Medical Bacteriology Examination

20 clinical MCQs in Microbiology. The heat-labile toxin of Enterotoxigenic Escherichia coli (ETEC) acts through which mechan

Questions, Answers & Explanations

1. Q1. The heat-labile toxin of Enterotoxigenic Escherichia coli (ETEC) acts through which mechanism?

   • Attachment and effacement
   • Activation of adenylyl cyclase
   • Aggregative adherence
   • Ribosomal dysfunction

   Answer: Activation of adenylyl cyclase

   Explanation: The heat-labile toxin (LT) of ETEC functions similarly to cholera toxin by activating adenylyl cyclase, increasing intracellular cAMP levels, leading to secretion of water and electrolytes.

2. Q2. Which of the following statements regarding Pseudomonas aeruginosa is correct?

   • P. aeruginosa is typically susceptible to penicillin G
   • P. aeruginosa grows readily in anaerobic blood cultures
   • P. aeruginosa penetrates intact skin using the enzyme "invasin"
   • P. aeruginosa seldom causes pneumonia
   • P. aeruginosa possesses fimbriae that promote attachment to epithelial cells

   Answer: P. aeruginosa possesses fimbriae that promote attachment to epithelial cells

   Explanation: P. aeruginosa has fimbriae (pili) that facilitate adherence to epithelial cells. It is an obligate aerobe and resistant to penicillin G.

3. Q3. Long-term carriage and chronic shedding are most likely after gastrointestinal infection with:

   • Escherichia coli O157:H7
   • Shigella dysenteriae
   • Vibrio cholerae
   • Campylobacter jejuni
   • Salmonella typhi

   Answer: Salmonella typhi

   Explanation: Salmonella typhi can establish chronic carriage in the gallbladder, with persistent shedding in feces.

4. Q4. A patient presents with acute non-bloody diarrhea for 12 hours. Which organism is least likely to cause his illness in a non-endemic area without travel history?

   • Salmonella typhimurium
   • Campylobacter jejuni
   • Shigella sonnei
   • Vibrio cholerae

   Answer: Vibrio cholerae

   Explanation: Vibrio cholerae is typically associated with specific endemic regions or outbreaks related to contaminated water sources.

5. Q5. A 19-year-old man has urethral discharge. Culture shows Neisseria gonorrhoeae that is β-lactamase positive and highly resistant to tetracycline. Which statement is correct?

   • Both β-lactamase and tetracycline resistance genes are plasmid-mediated
   • β-lactamase is chromosomal, tetracycline resistance is plasmid-mediated
   • β-lactamase is plasmid-mediated, tetracycline resistance is chromosomal
   • Both β-lactamase and tetracycline resistance genes are chromosomal

   Answer: β-lactamase is plasmid-mediated, tetracycline resistance is chromosomal

   Explanation: In N. gonorrhoeae, β-lactamase production is typically plasmid-mediated (TEM-1), while high-level tetracycline resistance is usually due to chromosomal mutations.

6. Q6. Which Neisseria gonorrhoeae structure mediates attachment to host epithelial cells?

   • Lipooligosaccharide
   • Fimbriae
   • IgA1 protease
   • Outer membrane porin proteins
   • Iron-binding proteins

   Answer: Fimbriae

   Explanation: Fimbriae (pili) are the primary adhesion structures for initial mucosal attachment.

7. Q7. A 6-year-old boy has meningitis. CSF culture grows Neisseria meningitidis serogroup B. What is recommended for household contacts?

   • No prophylaxis needed
   • Pilin vaccine
   • Serogroup B polysaccharide capsule vaccine
   • Rifampin prophylaxis
   • Sulfonamide prophylaxis

   Answer: Rifampin prophylaxis

   Explanation: Close contacts require chemoprophylaxis (rifampin, ciprofloxacin, or ceftriaxone) to eradicate nasopharyngeal carriage.

8. Q8. A common long-term complication of gonococcal pelvic inflammatory disease (PID) is:

   • Cervical cancer
   • Urethral stricture
   • Uterine fibroids
   • Infertility
   • Vaginal–rectal fistula

   Answer: Infertility

   Explanation: PID leads to scarring of fallopian tubes, resulting in tubal infertility or ectopic pregnancy.

