MICROBIOLOGY MCQS EXAM |ACTUAL
QUESTIONS AND ANSWERS|GRADED A+
SECTION 1: BACTERIOLOGY – GRAM-POSITIVE BACTERIA
1. Which of the following is the most common cause of community-acquired
pneumonia?
• A) Streptococcus pneumoniae
• B) Staphylococcus aureus
• C) Haemophilus influenzae
• D) Mycoplasma pneumoniae
Correct Answer: A
Rationale: Streptococcus pneumoniae (pneumococcus) is the most common cause
of community-acquired pneumonia (CAP). It is a Gram-positive, lancet-shaped
diplococcus. It is encapsulated (polysaccharide capsule), which is the major
virulence factor. The capsule prevents phagocytosis.
2. Which of the following is the most common cause of bacterial meningitis in
adults?
• A) Neisseria meningitidis
• B) Streptococcus pneumoniae
• C) Listeria monocytogenes
• D) Haemophilus influenzae
,Correct Answer: B
Rationale: Streptococcus pneumoniae is the most common cause of bacterial
meningitis in adults (and children > 2 months). Neisseria meningitidis is the second
most common cause. Listeria is more common in elderly, immunocompromised,
and neonates.
3. Which of the following bacteria is catalase-positive?
• A) Streptococcus pneumoniae
• B) Streptococcus pyogenes
• C) Staphylococcus aureus
• D) Streptococcus agalactiae
Correct Answer: C
Rationale: Staphylococcus aureus is catalase-positive (produces catalase enzyme).
Streptococci are catalase-negative. Catalase test is used to differentiate
staphylococci (positive) from streptococci (negative). Staph aureus is also
coagulase-positive.
4. Which of the following is the most common cause of skin and soft tissue
infections (cellulitis)?
• A) Staphylococcus aureus
• B) Streptococcus pyogenes
• C) Pseudomonas aeruginosa
• D) Escherichia coli
Correct Answer: B
Rationale: Streptococcus pyogenes (Group A Streptococcus) is the most common
cause of cellulitis (and erysipelas). It is a Gram-positive coccus in chains. It
,produces streptolysins and pyrogenic exotoxins. It is also the cause of impetigo,
pharyngitis, and necrotizing fasciitis.
5. Which of the following bacteria is the most common cause of pharyngitis?
• A) Streptococcus pneumoniae
• B) Streptococcus pyogenes
• C) Staphylococcus aureus
• D) Haemophilus influenzae
Correct Answer: B
Rationale: Streptococcus pyogenes (Group A Strep) is the most common bacterial
cause of pharyngitis (strep throat). It is transmitted by respiratory droplets.
Untreated infection can lead to rheumatic fever (and glomerulonephritis).
Diagnosis is by rapid antigen detection test (RADT) and throat culture.
6. Which of the following is the most common cause of neonatal sepsis and
meningitis?
• A) Streptococcus agalactiae (Group B Strep)
• B) Streptococcus pneumoniae
• C) Listeria monocytogenes
• D) Escherichia coli
Correct Answer: A
Rationale: Streptococcus agalactiae (Group B Streptococcus, GBS) is the most
common cause of neonatal sepsis and meningitis. It colonizes the vagina and is
transmitted to the neonate during delivery. Prophylactic antibiotics (penicillin) are
given during labor to GBS-positive mothers.
, 7. Which of the following bacteria produces a toxin that causes toxic shock
syndrome?
• A) Streptococcus pyogenes
• B) Staphylococcus aureus
• C) Streptococcus agalactiae
• D) Enterococcus faecalis
Correct Answer: B
*Rationale: Staphylococcus aureus produces toxic shock syndrome toxin-1 (TSST-
1), which causes toxic shock syndrome (TSS). TSS is characterized by fever,
hypotension, rash (desquamation), and multiorgan failure. It is associated with
tampon use and wound infections.*
8. Which of the following is a feature of methicillin-resistant Staphylococcus
aureus (MRSA)?
• A) Resistance to beta-lactam antibiotics (penicillins, cephalosporins)
• B) mecA gene (encodes altered penicillin-binding protein PBP2a)
• C) Nosocomial (HA-MRSA) and community-acquired (CA-MRSA) strains
• D) All of the above
Correct Answer: D
*Rationale: MRSA is resistant to beta-lactam antibiotics due to the mecA gene
(which encodes PBP2a, a modified penicillin-binding protein). HA-MRSA is
associated with healthcare settings; CA-MRSA is associated with skin and soft
tissue infections in healthy individuals. Treatment includes vancomycin, linezolid,
or daptomycin.*
9. Which of the following is the most common cause of infective endocarditis?
QUESTIONS AND ANSWERS|GRADED A+
SECTION 1: BACTERIOLOGY – GRAM-POSITIVE BACTERIA
1. Which of the following is the most common cause of community-acquired
pneumonia?
