MEDICAL BACTERIOLOGY (MBMM3311) - MCQs – 29 MCQs | Kenya MBChB

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

This MCQ set contains 29 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: 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.

Q3: 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.

Q4: 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.

Q5: 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.

Q6: "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.

Q7: "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.

Q8: 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.

Q9: 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.

Q10: 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.

Q11: "Quellung reaction" is used to identify capsule of

  1. A. Bacillus anthracis
  2. B. All of the above
  3. C. Klebsiella pneumoniae
  4. D. Streptococcus pneumoniae
  5. E. The capsule appears to swell under microscopy, allowing serotype identification.

Correct answer: B – All of the above

Quellung reaction (capsular swelling test) can identify capsules of any encapsulated bacteria when mixed with specific anticapsular antisera. The capsule appears to swell under microscopy, allowing serotype identification.

Q12: A comma-shaped gram-negative rod showing "shooting star" motility in hanging drop is

  1. A. Pseudomonas aeruginosa
  2. B. Helicobacter pylori
  3. C. Vibrio cholerae
  4. D. Campylobacter jejuni

Correct answer: C – Vibrio cholerae

V. cholerae shows characteristic rapid darting or shooting star motility in hanging drop preparation due to its single polar flagellum. This distinctive motility pattern aids in presumptive identification.

Q13: "Widal test" detects antibodies against

  1. A. Escherichia coli
  2. B. Vibrio cholerae
  3. C. Shigella dysenteriae
  4. D. Salmonella typhi
  5. E. and H (flagellar) antigens of Salmonella typhi in typhoid fever. A four-fold rise in titer is significant, though the test has limitations in endemic

Correct answer: D – Salmonella typhi

Widal test detects agglutinating antibodies against O (somatic) and H (flagellar) antigens of Salmonella typhi in typhoid fever. A four-fold rise in titer is significant, though the test has limitations in endemic areas.

Q14: Which organism produces "currant jelly" sputum?

  1. A. Staphylococcus aureus
  2. B. Klebsiella pneumoniae
  3. C. Mycobacterium tuberculosis
  4. D. Streptococcus pneumoniae
  5. E. This destructive pneumonia often affects alcoholics and diabetics.

Correct answer: B – Klebsiella pneumoniae

Klebsiella pneumonia produces thick, bloody, mucoid sputum resembling currant jelly due to extensive tissue necrosis and the organism's large polysaccharide capsule. This destructive pneumonia often affects alcoholics and diabetics.

Q15: "McFadyean reaction" showing pink-staining capsule is diagnostic for

  1. A. Clostridium perfringens
  2. B. Klebsiella pneumoniae
  3. C. Bacillus anthracis
  4. D. Streptococcus pneumoniae
  5. E. anthracis bacilli in blood or tissue smears. This is used for rapid diagnosis of anthrax in animals.

Correct answer: C – Bacillus anthracis

McFadyean polychrome methylene blue staining shows characteristic pink/purple capsule around blue B. anthracis bacilli in blood or tissue smears. This is used for rapid diagnosis of anthrax in animals.

Q16: Which organism shows "tumbling motility" at 25°C but not at 37°C?

  1. A. Proteus mirabilis
  2. B. Escherichia coli
  3. C. Listeria monocytogenes
  4. D. Salmonella typhi
  5. E. due to peritrichous flagella expression. Motility is reduced or absent at 37°C, which helps distinguish it from other motile gram-positive rods.

Correct answer: C – Listeria monocytogenes

Listeria exhibits characteristic tumbling or end-over-end motility at room temperature (20-25°C) due to peritrichous flagella expression. Motility is reduced or absent at 37°C, which helps distinguish it from other motile gram-positive rods.

Q17: "Satellite phenomenon" on blood agar indicates

  1. A. Bacterial inhibition
  2. B. Synergistic bacterial growth
  3. C. Streptococcus enhancing hemolysis
  4. D. Staphylococcus aureus providing factors for Haemophilus
  5. E. through hemolysis and heating of blood, while Haemophilus requires both X (hemin) and V factors.

Correct answer: D – Staphylococcus aureus providing factors for Haemophilus

Haemophilus influenzae grows as small satellite colonies around S. aureus colonies on blood agar. S. aureus provides V factor (NAD) through hemolysis and heating of blood, while Haemophilus requires both X (hemin) and V factors.