9. Q9. Which statement best describes the pathogenesis of Clostridium botulinum?

   • Toxin inhibits acetylcholine release at cholinergic synapses
   • Exotoxin is a lecithinase causing tissue necrosis
   • Capsule inhibits phagocytosis and aids CNS invasion
   • Toxin suppresses release of inhibitory neurotransmitters
   • Produces leukotoxin leading to abscess formation

   Answer: Toxin inhibits acetylcholine release at cholinergic synapses

   Explanation: Botulinum toxin cleaves SNARE proteins, blocking acetylcholine release and causing flaccid paralysis.

10. Q10. A patient with cavitary lung disease grows a photochromogenic acid-fast bacillus (orange pigment on light exposure). The organism is:

    • Mycobacterium tuberculosis
    • Mycobacterium kansasii
    • Mycobacterium gordonae
    • Mycobacterium avium complex
    • Mycobacterium fortuitum

    Answer: Mycobacterium kansasii

    Explanation: M. kansasii is a Runyon Group I photochromogen.

11. Q11. A patient has fever, weight loss, and granulomas with acid-fast bacilli in liver and bone marrow. Chest X-ray is normal. Most likely organism:

    • Mycobacterium leprae
    • Mycobacterium fortuitum
    • Mycobacterium ulcerans
    • Mycobacterium gordonae
    • Mycobacterium tuberculosis

    Answer: Mycobacterium tuberculosis

    Explanation: This describes disseminated (miliary) tuberculosis.

12. Q12. A major concern for the patient above is infection with a strain that is:

    • Susceptible only to isoniazid
    • Resistant to streptomycin
    • Resistant to clarithromycin
    • Resistant to isoniazid and rifampin

    Answer: Resistant to isoniazid and rifampin

    Explanation: Multidrug-resistant tuberculosis (MDR-TB) is defined as resistance to both isoniazid and rifampin.

13. Q13. A pregnant woman previously treated for syphilis has RPR non-reactive and TP-PA reactive. Interpretation:

    • Previous treatment was effective
    • Baby is at high risk for congenital syphilis
    • Mother requires re-treatment
    • Mother needs lumbar puncture

    Answer: Previous treatment was effective

    Explanation: Treponemal tests (TP-PA) stay positive for life; non-treponemal tests (RPR) become negative after successful treatment.

14. Q14. A 20-year-old woman has a 2-cm painless ulcer with raised border on the labia majora. Pathogen:

    • Haemophilus ducreyi
    • Papillomavirus
    • Neisseria gonorrhoeae
    • Chlamydia trachomatis

    Answer: Haemophilus ducreyi

15. Q15. Common causes of community-acquired pneumonia include:

    • Legionella pneumophila
    • Chlamydia pneumoniae
    • Streptococcus pneumoniae
    • Mycoplasma pneumoniae
    • All of the above

    Answer: All of the above

16. Q16. Mechanism of "rice-water" diarrhea in Cholera:

    • Clostridium difficile enterotoxin
    • Vibrio cholerae toxin (A/B subunits)
    • Shigella dysenteriae type 1 (Shiga toxin)
    • ETEC toxins

    Answer: Vibrio cholerae toxin (A/B subunits)

17. Q17. Mechanism of action of diphtheria toxin:

    • Forms pores in red cells
    • Degrades lecithin
    • Causes release of TNF
    • Inhibits elongation factor-2
    • Increases adenylate cyclase activity

    Answer: Inhibits elongation factor-2

    Explanation: It inactivates EF-2 via ADP-ribosylation, stopping protein synthesis.

18. Q18. Toxic shock syndrome (TSS) is caused by:

    • Lipopolysaccharide
    • Peptidoglycan
    • A superantigen toxin
    • A toxin with A/B subunits

    Answer: A superantigen toxin

19. Q19. Most likely organism responsible for TSS in a patient with nasal tampons:

    • Escherichia coli
    • Corynebacterium diphtheriae
    • Clostridium perfringens
    • Neisseria meningitidis
    • Staphylococcus aureus

    Answer: Staphylococcus aureus

20. Q20. Characteristic of Bacillus anthracis EXCEPT:

    • Motility
    • Medusa-head colonies
    • Poly-D-glutamic acid capsule
    • Susceptibility to penicillin

    Answer: Motility

    Explanation: B. anthracis is non-motile. ## SECTION B – SHORT ANSWER QUESTIONS