answer it correctly with question and answer format with correct answer marked in MCQ 1. Haemophilus influenzae requires which growth factors? A. Iron and glucose B. X and V factors C. X factor only D. V factor only 2. The major virulence factor of Clostridium perfringens is: A. Enterotoxin B. Beta toxin C. Alpha toxin D. Exotoxin A 3. Which of the following correctly describes Shigella? A. Gram-positive, motile B. Gram-negative, motile C. Gram-negative, non-motile D. Gram-positive, non-motile 4. Shigella infection most commonly affects: A. Neonates B. Children <5 years C. Adolescents D. Elderly 5. The virulence factor responsible for colonic epithelial invasion in Shigella is: A. Capsule B. Endotoxin C. Type III secretion system D. Pili 6. The Hib vaccine targets which bacterial component? A. Lipopolysaccharide B. Protein A C. PRP polysaccharide capsule D. Flagella 7. Helicobacter pylori infection is most strongly associated with: A. Ulcerative colitis B. Crohn disease C. Peptic ulcer disease D. Colon cancer 8. Uropathogenic E. coli adheres to urinary epithelium via: A. Capsule B. Endotoxin C. Pili (fimbriae) D. Flagella Microbiology Without ANS 7. Helicobacter pylori infection is most strongly associated with: A. Ulcerative colitis B. Crohn disease C. Peptic ulcer disease D. Colon cancer 8. Uropathogenic E. coli adheres to urinary epithelium via: A. Capsule B. Endotoxin C. Pili (fimbriae) D. Flagella 9. Unpasteurized dairy products are classically associated with infection by: A. Salmonella typhi B. Shigella dysenteriae C. Listeria monocytogenes D. Campylobacter jejuni 10. The characteristic lesion in Clostridioides difficile infection is: A. Ulceration B. Caseation C. Pseudomembranous colitis D. Granuloma 11. The major clinical manifestation of diphtheria is: A. Bloody diarrhea B. Gray pseudomembrane in throat C. Vesicular rash D. Parotitis 12. Corynebacterium diphtheriae appears black on which medium? A. Chocolate agar B. Blood agar C. Tellurite agar D. MacConkey agar 13. A child with fever, abdominal pain, and bloody mucoid diarrhea most likely has: A. Cholera B. Shigellosis C. Giardiasis D. Amebiasis 14. The most important immediate treatment in severe diphtheria is: A. Antibiotics only B. Corticosteroids C. Diphtheria antitoxin D. Vaccine booster 15. Acinetobacter baumannii infections are most commonly: A. Community acquired B. Zoonotic C. Hospital acquired D. Food borne 16. Haemophilus influenzae type B most commonly causes: A. Pneumonia in adults B. Otitis media C. Meningitis in children D. Diarrhea 17. Salmonella is commonly transmitted through: A. Air droplets B. Sexual contact C. Contaminated food and water D. Blood transfusion 18. Which E. coli strain causes hemorrhagic colitis and HUS? A. ETEC B. EPEC C. EHEC D. EAEC 19. Gram-positive, anaerobic, spore-forming bacilli include: A. Bacillus anthracis B. Clostridium tetani C. Listeria monocytogenes D. Corynebacterium diphtheriae 20. Epiglottitis caused by H. influenzae is mainly due to: A. Endotoxin release B. PRP capsule inhibiting phagocytosis C. Flagellar invasion D. Spore formation ✅ Answer: B — PRP capsule 21. A patient presents with rigid paralysis and lockjaw. The most likely organism is: A. Clostridium botulinum B. Clostridium perfringens C. Clostridium tetani D. Corynebacterium diphtheriae 22. Descending flaccid paralysis is characteristic of infection with: A. Clostridium tetani B. Clostridium botulinum C. Poliovirus D. Listeria monocytogenes 23. Crepitus with severe muscle pain is most suggestive of: A. Tetanus B. Cellulitis C. Gas gangrene D. Necrotizing fasciitis 24. A patient develops diarrhea after antibiotic use. The causative organism is: A. Salmonella B. Shigella C. Clostridioides difficile D. Vibrio cholerae 25. Black eschar at the site of infection is typical of: A. Tularemia B. Anthrax C. Plague D. Diphtheria 26. Fishy vaginal odor with clue cells on microscopy indicates: A. Candidiasis B. Trichomoniasis C. Bacterial vaginosis D. Gonorrhea 27. A pregnant woman with meningitis should be empirically treated for: