MICROBIOLOGY WITH LAB CASE
STUDY QUIZ 2 QUESTIONS AND
ANSWERS
1. A 24-year-old female presents with sudden high fever, hypotension, and a diffuse
‘sunburn-like’ rash that later peels. She mentions using high-absorbency tampons. Laboratory
results show elevation in ALT and AST. What is the primary mechanism of the toxin involved?
A. Activation of the adenylate cyclase pathway
B. Cleavage of SNARE proteins in motor neurons
C. Inhibition of protein synthesis via EF-2 inactivation
D. Binding to MHC II and T-cell receptors leading to a cytokine storm
Answer: D
Conceptual Explanation: The case describes Toxic Shock Syndrome caused by S. aureus
TSST-1, which acts as a superantigen, bridging MHC II and TCRs non-specifically to trigger
massive cytokine release.
2. A 65-year-old patient with a history of heart valve replacement presents with subacute
endocarditis. Blood cultures grow Gram-positive cocci that are catalase-negative and
optochin-resistant. Which of the following is the most likely pathogen?
A. Streptococcus pneumoniae
,B. Viridans group streptococci
C. Streptococcus pyogenes
D. Staphylococcus epidermidis
Answer: B
Conceptual Explanation: Viridans streptococci (like S. mutans) are alpha-hemolytic
(catalase-negative) and optochin-resistant, commonly causing subacute endocarditis in
damaged valves.
3. A neonate presents with irritability, poor feeding, and a bulging fontanelle. CSF analysis
shows high protein, low glucose, and Gram-positive bacilli with ‘tumbling motility’ at 25°C.
What was the likely source of infection?
A. Transmission during passage through the birth canal
B. Inhalation of respiratory droplets
C. Contaminated deli meats or unpasteurized cheese
D. Fecal-oral transmission from a pet turtle
Answer: C
Conceptual Explanation: Listeria monocytogenes is a Gram-positive rod with tumbling
motility at lower temperatures, often transmitted via contaminated food (dairy/meats) and
causing neonatal meningitis.
, 4. A 40-year-old intravenous drug user presents with fever and a new heart murmur. Blood
cultures are positive for Gram-positive cocci that are catalase-positive and coagulase-positive.
Which media is selective and differential for this organism?
A. MacConkey Agar
B. Thayer-Martin Agar
C. Mannitol Salt Agar
D. Hektoen Enteric Agar
Answer: C
Conceptual Explanation: Staphylococcus aureus (catalase+, coagulase+) grows on
Mannitol Salt Agar, fermenting mannitol to turn the phenol red indicator yellow.
5. A patient presents with a painless ulcer (chancre) on the genitals. Dark-field microscopy
reveals spiral-shaped organisms. What is the drug of choice for this condition?
A. Ciprofloxacin
B. Metronidazole
C. Azithromycin
D. Penicillin G
Answer: D
Conceptual Explanation: The symptoms describe primary syphilis caused by Treponema
pallidum. Penicillin G remains the gold standard treatment.
STUDY QUIZ 2 QUESTIONS AND
ANSWERS
1. A 24-year-old female presents with sudden high fever, hypotension, and a diffuse
‘sunburn-like’ rash that later peels. She mentions using high-absorbency tampons. Laboratory
results show elevation in ALT and AST. What is the primary mechanism of the toxin involved?
A. Activation of the adenylate cyclase pathway
B. Cleavage of SNARE proteins in motor neurons
C. Inhibition of protein synthesis via EF-2 inactivation
D. Binding to MHC II and T-cell receptors leading to a cytokine storm
Answer: D
Conceptual Explanation: The case describes Toxic Shock Syndrome caused by S. aureus
TSST-1, which acts as a superantigen, bridging MHC II and TCRs non-specifically to trigger
massive cytokine release.
2. A 65-year-old patient with a history of heart valve replacement presents with subacute
endocarditis. Blood cultures grow Gram-positive cocci that are catalase-negative and
optochin-resistant. Which of the following is the most likely pathogen?
A. Streptococcus pneumoniae
,B. Viridans group streptococci
C. Streptococcus pyogenes
D. Staphylococcus epidermidis
Answer: B
Conceptual Explanation: Viridans streptococci (like S. mutans) are alpha-hemolytic
(catalase-negative) and optochin-resistant, commonly causing subacute endocarditis in
damaged valves.
3. A neonate presents with irritability, poor feeding, and a bulging fontanelle. CSF analysis
shows high protein, low glucose, and Gram-positive bacilli with ‘tumbling motility’ at 25°C.
What was the likely source of infection?
A. Transmission during passage through the birth canal
B. Inhalation of respiratory droplets
C. Contaminated deli meats or unpasteurized cheese
D. Fecal-oral transmission from a pet turtle
Answer: C
Conceptual Explanation: Listeria monocytogenes is a Gram-positive rod with tumbling
motility at lower temperatures, often transmitted via contaminated food (dairy/meats) and
causing neonatal meningitis.
, 4. A 40-year-old intravenous drug user presents with fever and a new heart murmur. Blood
cultures are positive for Gram-positive cocci that are catalase-positive and coagulase-positive.
Which media is selective and differential for this organism?
A. MacConkey Agar
B. Thayer-Martin Agar
C. Mannitol Salt Agar
D. Hektoen Enteric Agar
Answer: C
Conceptual Explanation: Staphylococcus aureus (catalase+, coagulase+) grows on
Mannitol Salt Agar, fermenting mannitol to turn the phenol red indicator yellow.
5. A patient presents with a painless ulcer (chancre) on the genitals. Dark-field microscopy
reveals spiral-shaped organisms. What is the drug of choice for this condition?
A. Ciprofloxacin
B. Metronidazole
C. Azithromycin
D. Penicillin G
Answer: D
Conceptual Explanation: The symptoms describe primary syphilis caused by Treponema
pallidum. Penicillin G remains the gold standard treatment.
