Medical Bacteriology MCQs: ETEC, OmpathStudy

MOUNT KENYA UNIVERSITY DEPARTMENT OF MEDICAL MICROBIOLOGY BACHELOR OF MEDICINE AND BACHELOR OF SURGERY MBMM3311 MEDICAL BACTERIOLOGY --- SECTION A Answer all questions in this section 1. Heat-labile toxin of ETEC acts by which of the following mechanisms? (A) Attachment and effacement (B) Activation of adenylyl cyclase (C) Aggregative adherence (D) Ribosomal dysfunction (E) None of the above Answer: B Explanation: LT toxin ADP-ribosylates the Gs protein, permanently activating adenylate cyclase → ↑cAMP → fluid/electrolyte secretion. Same mechanism as cholera toxin. 2. Which of the following statements about Pseudomonas aeruginosa is correct? (A) P aeruginosa are typically susceptible to penicillin G (B) P aeruginosa are readily grown in anaerobic blood cultures (C) P aeruginosa are able to penetrate intact normal human skin by elaborating the enzyme invasin (D) P aeruginosa seldom, if ever, cause pneumonia (E) P aeruginosa have fimbriae, which promote attachment to epithelial cells Answer: C Explanation: P. aeruginosa is an obligate aerobe (rules out B) and intrinsically resistant to penicillin G (rules out A), and it commonly causes pneumonia (ventilated/CF patients — rules out D). It elaborates enzymes that let it penetrate intact skin/mucosa, key in burn-wound infection. 3. Long-term carriage and shedding is most likely after GI infection with which of the following species? (A) Escherichia coli O157:H7 (B) Shigella dysenteriae (C) Vibrio cholerae (D) Campylobacter jejuni (E) Salmonella typhi Answer: E Explanation: S. typhi can establish chronic carriage in the gallbladder (especially with gallstones), continuing to shed the organism in stool for months to years — the basis of the "Typhoid Mary" phenomenon. 4. A patient presents with non-bloody diarrhea for 12 hours, lives in Thika, no recent travel. Which is unlikely to be the cause? (A) Salmonella typhimurium (B) Campylobacter jejuni (C) Shigella sonnei (D) Vibrio cholerae Answer: D Explanation: V. cholerae causes severe watery diarrhea but requires a specific exposure — contaminated water/food from an epidemic/endemic source or travel. Without travel or a known outbreak, it's the least likely of the options. 5. N. gonorrhoeae is β-lactamase positive and has high-level tetracycline resistance. Which statement about these resistance factors is correct? (A) β-lactamase production and high-level tetracycline resistance are both mediated by genes on plasmids (B) β-lactamase production is mediated by a gene on the chromosome; tetracycline resistance by a gene on a plasmid (C) β-lactamase production is mediated by a gene on a plasmid; tetracycline resistance by a gene on the chromosome (D) β-lactamase production and high-level tetracycline resistance are both mediated by genes on the chromosome Answer: A Explanation: PPNG strains carry a plasmid-borne TEM-1 β-lactamase; TRNG strains carry the tetM determinant on a conjugative plasmid. Both resistance traits are plasmid-mediated, which is also why they spread so efficiently between strains. 6. Which cell component of N. gonorrhoeae is responsible for attachment to host cells? (A) Lipooligosaccharide (B) Fimbriae (C) IgA1 protease (D) Outer membrane porin protein (E) Iron-binding protein Answer: B Explanation: Pili (fimbriae) mediate initial attachment to mucosal epithelial cells and are also a major mechanism of antigenic variation to evade immunity. 7. A 6-year-old boy has meningococcal meningitis (N. meningitidis serogroup B). What should be considered for his household contacts? (A) No prophylaxis or other steps are necessary (B) They should be given N meningitidis pilin vaccine (C) They should be given N meningitidis serogroup B polysaccharide capsule vaccine (D) They should be given rifampin prophylaxis (E) They should be given sulfonamide prophylaxis Answer: D Explanation: Close/household contacts need chemoprophylaxis (rifampin, ciprofloxacin, or ceftriaxone) to eliminate carriage and prevent secondary cases. The serogroup B capsule is poorly immunogenic (it resembles host neural cell adhesion molecules), so no effective capsular vaccine exists for it — unlike serogroups A/C/W/Y. 8. An 18-year-old woman has gonococcal PID with a tubo-ovarian abscess. A common sequela of this infection is: (A) Cancer of the cervix (B) Urethral stricture (C) Uterine fibroid tumors (D) Infertility (E) Vaginal-rectal fistula Answer: D Explanation: PID causes tubal scarring/adhesions, leading to infertility or increased ectopic pregnancy risk — the major long-term complication of untreated gonococcal/chlamydial PID. 9. Which statement best describes the pathogenesis of Clostridium botulinum? (A) It elaborates a toxin that inhibits the release of acetylcholine at cholinergic synapses (B) It elaborates an exotoxin that is a lecithinase causing tissue necrosis (C) It produces a polysaccharide capsule that inhibits phagocytosis (D) It elaborates a toxin that suppresses the release of inhibitory neurotransmitters (E) It produces