BIOL 271 MICROBIOLOGY LAB CASE
STUDY QUIZ 6
1. A 45-year-old male presents with high fever, productive cough with rust-colored sputum,
and sharp chest pain. A Gram stain of the sputum reveals Gram-positive, lancet-shaped
diplococci. Which organism is most likely responsible?
A. Klebsiella pneumoniae
B. Mycoplasma pneumoniae
C. Streptococcus pneumoniae
D. Staphylococcus aureus
Answer: C
Conceptual Explanation: Streptococcus pneumoniae is classically associated with rust-
colored sputum and appears as Gram-positive lancet-shaped diplococci.
2. A patient with a history of intravenous drug use presents with a fever and a new heart
murmur. Blood cultures grow Gram-positive cocci in clusters that are catalase-positive and
coagulase-positive. What is the likely diagnosis?
A. Rheumatic fever caused by Streptococcus pyogenes
B. Subacute endocarditis caused by Streptococcus viridans
,C. Acute endocarditis caused by Staphylococcus aureus
D. Septicemia caused by Escherichia coli
Answer: C
Conceptual Explanation: S. aureus is coagulase-positive and a common cause of acute
endocarditis in IV drug users.
3. A 22-year-old college student is admitted with a severe headache, stiff neck, and a
petechial rash. The CSF shows high protein, low glucose, and many neutrophils. Which
virulence factor is most critical for the pathogen to enter the bloodstream from the
nasopharynx?
A. Lipopolysaccharide (LPS)
B. Hyaluronidase
C. Polysaccharide capsule
D. M protein
Answer: C
Conceptual Explanation: The polysaccharide capsule of Neisseria meningitidis is essential
for evading phagocytosis and surviving in the bloodstream.
, 4. After a 10-day course of clindamycin for a dental infection, a patient develops profuse,
foul-smelling diarrhea and abdominal cramping. What is the mechanism of the toxin
responsible for this condition?
A. Inhibition of protein synthesis by targeting the 50S ribosome
B. Disruption of tight junctions and induction of apoptosis in colonic cells
C. Activation of adenylate cyclase increasing cAMP levels
D. Cleavage of SNARE proteins to inhibit neurotransmitter release
Answer: B
Conceptual Explanation: Clostridioides difficile produces Toxin A and Toxin B, which
disrupt the actin cytoskeleton and tight junctions of the intestinal epithelium.
5. A child presents with a ‘bulls-eye’ rash (erythema migrans) after a camping trip in
Connecticut. Which laboratory technique is the gold standard for confirming this diagnosis in
the later stages?
A. Two-tier testing: ELISA followed by Western Blot
B. Darkfield microscopy of blood
C. Gram stain of skin biopsy
D. Acid-fast staining of joint fluid
Answer: A
STUDY QUIZ 6
1. A 45-year-old male presents with high fever, productive cough with rust-colored sputum,
and sharp chest pain. A Gram stain of the sputum reveals Gram-positive, lancet-shaped
diplococci. Which organism is most likely responsible?
A. Klebsiella pneumoniae
B. Mycoplasma pneumoniae
C. Streptococcus pneumoniae
D. Staphylococcus aureus
Answer: C
Conceptual Explanation: Streptococcus pneumoniae is classically associated with rust-
colored sputum and appears as Gram-positive lancet-shaped diplococci.
2. A patient with a history of intravenous drug use presents with a fever and a new heart
murmur. Blood cultures grow Gram-positive cocci in clusters that are catalase-positive and
coagulase-positive. What is the likely diagnosis?
A. Rheumatic fever caused by Streptococcus pyogenes
B. Subacute endocarditis caused by Streptococcus viridans
,C. Acute endocarditis caused by Staphylococcus aureus
D. Septicemia caused by Escherichia coli
Answer: C
Conceptual Explanation: S. aureus is coagulase-positive and a common cause of acute
endocarditis in IV drug users.
3. A 22-year-old college student is admitted with a severe headache, stiff neck, and a
petechial rash. The CSF shows high protein, low glucose, and many neutrophils. Which
virulence factor is most critical for the pathogen to enter the bloodstream from the
nasopharynx?
A. Lipopolysaccharide (LPS)
B. Hyaluronidase
C. Polysaccharide capsule
D. M protein
Answer: C
Conceptual Explanation: The polysaccharide capsule of Neisseria meningitidis is essential
for evading phagocytosis and surviving in the bloodstream.
, 4. After a 10-day course of clindamycin for a dental infection, a patient develops profuse,
foul-smelling diarrhea and abdominal cramping. What is the mechanism of the toxin
responsible for this condition?
A. Inhibition of protein synthesis by targeting the 50S ribosome
B. Disruption of tight junctions and induction of apoptosis in colonic cells
C. Activation of adenylate cyclase increasing cAMP levels
D. Cleavage of SNARE proteins to inhibit neurotransmitter release
Answer: B
Conceptual Explanation: Clostridioides difficile produces Toxin A and Toxin B, which
disrupt the actin cytoskeleton and tight junctions of the intestinal epithelium.
5. A child presents with a ‘bulls-eye’ rash (erythema migrans) after a camping trip in
Connecticut. Which laboratory technique is the gold standard for confirming this diagnosis in
the later stages?
A. Two-tier testing: ELISA followed by Western Blot
B. Darkfield microscopy of blood
C. Gram stain of skin biopsy
D. Acid-fast staining of joint fluid
Answer: A
