MGY377 – Microbiology: Pathogens and Pathogenesis( 200
Multiple-Choice Practice Questions With Answers &
Explanations)
1. Which of the following is the most common cause of community-acquired pneumonia in adults?
A) Klebsiella pneumoniae
B) Mycoplasma pneumoniae
C) Streptococcus pneumoniae
D) Legionella pneumophila
Answer: C
Explanation: Streptococcus pneumoniae (pneumococcus) is the leading cause of community-acquired
bacterial pneumonia. Mycoplasma is common in younger individuals but less frequent overall. Klebsiella
is typical in alcoholics/nosocomial; Legionella causes atypical pneumonia.
2. A Gram-positive coccus in pairs and chains, catalase-negative, alpha-hemolytic on blood agar,
optochin-sensitive. Which organism?
A) Streptococcus pyogenes
B) Staphylococcus aureus
C) Enterococcus faecalis
D) Streptococcus pneumoniae
Answer: D
Explanation: S. pneumoniae is alpha-hemolytic, optochin-sensitive, and catalase-negative. S. pyogenes is
beta-hemolytic, bacitracin-sensitive; S. aureus is catalase-positive; Enterococcus is gamma-hemolytic or
alpha, but optochin-resistant.
3. The Quellung reaction detects:
A) Bacterial motility
B) Capsular swelling
,C) Spore formation
D) Toxin production
Answer: B
Explanation: The Quellung reaction uses specific antisera to visualize the bacterial capsule under a
microscope; it is used to serotype Streptococcus pneumoniae and other encapsulated bacteria.
4. Which virulence factor of Neisseria meningitidis is most directly associated with septic shock and
purpura fulminans?
A) IgA protease
B) Endotoxin (Lipooligosaccharide, LOS)
C) Polysaccharide capsule
D) Pili
Answer: B
Explanation: LOS (endotoxin) triggers massive cytokine release (TNF-α, IL-1), leading to septic shock, DIC,
and purpura fulminans. Capsule and pili aid colonization/invasion; IgA protease helps evade mucosal
immunity.
5. A child presents with a "barky" cough, inspiratory stridor, and a steeple sign on neck X-ray. Most likely
pathogen?
A) Bordetella pertussis
B) Haemophilus influenzae type b
C) Parainfluenza virus
D) Respiratory syncytial virus
Answer: C
Explanation: Croup (laryngotracheobronchitis) is most often caused by parainfluenza virus (types 1 and
2). B. pertussis causes whooping cough; Hib causes epiglottitis; RSV causes bronchiolitis.
,6. The gram-negative rod that produces a blue-green pigment (pyocyanin) and has a fruity odor is:
A) Escherichia coli
B) Pseudomonas aeruginosa
C) Proteus mirabilis
D) Serratia marcescens
Answer: B
Explanation: P. aeruginosa produces pyocyanin (blue-green), pyoverdin (yellow-green), and a grape-like
odor. It is an opportunistic pathogen in burns, CF, and immunocompromised hosts.
7. Which of the following is a key feature of Mycobacterium tuberculosis that contributes to its acid-fast
staining property?
A) Thin peptidoglycan layer
B) Mycolic acid-rich cell wall
C) Presence of endospores
D) Teichoic acids
Answer: B
Explanation: The high lipid content (mycolic acids, waxes) resists conventional Gram stain and retains
carbolfuchsin after acid-alcohol decolorization (acid-fast).
8. A 25-year-old woman develops urinary frequency and dysuria. Urinalysis shows leukocyte esterase
and nitrites. Most common uropathogen?
A) Klebsiella pneumoniae
B) Proteus mirabilis
C) Staphylococcus saprophyticus
D) Escherichia coli
Answer: D
Explanation: E. coli accounts for ~80% of community-acquired UTIs. S. saprophyticus is second in young
sexually active women. Proteus is associated with struvite stones; Klebsiella is less common.
, 9. Which toxin is responsible for the "rice-water" stool characteristic of cholera?
A) Shiga toxin
B) Cholera toxin (CTX)
C) Tetanospasmin
D) Botulinum toxin
Answer: B
Explanation: Cholera toxin (an AB5 toxin) activates adenylate cyclase in intestinal epithelial cells, causing
massive secretion of fluid and electrolytes, leading to profuse watery diarrhea without inflammatory
cells.
10. A patient presents with bloody diarrhea and hemorrhagic colitis after eating undercooked ground
beef. No fever. Likely pathogen?
A) Salmonella enteritidis
B) Campylobacter jejuni
C) Shigella sonnei
D) Escherichia coli O157:H7
Answer: D
Explanation: E. coli O157:H7 produces Shiga-like toxin, causing hemorrhagic colitis; often from
contaminated beef. It can lead to HUS (hemolytic uremic syndrome).
