Gram staining is used to differentiate bacteria based on:

Correct Answer: C

Rationale: The correct answer is C: cell wall composition. Gram staining differentiates bacteria based on the structure of their cell walls. It classifies bacteria into Gram-positive (thick peptidoglycan layer) and Gram-negative (thin peptidoglycan layer). This differentiation is crucial for identifying bacterial species and guiding treatment choices. Size and shape (choice A) are not specific to Gram staining. Presence of flagella (choice B) is related to bacterial motility, not Gram staining. Genetic material (choice D) is not directly assessed by Gram staining. Overall, the unique composition of the cell wall is the key factor in distinguishing bacteria through Gram staining.

Correct Answer: A

Rationale: The correct answer is A: Clostridium perfringens. This bacterium is a Gram-positive rod that forms chains and produces gas due to its ability to ferment carbohydrates. It is also spore-forming, which helps it survive harsh conditions. Clostridium perfringens is commonly associated with severe tissue infections and gas gangrene. Choice B: Bacillus anthracis is also a spore-forming Gram-positive rod but is not typically associated with gas production or severe tissue infections. Choice C: Clostridium tetani is a spore-forming bacterium that causes tetanus, not severe tissue infections with gas production. Choice D: Pseudomonas aeruginosa is a Gram-negative bacterium and does not fit the description provided in the question.

Correct Answer: A

Rationale: Step 1: Gram-negative rod with safety pin appearance indicates bipolar staining. Step 2: Yersinia pestis is a Gram-negative rod causing bubonic plague. Step 3: Y. pestis has bipolar staining due to its unique F1 capsule. Step 4: The swollen lymph node and bacteremia are consistent with bubonic plague. Step 5: Therefore, the correct answer is A: Yersinia pestis. Summary: - B: Clostridium tetani causes tetanus, not characterized by a safety pin appearance. - C: Brucella melitensis causes brucellosis, not characterized by a safety pin appearance. - D: Pasteurella multocida causes animal bites infections, not characterized by a safety pin appearance.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. The presence of Gram-positive cocci in pairs with a capsule is characteristic of Streptococcus pneumoniae. Step 1: Gram-positive cocci in pairs indicate a Streptococcus species. Step 2: The presence of a capsule suggests S. pneumoniae specifically. Summary: B: Staphylococcus aureus is a Gram-positive cocci but does not typically present in pairs with a capsule. C: Neisseria gonorrhoeae is a Gram-negative diplococcus, not a Gram-positive cocci. D: Klebsiella pneumoniae is a Gram-negative rod, not a cocci.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This bacterium is a common cause of pneumonia and is characterized by being Gram-positive cocci arranged in pairs with a capsule, as described in the question. The presence of a capsule is a key characteristic of Streptococcus pneumoniae, distinguishing it from the other choices. Staphylococcus aureus (B) is a Gram-positive cocci, but it typically forms clusters, not pairs. Klebsiella pneumoniae (C) is a Gram-negative rod, not a Gram-positive cocci. Enterococcus faecalis (D) is a Gram-positive cocci, but it does not typically present with a capsule in the same way Streptococcus pneumoniae does.