Medical Bacteriology Examination MCQs

Question 1 The heat-labile toxin of Enterotoxigenic Escherichia coli (ETEC) acts through which mechanism? A. Attachment and effacement B. Activation of adenylyl cyclase C. Aggregative adherence D. Ribosomal dysfunction E. Primarily an incidental laboratory finding Answer: B. 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. --- Question 2 Which of the following statements regarding Pseudomonas aeruginosa is correct? A. P. aeruginosa is typically susceptible to penicillin G B. P. aeruginosa grows readily in anaerobic blood cultures C. P. aeruginosa penetrates intact skin using the enzyme "invasin" D. P. aeruginosa seldom causes pneumonia E. P. aeruginosa possesses fimbriae that promote attachment to epithelial cells Answer: E. 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. --- Question 3 Long-term carriage and chronic shedding are most likely after gastrointestinal infection with A. Escherichia coli O157:H7 B. Shigella dysenteriae C. Vibrio cholerae D. Campylobacter jejuni E. Salmonella typhi Answer: E. Salmonella typhi Explanation: Salmonella typhi can establish chronic carriage in the gallbladder, with persistent shedding in feces. --- Question 4 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? A. Salmonella typhimurium B. Campylobacter jejuni C. Shigella sonnei D. Vibrio cholerae E. Reduced by routine feedback control Answer: D. Vibrio cholerae Explanation: Vibrio cholerae is typically associated with specific endemic regions or outbreaks related to contaminated water sources. --- Question 5 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? A. Both β-lactamase and tetracycline resistance genes are plasmid-mediated B. β-lactamase is chromosomal, tetracycline resistance is plasmid-mediated C. β-lactamase is plasmid-mediated, tetracycline resistance is chromosomal D. Both β-lactamase and tetracycline resistance genes are chromosomal E. Occurs only after chronic exposure Answer: C. 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. --- Question 6 Which Neisseria gonorrhoeae structure mediates attachment to host epithelial cells? A. Lipooligosaccharide B. Fimbriae C. IgA1 protease D. Outer membrane porin proteins E. Iron-binding proteins Answer: B. Fimbriae Explanation: Fimbriae (pili) are the primary adhesion structures for initial mucosal attachment. --- Question 7 A 6-year-old boy has meningitis. CSF culture grows Neisseria meningitidis serogroup B. What is recommended for household contacts? A. No prophylaxis needed B. Pilin vaccine C. Serogroup B polysaccharide capsule vaccine D. Rifampin prophylaxis E. Sulfonamide prophylaxis Answer: D. Rifampin prophylaxis Explanation: Close contacts require chemoprophylaxis (rifampin, ciprofloxacin, or ceftriaxone) to eradicate nasopharyngeal carriage. --- Question 8 A common long-term complication of gonococcal pelvic inflammatory disease (PID) is A. Cervical cancer B. Urethral stricture C. Uterine fibroids D. Infertility E. Vaginal–rectal fistula Answer: D. Infertility Explanation: PID leads to scarring of fallopian tubes, resulting in tubal infertility or ectopic pregnancy. --- Question 9 Which statement best describes the pathogenesis of Clostridium botulinum? A. Botulinum toxin B. Exotoxin is a lecithinase causing tissue necrosis C. Capsule inhibits phagocytosis and aids CNS invasion D. Toxin suppresses release of inhibitory neurotransmitters E. Produces leukotoxin leading to abscess formation Answer: A. Botulinum toxin Explanation: Botulinum toxin cleaves SNARE proteins, blocking acetylcholine release and causing flaccid paralysis. --- Question 10 A patient with cavitary lung disease grows a photochromogenic acid-fast bacillus (orange pigment on light exposure). The organism is A. Mycobacterium tuberculosis B. M. kansasii C. Mycobacterium gordonae D. Mycobacterium avium complex E. Mycobacterium fortuitum Answer: B. M. kansasii Explanation: M. kansasii is a Runyon Group I photochromogen. --- Question 11 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 A. Mycobacterium leprae B. Mycobacterium fortuitum C. Mycobacterium ulcerans D. Mycobacterium gordonae E. Mycobacterium tuberculosis Answer: E