MEDICAL BACTERIOLOGY (MBMM3311) - MCQs – 50 MCQs | Kenya MBChB

50 Year 2: Microbiology exam questions on MEDICAL BACTERIOLOGY (MBMM3311) - MCQs for medical students. Includes MCQs, answers, explanations and written question

This MCQ set contains 50 questions on MEDICAL BACTERIOLOGY (MBMM3311) - MCQs in the Year 2: Microbiology unit. Each question includes the correct answer and a detailed explanation for active recall and exam preparation.

Q1: A gram-positive coccus isolated from a wound shows golden-yellow colonies on blood agar but is coagulase negative. The most likely organism is

  1. A. Streptococcus pyogenes
  2. B. Staphylococcus epidermidis
  3. C. Staphylococcus aureus
  4. D. Staphylococcus saprophyticus
  5. E. This organism is commonly associated with urinary tract infections in young women.

Correct answer: D – Staphylococcus saprophyticus

S. saprophyticus can produce yellow pigment but is coagulase negative, unlike S. aureus which is coagulase positive. This organism is commonly associated with urinary tract infections in young women.

Q2: A patient with "rice water" stool most likely has infection with

  1. A. Campylobacter jejuni
  2. B. Salmonella typhi
  3. C. Shigella dysenteriae
  4. D. Vibrio cholerae
  5. E. is characteristic of cholera caused by V. cholera

Correct answer: D – Vibrio cholerae

Rice water stool (watery with flecks of mucus resembling water in which rice has been washed) is characteristic of cholera caused by V. cholerae. The toxin causes massive fluid secretion leading to severe dehydration.

Q3: Which organism can grow at refrigeration temperature (4°C)?

  1. A. Bacillus anthracis
  2. B. Clostridium tetani
  3. C. Listeria monocytogenes
  4. D. Corynebacterium diphtheriae
  5. E. , making it a significant food safety concern in refrigerated ready-to-eat foods. This unique ability distinguishes it from most other pathogenic bact

Correct answer: C – Listeria monocytogenes

Listeria is psychrophilic and can multiply at refrigeration temperatures (4°C), making it a significant food safety concern in refrigerated ready-to-eat foods. This unique ability distinguishes it from most other pathogenic bacteria.

Q4: A 25-year-old sexually active female presents with urinary tract infection. The organism that is coagulase-negative and novobiocin resistant is

  1. A. Proteus mirabilis
  2. B. Escherichia coli
  3. C. Staphylococcus saprophyticus
  4. D. Staphylococcus epidermidis
  5. E. coli. S. epidermidis is novobiocin sensitive, which helps differentiate these two coagulase-negative staphylococci.

Correct answer: C – Staphylococcus saprophyticus

S. saprophyticus is novobiocin resistant and causes UTI in young sexually active women, being the second most common cause of UTI after E. coli. S. epidermidis is novobiocin sensitive, which helps differentiate these two coagulase-negative staphylococci.

Q5: The "string of pearls" appearance on Loeffler serum medium is characteristic of

  1. A. Listeria monocytogenes
  2. B. Clostridium perfringens
  3. C. Corynebacterium diphtheriae
  4. D. Bacillus anthracis

Correct answer: D – Bacillus anthracis

B. anthracis shows string of pearls appearance when grown in presence of penicillin on Loeffler medium. This occurs due to the formation of round bodies along the bacilli chain, creating a beaded appearance.

Q6: A neonate develops meningitis. CSF culture grows gram-negative bacilli with K1 capsular antigen. The organism is

  1. A. Escherichia coli
  2. B. Listeria monocytogenes
  3. C. Haemophilus influenzae
  4. D. Neisseria meningitidis

Correct answer: A – Escherichia coli

E. coli with K1 capsular antigen is the most common cause of neonatal meningitis. The K1 polysaccharide capsule helps the organism evade the immature neonatal immune system and cross the blood-brain barrier.

Q7: "Medusa head" colonies on blood agar are produced by

  1. A. Clostridium perfringens
  2. B. Bacillus anthracis
  3. C. Bacillus cereus
  4. D. Corynebacterium diphtheriae

Correct answer: B – Bacillus anthracis

B. anthracis produces characteristic medusa head or curled hair lock colonies due to chaining of bacilli. The colonies have irregular edges with comma-shaped projections resembling the snakes on Medusa's head.

Q8: Which test differentiates Staphylococcus from Streptococcus?

  1. A. Urease test
  2. B. Oxidase test
  3. C. Coagulase test
  4. D. Catalase test
  5. E. while Streptococcus is catalase negativ

Correct answer: D – Catalase test

Staphylococcus is catalase positive (produces bubbles when hydrogen peroxide is added) while Streptococcus is catalase negative. This is the primary test used to differentiate these two genera of gram-positive cocci.

