BIOL 271 MICROBIOLOGY CASE STUDY
ADVANCED QUIZ 5
1. A 25-year-old patient presents with severe ‘rice-water’ stools and rapid dehydration after
traveling. The pathogen identified is Vibrio cholerae. What is the molecular mechanism of the
cholera toxin?
A. Inhibition of protein synthesis via EF-2 ribosylation
B. Cleavage of SNARE proteins to prevent neurotransmitter release
C. Activation of adenylate cyclase through G-protein modification
D. Degradation of host cell membrane phospholipids
Answer: C
Conceptual Explanation: Cholera toxin ADP-ribosylates the Gs alpha subunit, keeping
adenylate cyclase active, which increases cAMP levels and leads to massive chloride and
water secretion.
2. A clinical isolate of Staphylococcus aureus is found to be resistant to methicillin (MRSA).
What genetic element is responsible for this resistance?
A. The mecA gene encoding an altered PBP2a
B. A plasmid-borne beta-lactamase gene
,C. Efflux pumps that remove the drug from the cytoplasm
D. Mutations in the 16S rRNA gene
Answer: A
Conceptual Explanation: MRSA resistance is mediated by the mecA gene, which encodes
PBP2a, a penicillin-binding protein with low affinity for beta-lactam antibiotics.
3. A patient with persistent cough and night sweats has a sputum sample that stains positive
with the Ziehl-Neelsen stain. What component of the cell wall causes this characteristic?
A. Mycolic acids
B. Teichoic acids
C. Thick layer of peptidoglycan
D. Lipopolysaccharide (LPS)
Answer: A
Conceptual Explanation: Mycobacterium species have waxy mycolic acids in their cell
walls, which resist decolorization by acid-alcohol, making them ‘acid-fast’.
4. An infant presents with flaccid paralysis after consuming honey. Clostridium botulinum is
suspected. What is the mechanism of the botulinum toxin?
A. Blocking the release of acetylcholine at the neuromuscular junction
B. Increasing cAMP levels in intestinal cells
C. Blocking the release of inhibitory neurotransmitters (GABA)
, D. Acting as a superantigen to trigger a cytokine storm
Answer: A
Conceptual Explanation: Botulinum toxin cleaves SNARE proteins, preventing the fusion
of synaptic vesicles containing acetylcholine, leading to flaccid paralysis.
5. A puncture wound leads to ‘lockjaw’ and muscle spasms. The causative agent is Clostridium
tetani. How does tetanospasmin differ from botulinum toxin?
A. It affects the peripheral nervous system only
B. It blocks the release of glycine and GABA in the CNS
C. It is an endotoxin rather than an exotoxin
D. It stimulates the release of acetylcholine
Answer: B
Conceptual Explanation: Tetanospasmin travels retrogradely to the CNS and prevents the
release of inhibitory neurotransmitters like GABA and glycine, causing rigid paralysis.
6. A 19-year-old female presents with painful urination and a purulent vaginal discharge.
Gram-negative diplococci are seen inside neutrophils. What is the primary virulence factor
preventing phagocytosis?
A. A thick polysaccharide capsule
B. Fimbriae (pili) for attachment and antigenic variation
C. IgA protease secretion
ADVANCED QUIZ 5
1. A 25-year-old patient presents with severe ‘rice-water’ stools and rapid dehydration after
traveling. The pathogen identified is Vibrio cholerae. What is the molecular mechanism of the
cholera toxin?
A. Inhibition of protein synthesis via EF-2 ribosylation
B. Cleavage of SNARE proteins to prevent neurotransmitter release
C. Activation of adenylate cyclase through G-protein modification
D. Degradation of host cell membrane phospholipids
Answer: C
Conceptual Explanation: Cholera toxin ADP-ribosylates the Gs alpha subunit, keeping
adenylate cyclase active, which increases cAMP levels and leads to massive chloride and
water secretion.
2. A clinical isolate of Staphylococcus aureus is found to be resistant to methicillin (MRSA).
What genetic element is responsible for this resistance?
A. The mecA gene encoding an altered PBP2a
B. A plasmid-borne beta-lactamase gene
,C. Efflux pumps that remove the drug from the cytoplasm
D. Mutations in the 16S rRNA gene
Answer: A
Conceptual Explanation: MRSA resistance is mediated by the mecA gene, which encodes
PBP2a, a penicillin-binding protein with low affinity for beta-lactam antibiotics.
3. A patient with persistent cough and night sweats has a sputum sample that stains positive
with the Ziehl-Neelsen stain. What component of the cell wall causes this characteristic?
A. Mycolic acids
B. Teichoic acids
C. Thick layer of peptidoglycan
D. Lipopolysaccharide (LPS)
Answer: A
Conceptual Explanation: Mycobacterium species have waxy mycolic acids in their cell
walls, which resist decolorization by acid-alcohol, making them ‘acid-fast’.
4. An infant presents with flaccid paralysis after consuming honey. Clostridium botulinum is
suspected. What is the mechanism of the botulinum toxin?
A. Blocking the release of acetylcholine at the neuromuscular junction
B. Increasing cAMP levels in intestinal cells
C. Blocking the release of inhibitory neurotransmitters (GABA)
, D. Acting as a superantigen to trigger a cytokine storm
Answer: A
Conceptual Explanation: Botulinum toxin cleaves SNARE proteins, preventing the fusion
of synaptic vesicles containing acetylcholine, leading to flaccid paralysis.
5. A puncture wound leads to ‘lockjaw’ and muscle spasms. The causative agent is Clostridium
tetani. How does tetanospasmin differ from botulinum toxin?
A. It affects the peripheral nervous system only
B. It blocks the release of glycine and GABA in the CNS
C. It is an endotoxin rather than an exotoxin
D. It stimulates the release of acetylcholine
Answer: B
Conceptual Explanation: Tetanospasmin travels retrogradely to the CNS and prevents the
release of inhibitory neurotransmitters like GABA and glycine, causing rigid paralysis.
6. A 19-year-old female presents with painful urination and a purulent vaginal discharge.
Gram-negative diplococci are seen inside neutrophils. What is the primary virulence factor
preventing phagocytosis?
A. A thick polysaccharide capsule
B. Fimbriae (pili) for attachment and antigenic variation
C. IgA protease secretion
