Question 1

Which structural feature of Staphylococcus aureus allows it to evade phagocytosis by binding to immunoglobulin G (IgG) and inhibiting opsonization and the complement cascade?

Accepted Answer

Protein A

Question 2

A patient presents with a skin infection characterized by pus-filled lesions on exposed skin. Laboratory tests reveal Gram-positive cocci in chains. Which organism is most likely responsible, and what is the name of this condition?

Accepted Answer

Streptococcus pyogenes; pyoderma

Question 3

Which enzyme produced by Staphylococcus aureus allows it to grow on human skin and in oil glands by breaking down lipids?

Accepted Answer

Lipase

Question 4

A newborn develops bacteremia and meningitis shortly after birth. The mother was not treated with prophylactic antibiotics during delivery. Which organism is most likely responsible for the infection?

Accepted Answer

Streptococcus agalactiae (Group B)

Question 5

Which toxin produced by Staphylococcus aureus is responsible for the symptoms of food poisoning, such as vomiting and diarrhea?

Accepted Answer

Enterotoxin

Question 6

A patient with a history of broad-spectrum antibiotic use develops severe diarrhea and pseudomembranous colitis. Which organism is most likely responsible, and what is the primary virulence factor?

Accepted Answer

Clostridium difficile; production of toxins

Question 7

Which Gram-positive bacillus is associated with the formation of an eschar in cutaneous infections and is a potential bioterrorism agent?

Accepted Answer

Bacillus anthracis

Question 8

Which of the following is a key difference between Streptococcus pneumoniae and Enterococcus faecalis in laboratory diagnosis?

Accepted Answer

S. pneumoniae forms unpigmented, alpha-hemolytic colonies and lacks Lancefield antigens, while E. faecalis is distinguished by sensitivity to bile

Question 9

A patient presents with muscle spasms and lockjaw after sustaining a puncture wound contaminated with soil. Which organism is responsible, and what is the mechanism of its toxin?

Accepted Answer

Clostridium tetani; toxin prevents inhibitory neurotransmitter release, causing continuous muscle contraction

Question 10

Which of the following best explains why Mycobacterium tuberculosis is resistant to many antimicrobial drugs and desiccation?

Accepted Answer

It has a waxy cell wall containing mycolic acid

Question 11

A patient with AIDS develops disseminated infection affecting multiple organs after ingesting contaminated water. Which organism is most likely responsible?

Accepted Answer

Mycobacterium avium-intracellulare

Question 12

Which Gram-positive bacterium is a common cause of dental caries and can lead to endocarditis if it enters the bloodstream?

Accepted Answer

Streptococcus viridans group

Question 13

Which of the following is the most effective method for preventing diphtheria in populations?

Accepted Answer

Immunization with diphtheria toxoid vaccine

Question 14

A patient develops pneumonia with unpigmented, alpha-hemolytic colonies on blood agar. Which virulence factor allows the causative organism to evade host immune defenses?

Accepted Answer

Polysaccharide capsule

Question 15

Which of the following equations best represents the calculation of the minimum inhibitory concentration (MIC) for an antibiotic in a laboratory setting?

Accepted Answer

$MIC = \frac{C_{antibiotic}}{V_{medium}}$

Question 16

Which of the following is a distinguishing feature of Mycoplasma pneumoniae compared to other bacteria?

Accepted Answer

It lacks a cell wall and contains sterols in its cytoplasmic membrane

Question 17

A patient presents with a green pigment on blood agar and a history of dental procedures. Which organism is most likely responsible for the infection?

Accepted Answer

Streptococcus viridans group

Question 18

Which of the following best describes the pathogenesis of Listeria monocytogenes in causing meningitis in at-risk populations?

Accepted Answer

It is an intracellular pathogen that grows in phagocytes and uses listeriolysin S to avoid digestion

Question 19

Which of the following is the primary reason why Enterococcus faecalis is difficult to treat in healthcare settings?

Accepted Answer

It is often resistant to multiple antimicrobials

Pathogenic Gram-Positive Bacteria: Structure, Physiology, and Pathogenicity

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