Q9: A patient develops pseudomembranous colitis after prolonged antibiotic therapy. The causative organism is

  1. A. Clostridium botulinum
  2. B. Clostridium difficile
  3. C. Clostridium perfringens
  4. D. Clostridium tetani
  5. E. difficile produces toxins A (enterotoxin) and B (cytotoxin) causing antibiotic-associated pseudomembranous colitis. The condition develops when normal

Correct answer: B – Clostridium difficile

C. difficile produces toxins A (enterotoxin) and B (cytotoxin) causing antibiotic-associated pseudomembranous colitis. The condition develops when normal gut flora is disrupted by antibiotics, allowing C. difficile overgrowth.

Q10: "Chinese letter" or "V and L shaped" arrangement is seen in

  1. A. Bacillus anthracis
  2. B. Listeria monocytogenes
  3. C. Mycobacterium tuberculosis
  4. D. Corynebacterium diphtheriae

Correct answer: D – Corynebacterium diphtheriae

C. diphtheriae shows characteristic angular arrangement resembling Chinese letters or cuneiform writing due to snapping division. The cells remain attached at angles after division, creating V or L shapes.

Q11: A burn patient develops green-blue pus with a fruity odor. The organism is

  1. A. Klebsiella pneumoniae
  2. B. Staphylococcus aureus
  3. C. Pseudomonas aeruginosa
  4. D. Escherichia coli

Correct answer: C – Pseudomonas aeruginosa

Pseudomonas produces pyocyanin (blue-green pigment) and pyoverdine giving green-blue pus with a characteristic grape-like or fruity odor. This organism is a common opportunistic pathogen in burn wounds.

Q12: The CAMP test is positive for

  1. A. Streptococcus pneumoniae
  2. B. Enterococcus faecalis
  3. C. Streptococcus agalactiae
  4. D. Streptococcus pyogenes
  5. E. shows enhanced hemolysis (arrowhead-shaped zon

Correct answer: C – Streptococcus agalactiae

Group B Streptococcus (S. agalactiae) shows enhanced hemolysis (arrowhead-shaped zone) when grown near S. aureus on blood agar. This CAMP (Christie, Atkins, Munch-Petersen) test is used for definitive identification.

Q13: Which organism produces black colonies on bismuth sulfite agar?

  1. A. Vibrio cholerae
  2. B. Escherichia coli
  3. C. Salmonella typhi
  4. D. Shigella dysenteriae

Correct answer: C – Salmonella typhi

S. typhi produces hydrogen sulfide (H2S) which reacts with bismuth sulfite to form black bismuth sulfide, producing characteristic black colonies with metallic sheen. This is used for selective isolation of S. typhi.

Q14: A patient presents with flaccid paralysis and descending weakness. The organism that causes this by blocking acetylcholine release is

  1. A. Clostridium perfringens
  2. B. Corynebacterium diphtheriae
  3. C. Clostridium tetani
  4. D. Clostridium botulinum

Correct answer: D – Clostridium botulinum

C. botulinum toxin blocks acetylcholine release at neuromuscular junctions causing flaccid paralysis with descending pattern (starting from cranial nerves), unlike tetanus which causes spastic paralysis with muscle rigidity.

Q15: "Swarming" motility on blood agar is characteristic of

  1. A. Salmonella typhi
  2. B. Proteus species
  3. C. Klebsiella pneumoniae
  4. D. Escherichia coli

Correct answer: B – Proteus species

Proteus shows swarming motility creating concentric waves or ripples on agar due to highly active flagella and differentiation into elongated swarmer cells. This can be inhibited by increasing agar concentration or adding bile salts.

Q16: A gram-negative coccus that ferments maltose but not glucose is

  1. A. Moraxella catarrhalis
  2. B. None of the above
  3. C. Neisseria meningitidis
  4. D. Neisseria gonorrhoeae
  5. E. N. gonorrhoeae ferments only glucose, while N. meningitidis ferments both glucose and maltos

Correct answer: B – None of the above

This is a trick question - Neisseria species always ferment glucose first before maltose. N. gonorrhoeae ferments only glucose, while N. meningitidis ferments both glucose and maltose. No Neisseria ferments maltose without fermenting glucose.

Q17: "Elek test" is used to detect toxigenicity of

  1. A. Vibrio cholerae
  2. B. Corynebacterium diphtheriae
  3. C. Bacillus anthracis
  4. D. Clostridium tetani
  5. E. diphtheriae strains. Lines of precipitation form between toxin-producing colonies and antitoxin-soaked filter paper strips on agar.