A. Neisseria meningitidis B. Streptococcus pneumoniae C. Listeria monocytogenes D. Haemophilus influenzae 28. Drug of choice for Listeria monocytogenes infection is: A. Ceftriaxone B. Vancomycin C. Ampicillin D. Metronidazole 29. An immunocompromised patient with brain abscess is most likely infected by: A. Toxoplasma gondii B. Nocardia C. Listeria D. Aspergillus 30. Drug of choice for Nocardia infection is: A. Penicillin B. Metronidazole C. TMP-SMX D. Amphotericin B 31. A child with gray membrane in throat — first step in management? A. Antibiotics B. Vaccination C. Diphtheria antitoxin D. Steroids 32. Fish or meat handlers with a violaceous skin lesion are infected with: A. Bacillus anthracis B. Erysipelothrix rhusiopathiae C. Staphylococcus aureus D. Streptococcus pyogenes 33. Treatment of Erysipelothrix rhusiopathiae infection is: A. Metronidazole B. TMP-SMX C. Penicillin D. Vancomycin 34. Fishy odor + clue cells are caused by: A. Candida albicans B. Gardnerella vaginalis C. Trichomonas vaginalis D. Chlamydia trachomatis 35. Treatment of Gardnerella vaginalis infection is: A. Fluconazole B. Penicillin C. Metronidazole D. Azithromycin 36. Helicobacter pylori infection is most commonly associated with: A. Ulcerative colitis B. Peptic ulcer disease C. Appendicitis D. IBS 37. The most characteristic feature of bacterial vaginosis is: A. Thick curdy discharge B. Green frothy discharge C. Fishy odor D. Genital ulcers 38. Uropathogenic E. coli causes UTI primarily due to: A. Capsule B. Endotoxin C. Pili (fimbriae) D. Flagella
Microbiology MCQs
- A. Iron and glucose
- B. X and V factors ✅
- C. X factor only
- D. V factor only
X factor (hemin) and V factor (NAD) are both required for growth of H. influenzae.
- A. Enterotoxin
- B. Beta toxin
- C. Alpha toxin ✅
- D. Exotoxin A
Alpha toxin (lecithinase/phospholipase C) is the main virulence factor, causing gas gangrene and hemolysis.
- A. Gram-positive, motile
- B. Gram-negative, motile
- C. Gram-negative, non-motile ✅
- D. Gram-positive, non-motile
Shigella is a Gram-negative, non-motile, non-lactose fermenting rod.
- A. Neonates
- B. Children <5 years ✅
- C. Adolescents
- D. Elderly
Shigellosis has the highest incidence in children under 5 years in endemic areas.
- A. Capsule
- B. Endotoxin
- C. Type III secretion system ✅
- D. Pili
Shigella uses a Type III secretion system (T3SS) encoded on a large virulence plasmid to inject proteins into epithelial cells and invade the colonic mucosa.
- A. Lipopolysaccharide
- B. Protein A
- C. PRP polysaccharide capsule ✅
- D. Flagella
The Hib vaccine contains polyribosylribitol phosphate (PRP), the capsular polysaccharide of H. influenzae type b, conjugated to a carrier protein.
- A. Ulcerative colitis
- B. Crohn disease
- C. Peptic ulcer disease ✅
- D. Colon cancer
H. pylori is the leading cause of peptic ulcer disease, responsible for >90% of duodenal ulcers and ~70-80% of gastric ulcers.
- A. Capsule
- B. Endotoxin
- C. Pili (fimbriae) ✅
- D. Flagella
Type 1 and P fimbriae (pili) mediate adhesion of uropathogenic E. coli to uroepithelial cells, a key step in UTI pathogenesis.
- A. Salmonella typhi
- B. Shigella dysenteriae
- C. Listeria monocytogenes ✅
- D. Campylobacter jejuni
Listeria monocytogenes thrives in cold temperatures (psychrotrophic) and is classically linked to unpasteurized dairy, soft cheeses, and deli meats.
- A. Ulceration
- B. Caseation
- C. Pseudomembranous colitis ✅
- D. Granuloma
C. difficile toxins A and B cause pseudomembranous colitis, characterized by yellow-white plaques (pseudomembranes) on the colonic mucosa.
- A. Bloody diarrhea
- B. Gray pseudomembrane in throat ✅
- C. Vesicular rash
- D. Parotitis
Corynebacterium diphtheriae produces a tough gray-white fibrinous pseudomembrane over the tonsillar/pharyngeal mucosa that bleeds on removal.
- A. Chocolate agar
- B. Blood agar
- C. Tellurite agar ✅
- D. MacConkey agar
C. diphtheriae reduces potassium tellurite to metallic tellurium, producing black colonies on tellurite-containing media (e.g., Hoyle's or modified Tinsdale agar).