Q18: A patient with facial nerve palsy, meningitis, and rash has CSF showing gram-negative diplococci. The organism is

  1. A. Streptococcus pneumoniae
  2. B. Haemophilus influenzae
  3. C. Neisseria meningitidis
  4. D. Neisseria gonorrhoeae
  5. E. The organism appears as gram-negative kidney bean-shaped diplococci in CSF.

Correct answer: C – Neisseria meningitidis

N. meningitidis causes meningococcemia with petechial/purpuric rash, meningitis, and can affect cranial nerves including the facial nerve. The organism appears as gram-negative kidney bean-shaped diplococci in CSF.

Q19: A diabetic patient develops malignant external otitis. The organism is

  1. A. Escherichia coli
  2. B. Staphylococcus aureus
  3. C. Proteus mirabilis
  4. D. Pseudomonas aeruginosa

Correct answer: D – Pseudomonas aeruginosa

P. aeruginosa causes severe, invasive external otitis in diabetic and elderly immunocompromised patients. The infection extends from ear canal to temporal bone (osteomyelitis) and can involve cranial nerves with high morbidity.

Q20: "Leonine facies" (lion-like face) is associated with

  1. A. Actinomycosis
  2. B. Tuberculoid leprosy
  3. C. Lepromatous leprosy
  4. D. Diphtheria

Correct answer: C – Lepromatous leprosy

Lepromatous leprosy causes progressive facial skin thickening, nodule formation, and loss of eyebrows creating a characteristic lion-like appearance (leonine facies). This is caused by Mycobacterium leprae, though the question relates to bacterial morphology concepts.

Q21: A food handler with boils transmits food poisoning with rapid onset vomiting. The organism is

  1. A. Bacillus cereus
  2. B. Clostridium perfringens
  3. C. Staphylococcus aureus
  4. D. Salmonella enteritidis

Correct answer: C – Staphylococcus aureus

S. aureus produces heat-stable enterotoxin in contaminated food causing rapid onset (2-6 hours) food poisoning characterized by severe nausea, vomiting, and abdominal cramps. The toxin is preformed and not destroyed by reheating.

Q22: "Kanagawa phenomenon" (beta-hemolysis on Wagatsuma agar) is positive for pathogenic strains of

  1. A. Shigella sonnei
  2. B. Vibrio cholerae
  3. C. Vibrio parahaemolyticus
  4. D. Campylobacter jejuni

Correct answer: C – Vibrio parahaemolyticus

Kanagawa test differentiates pathogenic (hemolytic, toxin-producing) from non-pathogenic V. parahaemolyticus strains. Pathogenic strains produce thermostable direct hemolysin (TDH) causing beta-hemolysis on special Wagatsuma blood agar.

Q23: Which organism ferments lactose slowly (late lactose fermenter)?

  1. A. Citrobacter freundii
  2. B. Klebsiella pneumoniae
  3. C. Shigella sonnei
  4. D. Escherichia coli
  5. E. coli which ferments lactose rapidly.

Correct answer: A – Citrobacter freundii

Citrobacter is a late lactose fermenter, taking more than 24 hours to ferment lactose on MacConkey agar. It initially appears as colorless (non-lactose fermenting) colonies that gradually turn pink, unlike E. coli which ferments lactose rapidly.

Q24: A patient with chronic gastritis and peptic ulcer has a urease-positive curved rod. The organism is

  1. A. Proteus mirabilis
  2. B. Vibrio cholerae
  3. C. Helicobacter pylori
  4. D. Campylobacter jejuni
  5. E. It causes chronic gastritis, peptic ulcers, and is associated with gastric cancer.

Correct answer: C – Helicobacter pylori

H. pylori produces abundant urease enzyme creating an alkaline microenvironment that neutralizes gastric acid, allowing colonization of gastric mucosa. It causes chronic gastritis, peptic ulcers, and is associated with gastric cancer.

Q25: "Strawberry tongue" and "sandpaper rash" are features of infection with

  1. A. Corynebacterium diphtheriae
  2. B. Streptococcus pyogenes
  3. C. Streptococcus pneumoniae
  4. D. Staphylococcus aureus
  5. E. , circumoral pallor, and fine sandpaper-like rash that blanches on pressure.

Correct answer: B – Streptococcus pyogenes

Scarlet fever, caused by erythrogenic toxin-producing Group A Streptococcus (S. pyogenes), presents with strawberry tongue (red with prominent papillae), circumoral pallor, and fine sandpaper-like rash that blanches on pressure.

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