Bacteriology Practice Questions

Q: What is the most common causative organism of acute pyelonephritis in pregnancy?

E. coli. **Explanation:** **1. Why E. coli is correct:** *Escherichia coli* is the most common causative organism for acute pyelonephritis in pregnancy, accounting for approximately **70–80% of cases**. The primary mechanism is the ascending route of infection. During pregnancy, physiological changes such as progesterone-induced smooth muscle relaxation (leading to ureteral dilatation) and mechanical compression of the ureters by the gravid uterus cause **urinary stasis**. This environment, combined with pregnancy-induced glycosuria and aminoaciduria, facilitates the upward migration of fecal flora (E. coli) from the perineum to the upper urinary tract. **2. Why the other options are incorrect:** * **B & C (Klebsiella and Enterobacter):** While these are Gram-negative bacilli that can cause UTIs, they are significantly less common than E. coli, typically accounting for only 3–5% of cases each. They are more frequently seen in recurrent infections or hospital-acquired settings. * **D (Staphylococcus group):** Gram-positive organisms like *Staphylococcus saprophyticus* or Group B Streptococcus (GBS) are occasional causes of UTI in pregnancy, but they represent a small minority of pyelonephritis cases compared to the overwhelming prevalence of E. coli. **3. NEET-PG High-Yield Pearls:** * **Most common medical complication** requiring hospitalization during pregnancy: Acute Pyelonephritis. * **Most common site:** Right side (due to dextrorotation of the uterus and protection of the left ureter by the sigmoid colon). * **Screening:** All pregnant women should be screened for **Asymptomatic Bacteriuria (ASB)** at the first prenatal visit (12–16 weeks). If untreated, 25–40% of ASB cases progress to acute pyelonephritis. * **Complications:** Can lead to preterm labor, maternal sepsis, and ARDS.

Q: Chronic Burrowing ulcer is caused by which of the following?

Microaerophilic streptococci. **Explanation:** **Chronic Burrowing Ulcer**, also known as **Meleney’s Gangrene** (or Meleney’s synergistic gangrene), is a progressive, necrotizing infection of the skin and subcutaneous tissues. The correct answer is **Microaerophilic streptococci** because they are the primary causative agents identified in these lesions. 1. **Why Microaerophilic streptococci is correct:** These organisms thrive in low-oxygen environments. In Meleney’s gangrene, they typically act in **synergy** with other bacteria (often *Staphylococcus aureus* or Proteus). The infection is characterized by a slow-spreading, painful ulcer with undermined (burrowing) edges, often occurring post-operatively after abdominal or thoracic surgery. 2. **Why other options are incorrect:** * **Peptostreptococcus:** While these are obligate anaerobes often found in polymicrobial abscesses, they are not the classic primary agent described for the specific clinical entity of a "chronic burrowing ulcer." * **Streptococcus viridans:** These are commensals of the oral cavity and are primarily associated with subacute bacterial endocarditis (SBE), not necrotizing skin ulcers. * **Streptococcus pyogenes:** This organism causes **acute** infections like Erysipelas, Cellulitis, or "Flesh-eating" Necrotizing Fasciitis (Type II). Unlike the chronic, slow-progressing burrowing ulcer, *S. pyogenes* infections are rapidly fulminant. **High-Yield Clinical Pearls for NEET-PG:** * **Meleney’s Gangrene:** Look for keywords like "synergistic," "burrowing," and "post-operative." * **Fournier’s Gangrene:** A similar necrotizing infection specifically involving the perineum and scrotum. * **Treatment:** Management requires aggressive surgical debridement and broad-spectrum antibiotics. * **Differentiation:** Do not confuse this with **Tropical Ulcer** (caused by *Fusobacterium necrophorum* and *Borrelia vincentii*).

Q: What is the most common cause of urinary tract infection in children?

Escherichia coli. **Explanation:** **Escherichia coli (Option A)** is the most common cause of urinary tract infection (UTI) in children, accounting for approximately **80–90% of all cases**. The primary mechanism is the ascending route, where fecal flora colonize the perineum and enter the bladder. *E. coli* possesses specific virulence factors, such as **P-fimbriae (pyelonephritis-associated pili)**, which allow the bacteria to adhere to the uroepithelial cells and resist being washed away by urine flow. **Why other options are incorrect:** * **Klebsiella species (Option B):** While it is the second most common Gram-negative cause, it typically occurs in children with recurrent UTIs, those who have received prior antibiotics, or those with structural abnormalities. * **Pseudomonas aeruginosa (Option C):** This is an opportunistic pathogen. It is rarely a cause in healthy children and is usually associated with hospital-acquired infections, urinary tract instrumentation (catheterization), or chronic structural malformations. * **Candida (Option D):** Fungal UTIs are uncommon in immunocompetent children. They are typically seen in neonates in the NICU, children on prolonged broad-spectrum antibiotics, or those with indwelling catheters. **High-Yield Clinical Pearls for NEET-PG:** * **Most common route of infection:** Ascending route (except in neonates, where hematogenous spread is more common). * **Most common viral cause:** Adenovirus (typically causes acute hemorrhagic cystitis). * **Gold Standard Diagnosis:** Urine culture obtained via suprapubic aspiration (any growth is significant) or transurethral catheterization (>50,000 CFU/mL). * **Proteus mirabilis:** Often associated with UTIs in uncircumcised males and is linked to the formation of struvite (staghorn) calculi due to its urease-producing nature.