- A. Cholera
- B. Shigellosis ✅
- C. Giardiasis
- D. Amebiasis
Shigellosis classically presents with fever, abdominal cramps, and bloody mucoid (dysenteric) diarrhea, especially in children.
- A. Antibiotics only
- B. Corticosteroids
- C. Diphtheria antitoxin ✅
- D. Vaccine booster
Diphtheria antitoxin (equine) must be given immediately to neutralize unbound exotoxin; antibiotics (penicillin/erythromycin) are adjunctive to eliminate the organism.
- A. Community acquired
- B. Zoonotic
- C. Hospital acquired ✅
- D. Food borne
A. baumannii is a notorious nosocomial (hospital-acquired) pathogen, particularly causing VAP, bloodstream infections, and wound infections in ICU patients.
- A. Pneumonia in adults
- B. Otitis media
- C. Meningitis in children ✅
- D. Diarrhea
Before widespread vaccination, Hib was the most common cause of bacterial meningitis in children aged 2 months to 5 years.
- A. Air droplets
- B. Sexual contact
- C. Contaminated food and water ✅
- D. Blood transfusion
Salmonella is a foodborne pathogen transmitted via contaminated poultry, eggs, meat, and water (fecal-oral route).
- A. ETEC
- B. EPEC
- C. EHEC ✅
- D. EAEC
Enterohemorrhagic E. coli (EHEC), especially O157:H7, produces Shiga toxin causing hemorrhagic colitis and hemolytic uremic syndrome (HUS).
- A. Bacillus anthracis
- B. Clostridium tetani ✅
- C. Listeria monocytogenes
- D. Corynebacterium diphtheriae
Clostridium species are Gram-positive, anaerobic, spore-forming rods. (Bacillus anthracis is aerobic/facultative, not anaerobic.)
- A. Endotoxin release
- B. PRP capsule inhibiting phagocytosis ✅
- C. Flagellar invasion
- D. Spore formation
The PRP (polyribosylribitol phosphate) capsule of Hib is antiphagocytic, enabling the organism to evade host defenses and cause invasive disease including epiglottitis.
- A. Clostridium botulinum
- B. Clostridium perfringens
- C. Clostridium tetani ✅
- D. Corynebacterium diphtheriae
C. tetani produces tetanospasmin, which blocks inhibitory neurotransmitters (GABA/glycine), causing spastic/rigid paralysis and trismus (lockjaw).
- A. Clostridium tetani
- B. Clostridium botulinum ✅
- C. Poliovirus
- D. Listeria monocytogenes
Botulinum toxin blocks ACh release at the neuromuscular junction, causing descending flaccid paralysis starting with cranial nerve involvement (diplopia, dysarthria, dysphagia).
- A. Tetanus
- B. Cellulitis
- C. Gas gangrene ✅
- D. Necrotizing fasciitis
Gas gangrene (myonecrosis) caused by C. perfringens produces CO₂ and H₂ gas, causing crepitus (crackling on palpation) with severe muscle pain.
- A. Salmonella
- B. Shigella
- C. Clostridioides difficile ✅
- D. Vibrio cholerae
Antibiotic-associated diarrhea/pseudomembranous colitis is classically caused by C. difficile overgrowth following disruption of normal gut flora.
- A. Tularemia
- B. Anthrax ✅
- C. Plague
- D. Diphtheria
Cutaneous anthrax (Bacillus anthracis) produces a painless black eschar (malignant pustule) due to tissue necrosis from anthrax toxins.
- A. Candidiasis
- B. Trichomoniasis
- C. Bacterial vaginosis ✅
- D. Gonorrhea
Bacterial vaginosis (Gardnerella vaginalis) produces a fishy amine odor (positive whiff test) and clue cells (epithelial cells studded with bacteria) on wet mount.
- A. Neisseria meningitidis
- B. Streptococcus pneumoniae
- C. Listeria monocytogenes ✅
- D. Haemophilus influenzae
Pregnant women are at high risk for Listeria meningitis/septicemia; empiric coverage with ampicillin is added since cephalosporins do NOT cover Listeria.
- A. Ceftriaxone
- B. Vancomycin
- C. Ampicillin ✅
- D. Metronidazole
Ampicillin (with or without gentamicin for synergy) is the drug of choice for Listeria. Cephalosporins are ineffective.
- A. Toxoplasma gondii
- B. Nocardia ✅
- C. Listeria
- D. Aspergillus
Nocardia (especially N. asteroides) causes brain abscesses in immunocompromised patients (e.g., transplant, HIV, steroids). Note: Toxoplasma causes ring-enhancing lesions in AIDS, but as a bacterial MCQ context, Nocardia is the intended answer.
