Question 1

Which of the following best explains why Staphylococcus aureus is able to cause food intoxication even after food has been cooked?

Accepted Answer

It produces heat-stable enterotoxins that are not destroyed by cooking.

Question 2

A patient presents with a cluster of boils on the back of their neck. The lesions are deep, interconnected, and larger than a typical furuncle. What is the most likely diagnosis?

Accepted Answer

Carbuncle

Question 3

Which laboratory test would you use to distinguish Staphylococcus from Streptococcus species, and what is the expected result for Staphylococcus?

Accepted Answer

Catalase test; Staphylococcus is catalase positive ($2H_2O_2 \rightarrow 2H_2O + O_2$)

Question 4

A hospital outbreak of endocarditis is traced to a coagulase-negative, Gram-positive coccus that is part of the normal skin flora. Which organism is most likely responsible?

Accepted Answer

Staphylococcus epidermidis

Question 5

Which of the following is a major virulence factor of Streptococcus pyogenes that helps it evade phagocytosis?

Accepted Answer

M-protein

Question 6

A child develops a bright red rash and fever after a sore throat. Which toxin is responsible for this presentation, and which organism produces it?

Accepted Answer

Erythrogenic (pyrogenic) toxin produced by Streptococcus pyogenes

Question 7

Which of the following best describes the transmission route and risk group for Neisseria meningitidis infection?

Accepted Answer

Airborne transmission; high risk in closed populations such as college dormitories

Question 8

A patient with a history of rheumatic fever is prescribed long-term prophylaxis. Which drug is most appropriate for this purpose?

Accepted Answer

Penicillin

Question 9

Which of the following statements about MRSA (methicillin-resistant Staphylococcus aureus) is correct?

Accepted Answer

MRSA strains are resistant to all major drug groups except vancomycin.

Question 10

A Gram-positive coccus is isolated from a patient with pneumonia. The organism grows best in 5-10% CO$_2$, is alpha-hemolytic, and is optochin sensitive. What is the most likely organism?

Accepted Answer

Streptococcus pneumoniae

Question 11

Which of the following is a long-term complication of untreated Streptococcus pyogenes pharyngitis?

Accepted Answer

Rheumatic fever

Question 12

A laboratory technician is asked to perform a Quellung test on a sputum sample. What is the purpose of this test?

Accepted Answer

To detect capsular swelling in Streptococcus pneumoniae

Question 13

Which of the following best explains why abscesses caused by Staphylococcus aureus often require surgical intervention?

Accepted Answer

Antibiotics have poor penetration into abscesses due to the fibrous capsule.

Question 14

Which enzyme produced by Streptococcus pyogenes is responsible for digesting fibrin clots and facilitating the spread of infection?

Accepted Answer

Streptokinase

Question 15

A 7-year-old child presents with nephritis and increased blood pressure following a recent episode of strep throat. Which post-infectious complication is most likely?

Accepted Answer

Acute glomerulonephritis

Question 16

Which of the following is a distinguishing feature of Neisseria species compared to Staphylococcus and Streptococcus?

Accepted Answer

Gram-negative, bean-shaped diplococci

Question 17

A patient with a history of chronic ear infections is found to have a Gram-positive, alpha-hemolytic coccus that is optochin sensitive. Which vaccine would be most appropriate for prevention in high-risk individuals?

Accepted Answer

Capsular antigen vaccine for Streptococcus pneumoniae

Question 18

Which of the following best describes the pathogenesis of toxic shock syndrome caused by Staphylococcus aureus?

Accepted Answer

Toxemia due to superantigen toxins leading to shock and organ failure

Question 19

A college student living in a dormitory develops sudden fever, headache, and a petechial rash. Which organism is most likely responsible, and what is the recommended immediate treatment?

Accepted Answer

Neisseria meningitidis; treat with penicillin G or cephalosporin

Question 20

Which of the following is a key reason why Streptococcus pneumoniae cultures die in the presence of oxygen?

Accepted Answer

They lack catalase and peroxidases to detoxify reactive oxygen species.

Gram-Positive and Gram-Negative Cocci of Medical Importance: Staphylococcus, Streptococcus, and Neisseria

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