Correct answer: B – Corynebacterium diphtheriae

Elek immunoprecipitation test detects diphtheria toxin production by C. diphtheriae strains. Lines of precipitation form between toxin-producing colonies and antitoxin-soaked filter paper strips on agar.

Q18: Which organism produces "boxcar" shaped gram-positive rods?

  1. A. Listeria monocytogenes
  2. B. Bacillus anthracis
  3. C. Clostridium perfringens
  4. D. Corynebacterium diphtheriae

Correct answer: C – Clostridium perfringens

C. perfringens appears as large, rectangular, boxcar-shaped or brick-shaped gram-positive rods. They are non-motile and characteristically lack spores when grown in tissue (though they form spores in culture).

Q19: A patient develops hemolytic uremic syndrome (HUS) after eating undercooked hamburger. The organism is

  1. A. Shigella dysenteriae
  2. B. Escherichia coli O157:H7
  3. C. Salmonella enteritidis
  4. D. Campylobacter jejuni
  5. E. coli (EHEC), particularly O157:H7, produces Shiga toxin causing hemorrhagic colitis that can progress to HUS with acute renal failure, thrombocytopeni

Correct answer: B – Escherichia coli O157:H7

Enterohemorrhagic E. coli (EHEC), particularly O157:H7, produces Shiga toxin causing hemorrhagic colitis that can progress to HUS with acute renal failure, thrombocytopenia, and microangiopathic hemolytic anemia. Transmitted via contaminated beef.

Q20: "Drumstick" or "tennis racket" appearance of spores is seen in

  1. A. Bacillus anthracis
  2. B. Clostridium tetani
  3. C. Clostridium perfringens
  4. D. Clostridium botulinum
  5. E. tetani has terminal round spores that give the bacillus a characteristic drumstick or tennis racket appearanc

Correct answer: B – Clostridium tetani

C. tetani has terminal round spores that give the bacillus a characteristic drumstick or tennis racket appearance. This terminal spore position distinguishes it from C. botulinum which has subterminal spores.

Q21: The most common cause of acute bacterial meningitis in adults is

  1. A. Listeria monocytogenes
  2. B. Haemophilus influenzae
  3. C. Streptococcus pneumoniae
  4. D. Neisseria meningitidis

Correct answer: C – Streptococcus pneumoniae

S. pneumoniae (pneumococcus) is the leading cause of bacterial meningitis in adults and elderly, accounting for approximately 50% of cases. It also causes pneumonia, otitis media, and sinusitis.

Q22: A stool sample shows gram-negative curved rods with "gull wing" or "S" shape. The organism is

  1. A. Escherichia coli
  2. B. Helicobacter pylori
  3. C. Campylobacter jejuni
  4. D. Vibrio cholerae

Correct answer: C – Campylobacter jejuni

Campylobacter has characteristic S-shaped, gull wing, or spiral morphology and shows rapid darting corkscrew motility. It is a leading cause of bacterial gastroenteritis worldwide, transmitted through contaminated poultry.

Q23: Which organism is urease positive and causes "alkaline tide" in urine?

  1. A. Pseudomonas aeruginosa
  2. B. Proteus mirabilis
  3. C. Escherichia coli
  4. D. Klebsiella pneumoniae
  5. E. . This promotes formation of struvite (magnesium ammonium phosphat

Correct answer: B – Proteus mirabilis

Proteus produces urease enzyme that splits urea to ammonia and carbon dioxide, raising urine pH (alkaline tide). This promotes formation of struvite (magnesium ammonium phosphate) kidney stones and allows organism identification.

Q24: "Nagler reaction" is positive for

  1. A. Clostridium botulinum
  2. B. Clostridium difficile
  3. C. Clostridium perfringens
  4. D. Clostridium tetani
  5. E. production by

Correct answer: C – Clostridium perfringens

Nagler test detects lecithinase (alpha toxin/phospholipase C) production by C. perfringens. The organism causes opacity on one half of egg yolk agar, while the other half (containing antitoxin) remains clear.

Q25: A patient with infective endocarditis has blood culture showing catalase negative, gram-positive cocci resistant to optochin and not lysed by bile. The organism is

  1. A. Enterococcus faecalis
  2. B. Staphylococcus epidermidis
  3. C. Streptococcus pneumoniae
  4. D. Streptococcus pyogenes
  5. E. . Enterococci are common causes of subacute endocarditis and UTIs.

Correct answer: A – Enterococcus faecalis

Enterococcus is optochin resistant, bile insoluble, and catalase negative, distinguishing it from S. pneumoniae (optochin sensitive, bile soluble). Enterococci are common causes of subacute endocarditis and UTIs.

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