• A) Streptococcus pneumoniae
• B) Staphylococcus aureus
• C) Haemophilus influenzae
• D) Mycoplasma pneumoniae
Correct Answer: A
Rationale: Streptococcus pneumoniae (pneumococcus) is the most common cause
of community-acquired pneumonia (CAP). It is a Gram-positive, lancet-shaped
diplococcus. It is encapsulated (polysaccharide capsule), which is the major
virulence factor. The capsule prevents phagocytosis.
2. Which of the following is the most common cause of bacterial meningitis in
adults?
• A) Neisseria meningitidis
• B) Streptococcus pneumoniae
• C) Listeria monocytogenes
• D) Haemophilus influenzae
,Correct Answer: B
Rationale: Streptococcus pneumoniae is the most common cause of bacterial
meningitis in adults (and children > 2 months). Neisseria meningitidis is the second
most common cause. Listeria is more common in elderly, immunocompromised,
and neonates.
3. Which of the following bacteria is catalase-positive?
• A) Streptococcus pneumoniae
• B) Streptococcus pyogenes
• C) Staphylococcus aureus
• D) Streptococcus agalactiae
Correct Answer: C
Rationale: Staphylococcus aureus is catalase-positive (produces catalase enzyme).
Streptococci are catalase-negative. Catalase test is used to differentiate
staphylococci (positive) from streptococci (negative). Staph aureus is also
coagulase-positive.
4. Which of the following is the most common cause of skin and soft tissue
infections (cellulitis)?
• A) Staphylococcus aureus
• B) Streptococcus pyogenes
• C) Pseudomonas aeruginosa
• D) Escherichia coli
Correct Answer: B
Rationale: Streptococcus pyogenes (Group A Streptococcus) is the most common
cause of cellulitis (and erysipelas). It is a Gram-positive coccus in chains. It
,produces streptolysins and pyrogenic exotoxins. It is also the cause of impetigo,
pharyngitis, and necrotizing fasciitis.
5. Which of the following bacteria is the most common cause of pharyngitis?
• A) Streptococcus pneumoniae
• B) Streptococcus pyogenes
• C) Staphylococcus aureus
• D) Haemophilus influenzae
Correct Answer: B
Rationale: Streptococcus pyogenes (Group A Strep) is the most common bacterial
cause of pharyngitis (strep throat). It is transmitted by respiratory droplets.
Untreated infection can lead to rheumatic fever (and glomerulonephritis).
Diagnosis is by rapid antigen detection test (RADT) and throat culture.
6. Which of the following is the most common cause of neonatal sepsis and
meningitis?
• A) Streptococcus agalactiae (Group B Strep)
• B) Streptococcus pneumoniae
• C) Listeria monocytogenes
• D) Escherichia coli
Correct Answer: A
Rationale: Streptococcus agalactiae (Group B Streptococcus, GBS) is the most
common cause of neonatal sepsis and meningitis. It colonizes the vagina and is
transmitted to the neonate during delivery. Prophylactic antibiotics (penicillin) are
given during labor to GBS-positive mothers.
, 7. Which of the following bacteria produces a toxin that causes toxic shock
syndrome?
• A) Streptococcus pyogenes
• B) Staphylococcus aureus
• C) Streptococcus agalactiae
• D) Enterococcus faecalis
Correct Answer: B
*Rationale: Staphylococcus aureus produces toxic shock syndrome toxin-1 (TSST-
1), which causes toxic shock syndrome (TSS). TSS is characterized by fever,
hypotension, rash (desquamation), and multiorgan failure. It is associated with
tampon use and wound infections.*
8. Which of the following is a feature of methicillin-resistant Staphylococcus
aureus (MRSA)?
• A) Resistance to beta-lactam antibiotics (penicillins, cephalosporins)
• B) mecA gene (encodes altered penicillin-binding protein PBP2a)
• C) Nosocomial (HA-MRSA) and community-acquired (CA-MRSA) strains
• D) All of the above
Correct Answer: D
*Rationale: MRSA is resistant to beta-lactam antibiotics due to the mecA gene
(which encodes PBP2a, a modified penicillin-binding protein). HA-MRSA is
associated with healthcare settings; CA-MRSA is associated with skin and soft
tissue infections in healthy individuals. Treatment includes vancomycin, linezolid,
or daptomycin.*
9. Which of the following is the most common cause of infective endocarditis?