11. Which organism causes "consumption" of the complement factor C3 via its capsule?
A) Streptococcus pyogenes
B) Streptococcus agalactiae
Multiple-Choice Practice Questions With Answers &
Explanations)
1. Which of the following is the most common cause of community-acquired pneumonia in adults?
A) Klebsiella pneumoniae
B) Mycoplasma pneumoniae
C) Streptococcus pneumoniae
D) Legionella pneumophila
Answer: C
Explanation: Streptococcus pneumoniae (pneumococcus) is the leading cause of community-acquired
bacterial pneumonia. Mycoplasma is common in younger individuals but less frequent overall. Klebsiella
is typical in alcoholics/nosocomial; Legionella causes atypical pneumonia.
2. A Gram-positive coccus in pairs and chains, catalase-negative, alpha-hemolytic on blood agar,
optochin-sensitive. Which organism?
A) Streptococcus pyogenes
B) Staphylococcus aureus
C) Enterococcus faecalis
D) Streptococcus pneumoniae
Answer: D
Explanation: S. pneumoniae is alpha-hemolytic, optochin-sensitive, and catalase-negative. S. pyogenes is
beta-hemolytic, bacitracin-sensitive; S. aureus is catalase-positive; Enterococcus is gamma-hemolytic or
alpha, but optochin-resistant.
3. The Quellung reaction detects:
A) Bacterial motility
B) Capsular swelling
,C) Spore formation
D) Toxin production
Answer: B
Explanation: The Quellung reaction uses specific antisera to visualize the bacterial capsule under a
microscope; it is used to serotype Streptococcus pneumoniae and other encapsulated bacteria.
4. Which virulence factor of Neisseria meningitidis is most directly associated with septic shock and
purpura fulminans?
A) IgA protease
B) Endotoxin (Lipooligosaccharide, LOS)
C) Polysaccharide capsule
D) Pili
Answer: B
Explanation: LOS (endotoxin) triggers massive cytokine release (TNF-α, IL-1), leading to septic shock, DIC,
and purpura fulminans. Capsule and pili aid colonization/invasion; IgA protease helps evade mucosal
immunity.
5. A child presents with a "barky" cough, inspiratory stridor, and a steeple sign on neck X-ray. Most likely
pathogen?
A) Bordetella pertussis
B) Haemophilus influenzae type b
C) Parainfluenza virus
D) Respiratory syncytial virus
Answer: C
Explanation: Croup (laryngotracheobronchitis) is most often caused by parainfluenza virus (types 1 and
2). B. pertussis causes whooping cough; Hib causes epiglottitis; RSV causes bronchiolitis.
,6. The gram-negative rod that produces a blue-green pigment (pyocyanin) and has a fruity odor is:
A) Escherichia coli
B) Pseudomonas aeruginosa
C) Proteus mirabilis
D) Serratia marcescens
Answer: B
Explanation: P. aeruginosa produces pyocyanin (blue-green), pyoverdin (yellow-green), and a grape-like
odor. It is an opportunistic pathogen in burns, CF, and immunocompromised hosts.
7. Which of the following is a key feature of Mycobacterium tuberculosis that contributes to its acid-fast
staining property?
A) Thin peptidoglycan layer
B) Mycolic acid-rich cell wall
C) Presence of endospores
D) Teichoic acids
Answer: B
Explanation: The high lipid content (mycolic acids, waxes) resists conventional Gram stain and retains
carbolfuchsin after acid-alcohol decolorization (acid-fast).
8. A 25-year-old woman develops urinary frequency and dysuria. Urinalysis shows leukocyte esterase
and nitrites. Most common uropathogen?
A) Klebsiella pneumoniae
B) Proteus mirabilis
C) Staphylococcus saprophyticus
D) Escherichia coli
Answer: D
Explanation: E. coli accounts for ~80% of community-acquired UTIs. S. saprophyticus is second in young
sexually active women. Proteus is associated with struvite stones; Klebsiella is less common.
, 9. Which toxin is responsible for the "rice-water" stool characteristic of cholera?
A) Shiga toxin
B) Cholera toxin (CTX)
C) Tetanospasmin
D) Botulinum toxin
Answer: B
Explanation: Cholera toxin (an AB5 toxin) activates adenylate cyclase in intestinal epithelial cells, causing
massive secretion of fluid and electrolytes, leading to profuse watery diarrhea without inflammatory
cells.
10. A patient presents with bloody diarrhea and hemorrhagic colitis after eating undercooked ground
beef. No fever. Likely pathogen?
A) Salmonella enteritidis
B) Campylobacter jejuni
C) Shigella sonnei
D) Escherichia coli O157:H7
Answer: D
Explanation: E. coli O157:H7 produces Shiga-like toxin, causing hemorrhagic colitis; often from
contaminated beef. It can lead to HUS (hemolytic uremic syndrome).
11. Which organism causes "consumption" of the complement factor C3 via its capsule?
A) Streptococcus pyogenes
B) Streptococcus agalactiae
