Answer

Staphylococcus aureus

Answer

Diphtheroids

Answer

Staphylococcus epidermidis

Question 3

Which of the following is not a fungal infection?

Accepted Answer

Actinomycosis

Answer

Blastomycosis

Answer

Cryptococcus

Answer

Histoplasmosis

Question 4

A mother with a reactive VDRL test gave birth to an infant. Which of the following would help in determining the risk of transmission to the infant, EXCEPT?

Accepted Answer

TPHA test on the serum sample of the infant

Answer

TPHA test on the serum sample of the mother

Answer

VDRL on the paired serum sample of the infant and mother

Answer

Time interval between the treatment of the mother and her delivery

Question 5

Which of the following statements best describes Helicobacter pylori?

Accepted Answer

An obligate parasite of the gastric mucosa

Answer

A Gram-negative, non-flagellated spiral organism

Answer

Does not infect the duodenal mucosa

Answer

Unresponsive to antibiotic therapy

Question 6

Vi antigen is present in all of the following bacteria except?

Accepted Answer

S.paratyphi A

Answer

S.typhi

Answer

S.paratyphi C

Answer

S.dublin

Question 7

Atypical pneumonia is caused by all of the following pathogens except:

Accepted Answer

Haemophilus

Answer

Mycoplasma

Answer

Adenovirus

Answer

Chlamydia

Question 8

What is the recommended approach for BCG vaccination in an HIV-positive newborn?

Accepted Answer

Contraindicated

Answer

Double dilution

Answer

Half dilution

Answer

Dose doubled

Question 9

Which serovar of Chlamydia trachomatis causes Lymphogranuloma venereum (LGV)?

Accepted Answer

Lymphogranuloma venereum (LGV)

Answer

Trachoma

Answer

Inclusion conjunctivitis

Answer

Non-gonococcal urethritis (NGU)

Question 10

Which halophilic Vibrio species is implicated in gastroenteritis?

Accepted Answer

Vibrio Parahaemolyticus

Answer

Vibrio Mimicus

Answer

Vibrio Vulnificus

Answer

Vibrio Alginolyticus

Bacteriology — MCQs

Bacteriology — MCQs

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