MICROBIOLOGY CASE STUDY FINAL
ASSESSMENT QUESTIONS AND
ANSWERS
1. A 25-year-old male presents with a high fever, severe headache, and a stiff neck. A lumbar
puncture reveals cloudy CSF with high protein, low glucose, and Gram-negative diplococci.
What is the most likely pathogen?
A. Streptococcus pneumoniae
B. Neisseria meningitidis
C. Haemophilus influenzae
D. Listeria monocytogenes
Answer: B
Conceptual Explanation: Neisseria meningitidis is a Gram-negative diplococcus and a
leading cause of bacterial meningitis in young adults.
2. A patient presents with a deep wound from a rusted nail. Several days later, they
experience muscle spasms and ‘lockjaw.’ Which toxin is responsible for these symptoms?
A. Alpha toxin
B. Botulinum toxin
,C. Tetanospasmin
D. Enterotoxin B
Answer: C
Conceptual Explanation: Clostridium tetani produces tetanospasmin, a neurotoxin that
blocks inhibitory neurotransmitters, leading to spastic paralysis.
3. An elderly patient develops diarrhea after a course of broad-spectrum antibiotics. Stool
analysis reveals the presence of Toxin A and Toxin B. Which organism is responsible?
A. Salmonella enterica
B. Campylobacter jejuni
C. Shigella sonnei
D. Clostridioides difficile
Answer: D
Conceptual Explanation: C. difficile causes pseudomembranous colitis following antibiotic
use due to the disruption of normal gut flora.
4. A 30-year-old female presents with dysuria and frequency. The lab reports a Gram-negative
rod that is indole positive and ferments lactose (pink colonies on MacConkey agar). What is
the organism?
A. Proteus mirabilis
B. Klebsiella pneumoniae
, C. Pseudomonas aeruginosa
D. Escherichia coli
Answer: D
Conceptual Explanation: E. coli is the most common cause of UTIs and is a lactose
fermenter that is indole positive.
5. A patient with a history of intravenous drug use presents with high fever and a new heart
murmur. Blood cultures grow Gram-positive cocci in clusters that are catalase and coagulase
positive. What is the diagnosis?
A. Staphylococcus aureus endocarditis
B. Streptococcus pyogenes pharyngitis
C. Staphylococcus epidermidis endocarditis
D. Enterococcus faecalis infection
Answer: A
Conceptual Explanation: Staphylococcus aureus is coagulase-positive and a common
cause of acute endocarditis in IV drug users.
6. A child presents with a ‘honey-colored’ crusted lesion around the mouth. Gram stain shows
Gram-positive cocci in chains. Which organism is most likely?
A. Staphylococcus aureus
B. Streptococcus pyogenes
ASSESSMENT QUESTIONS AND
ANSWERS
1. A 25-year-old male presents with a high fever, severe headache, and a stiff neck. A lumbar
puncture reveals cloudy CSF with high protein, low glucose, and Gram-negative diplococci.
What is the most likely pathogen?
A. Streptococcus pneumoniae
B. Neisseria meningitidis
C. Haemophilus influenzae
D. Listeria monocytogenes
Answer: B
Conceptual Explanation: Neisseria meningitidis is a Gram-negative diplococcus and a
leading cause of bacterial meningitis in young adults.
2. A patient presents with a deep wound from a rusted nail. Several days later, they
experience muscle spasms and ‘lockjaw.’ Which toxin is responsible for these symptoms?
A. Alpha toxin
B. Botulinum toxin
,C. Tetanospasmin
D. Enterotoxin B
Answer: C
Conceptual Explanation: Clostridium tetani produces tetanospasmin, a neurotoxin that
blocks inhibitory neurotransmitters, leading to spastic paralysis.
3. An elderly patient develops diarrhea after a course of broad-spectrum antibiotics. Stool
analysis reveals the presence of Toxin A and Toxin B. Which organism is responsible?
A. Salmonella enterica
B. Campylobacter jejuni
C. Shigella sonnei
D. Clostridioides difficile
Answer: D
Conceptual Explanation: C. difficile causes pseudomembranous colitis following antibiotic
use due to the disruption of normal gut flora.
4. A 30-year-old female presents with dysuria and frequency. The lab reports a Gram-negative
rod that is indole positive and ferments lactose (pink colonies on MacConkey agar). What is
the organism?
A. Proteus mirabilis
B. Klebsiella pneumoniae
, C. Pseudomonas aeruginosa
D. Escherichia coli
Answer: D
Conceptual Explanation: E. coli is the most common cause of UTIs and is a lactose
fermenter that is indole positive.
5. A patient with a history of intravenous drug use presents with high fever and a new heart
murmur. Blood cultures grow Gram-positive cocci in clusters that are catalase and coagulase
positive. What is the diagnosis?
A. Staphylococcus aureus endocarditis
B. Streptococcus pyogenes pharyngitis
C. Staphylococcus epidermidis endocarditis
D. Enterococcus faecalis infection
Answer: A
Conceptual Explanation: Staphylococcus aureus is coagulase-positive and a common
cause of acute endocarditis in IV drug users.
6. A child presents with a ‘honey-colored’ crusted lesion around the mouth. Gram stain shows
Gram-positive cocci in chains. Which organism is most likely?
A. Staphylococcus aureus
B. Streptococcus pyogenes
