A patient with a wound infection had a Gram-positive cocci isolated from the sample. The bacteria were catalase-positive and coagulase-positive. What is the likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus aureus. This is because Staphylococcus aureus is Gram-positive, catalase-positive, and coagulase-positive, which matches the characteristics described in the question. Staphylococcus epidermidis is also Gram-positive but is coagulase-negative. Streptococcus pneumoniae is Gram-positive but is catalase-negative. Enterococcus faecalis is Gram-positive but is catalase-negative and coagulase-negative. Therefore, based on the given information, Staphylococcus aureus is the likely causative agent of the wound infection.

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.

Correct Answer: A

Rationale: The correct answer is A: Neisseria meningitidis. This bacterium fits the description of Gram-negative diplococci that are oxidase-positive and fermentative. Neisseria meningitidis commonly causes pneumonia and meningitis. Choice B: Neisseria gonorrhoeae is also a Gram-negative diplococci but is not fermentative. Choice C: Moraxella catarrhalis is oxidase-positive but not fermentative. Choice D: Haemophilus influenzae is not fermentative and typically appears as pleomorphic rods, not diplococci.