- A. Penicillin
- B. Metronidazole
- C. TMP-SMX ✅
- D. Amphotericin B
Trimethoprim-sulfamethoxazole (TMP-SMX) is the first-line treatment for Nocardia infections.
- A. Antibiotics
- B. Vaccination
- C. Diphtheria antitoxin ✅
- D. Steroids
Diphtheria antitoxin is the priority to neutralize circulating toxin; it must be given before laboratory confirmation in clinically suspected cases.
- A. Bacillus anthracis
- B. Erysipelothrix rhusiopathiae ✅
- C. Staphylococcus aureus
- D. Streptococcus pyogenes
Erysipelothrix rhusiopathiae causes erysipeloid, a violaceous (purple-red) skin infection in fish handlers, butchers, and veterinarians.
- A. Metronidazole
- B. TMP-SMX
- C. Penicillin ✅
- D. Vancomycin
Penicillin G is the drug of choice for Erysipelothrix rhusiopathiae (erysipeloid). It is notably resistant to vancomycin.
- A. Candida albicans
- B. Gardnerella vaginalis ✅
- C. Trichomonas vaginalis
- D. Chlamydia trachomatis
Gardnerella vaginalis is the primary organism in bacterial vaginosis, producing the characteristic fishy odor and clue cells.
- A. Fluconazole
- B. Penicillin
- C. Metronidazole ✅
- D. Azithromycin
Metronidazole (oral or vaginal gel) is the first-line treatment for bacterial vaginosis caused by Gardnerella vaginalis. Clindamycin is an alternative.
- A. Ulcerative colitis
- B. Peptic ulcer disease ✅
- C. Appendicitis
- D. IBS
H. pylori is the principal cause of peptic ulcer disease and is also associated with gastric adenocarcinoma and MALT lymphoma.
- A. Thick curdy discharge
- B. Green frothy discharge
- C. Fishy odor ✅
- D. Genital ulcers
The hallmark of bacterial vaginosis is a fishy (amine) odor, especially after adding KOH (positive whiff test). Thick curdy discharge = Candida; green frothy = Trichomonas.
- A. Capsule
- B. Endotoxin
- C. Pili (fimbriae) ✅
- D. Flagella
P fimbriae (type P pili) and type 1 fimbriae allow UPEC to adhere to uroepithelial receptors, resisting urinary flow washout and initiating infection.
Quick Reference Summary
| Q | Answer | Key Concept |
|---|---|---|
| 1 | B | X + V factors for H. influenzae |
| 2 | C | Alpha toxin = main C. perfringens virulence factor |
| 3 | C | Shigella: Gram-negative, non-motile |
| 4 | B | Children <5 years most affected |
| 5 | C | T3SS for Shigella invasion |
| 6 | C | PRP capsule in Hib vaccine |
| 7 | C | H. pylori → peptic ulcer disease |
| 8 | C | Pili for UPEC adhesion |
| 9 | C | Listeria from unpasteurized dairy |
| 10 | C | C. diff → pseudomembranous colitis |
| 11 | B | Gray pseudomembrane in diphtheria |
| 12 | C | Black colonies on tellurite agar |
| 13 | B | Bloody mucoid diarrhea = Shigella |
| 14 | C | Antitoxin first in diphtheria |
| 15 | C | Acinetobacter = nosocomial |
| 16 | C | Hib → meningitis in children |
| 17 | C | Salmonella via contaminated food/water |
| 18 | C | EHEC → HUS |
| 19 | B | Clostridium = anaerobic spore-former |
| 20 | B | PRP capsule = antiphagocytic |
| 21 | C | Rigid paralysis = C. tetani |
| 22 | B | Descending flaccid = botulinum |
| 23 | C | Crepitus = gas gangrene |
| 24 | C | Post-antibiotic diarrhea = C. difficile |
| 25 | B | Black eschar = anthrax |
| 26 | C | Clue cells + fishy = BV |
| 27 | C | Pregnant + meningitis → Listeria coverage |
| 28 | C | Ampicillin for Listeria |
| 29 | B | Nocardia → brain abscess (immunocompromised) |
| 30 | C | TMP-SMX for Nocardia |
| 31 | C | Antitoxin first in diphtheria |
| 32 | B | Erysipelothrix in fish/meat handlers |
| 33 | C | Penicillin for Erysipelothrix |
| 34 | B | Gardnerella = clue cells + fishy odor |
| 35 | C | Metronidazole for BV |
| 36 | B | H. pylori → peptic ulcer |
| 37 | C | Fishy odor = hallmark of BV |
| 38 | C | Pili = UPEC virulence |