A patient with a wound infection had Gram-positive cocci in clusters isolated from a wound culture. The organism was catalase-positive and coagulase-positive. What is the likely causative agent?

Correct Answer: A

Rationale: 1. Gram-positive cocci in clusters suggest Staphylococcus species. 2. Catalase-positive and coagulase-positive characteristics match with Staphylococcus aureus. 3. Staphylococcus aureus is a common cause of wound infections due to its virulence factors. 4. Other choices are incorrect: B) S. epidermidis is coagulase-negative and a common contaminant, C) S. pyogenes is catalase-negative, D) Enterococcus faecalis is not typically associated with wound infections.

Correct Answer: A

Rationale: The correct answer is A, Staphylococcus aureus (MRSA), because it is known to be resistant to many antibiotics commonly used in hospitals. MRSA poses a significant threat in healthcare settings due to its resistance, making it difficult to treat. Choice B, Escherichia coli, is not typically resistant to antibiotics. Choice C, Salmonella enterica, and Choice D, Vibrio cholerae, are not commonly associated with antibiotic resistance in hospitals. Therefore, A is the correct answer based on its well-documented antibiotic resistance in healthcare settings.

Correct Answer: A

Rationale: The correct answer is A: Yersinia pestis. Y. pestis is a Gram-negative rod that exhibits a 'safety pin' appearance on staining due to bipolar staining. This bacterium is the causative agent of plague, which can present as painful ulcers known as buboes. Y. pestis is transmitted by fleas and can cause outbreaks in humans. Summary: - B: Francisella tularensis causes tularemia, not typically associated with painful ulcers. - C: Brucella abortus causes brucellosis, which does not typically present with painful ulcers. - D: Clostridium botulinum causes botulism, characterized by muscle weakness and paralysis, not painful ulcers.

Correct Answer: B

Rationale: The correct answer is B: Bacillus subtilis. Bacillus subtilis is a Gram-positive bacterium known for forming endospores, which are resistant structures that allow the bacteria to survive harsh conditions like heat, radiation, and desiccation. This ability to form spores helps Bacillus subtilis to endure unfavorable environments. Staphylococcus aureus (A), Pseudomonas aeruginosa (C), and Neisseria gonorrhoeae (D) do not form spores and lack the same level of resistance to extreme conditions as Bacillus subtilis.

Correct Answer: A

Rationale: The correct answer is A: Escherichia coli. Step-by-step rationale: 1. Gram-negative rods: E. coli is a Gram-negative rod bacterium. 2. Fermented lactose: E. coli is known to ferment lactose. 3. Common cause of diarrhea: E. coli is a common causative agent of infectious diarrhea. Summary: B: Salmonella typhi causes typhoid fever, not typically associated with lactose fermentation. C: Shigella dysenteriae causes dysentery, not known for lactose fermentation. D: Proteus vulgaris is not a common cause of infectious diarrhea and does not typically ferment lactose.