Q: Which of the following is a feature of Streptococcus agalactiae rather than Staphylococcus aureus?

Coagulase negative. **Explanation:** The core distinction between the genera *Staphylococcus* and *Streptococcus* lies in their biochemical profiles. **Streptococcus agalactiae** (Group B Streptococcus) is a Gram-positive coccus that is **Coagulase negative**, whereas *Staphylococcus aureus* is the primary human pathogen that is **Coagulase positive**. 1. **Why Option C is Correct:** The Coagulase test identifies the ability of an organism to convert fibrinogen to fibrin. *S. aureus* is the "gold standard" for a positive coagulase test. In contrast, all Streptococci, including *S. agalactiae*, lack this enzyme, making them coagulase negative. 2. **Why Options A, B, and D are Incorrect:** * **Catalase positive (A):** This is the primary test to differentiate the two families. *Staphylococci* are Catalase positive, while *Streptococci* are **Catalase negative**. * **Bacitracin resistant (B):** While *S. agalactiae* is indeed Bacitracin resistant, this does not distinguish it from *S. aureus* (which is also typically resistant). Bacitracin sensitivity is specifically used to differentiate *S. pyogenes* (Sensitive) from other Streptococci. * **Alpha hemolysis (D):** *S. agalactiae* typically exhibits **narrow-zone Beta-hemolysis**. *S. aureus* also shows Beta-hemolysis. Alpha hemolysis is characteristic of *S. pneumoniae* and Viridans group Streptococci. **NEET-PG High-Yield Pearls:** * **CAMP Test:** *S. agalactiae* produces the "CAMP factor," which enlarges the zone of hemolysis produced by *S. aureus* (Arrowhead lethality). * **Clinical Significance:** *S. agalactiae* is the leading cause of **neonatal sepsis and meningitis**. Screening is done at 35–37 weeks of pregnancy. * **Hippurate Hydrolysis:** *S. agalactiae* is positive for hippurate hydrolysis, a key lab differentiator.

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).

Question 1

What is the most common causative organism of acute pyelonephritis in pregnancy?

Accepted Answer

E. coli

Answer

Klebsiella pneumoniae

Answer

Enterobacter

Answer

Staphylococcus group

Question 2

Chronic Burrowing ulcer is caused by which of the following?

Accepted Answer

Microaerophilic streptococci

Answer

Peptostreptococcus

Answer

Streptococcus viridans

Answer

Streptococcus pyogenes

Question 3

Which of the following statements is true regarding Klebsiella infections?

Accepted Answer

Predisposing factors for Klebsiella pneumonia include alcoholism and diabetes mellitus.

Answer

Most clinical isolates are obtained from the respiratory tract.

Answer

Klebsiella is closely related to Pseudomonas.

Answer

Detecting Klebsiella growth from a sputum culture obtained from an intubated patient mandates treatment with an aminoglycoside or a third-generation cephalosporin.

Question 4

What is the most common cause of urinary tract infection in children?

Accepted Answer

Escherichia coli

Answer

Klebsiella species

Answer

Pseudomonas aeruginosa

Answer

Candida

Question 5

Which of the following is a feature of Streptococcus agalactiae rather than Staphylococcus aureus?

Accepted Answer

Coagulase negative

Answer

Catalase positive

Answer

Bacitracin resistant

Answer

Alpha hemolysis

Question 6

A 28-year-old woman presented with high-grade fever, cough, diarrhea, and mental confusion for 4 days. Chest X-ray revealed bilateral pneumonitis. What etiology is most likely?

Accepted Answer

Legionella pneumophila

Answer

Streptococcus pneumoniae

Answer

Staphylococcus aureus

Answer

Pseudomonas aeruginosa

Question 7

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 8

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

Accepted Answer

Streptococcus viridians

Answer

Staphylococcus aureus

Answer

Diphtheroids

Answer

Staphylococcus epidermidis

Question 9

Which of the following is not a fungal infection?

Accepted Answer

Actinomycosis

Answer

Blastomycosis

Answer

Cryptococcus

Answer

Histoplasmosis

Question 10

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

Bacteriology — MCQs

Bacteriology — MCQs

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