A patient with a sore throat and fever had a throat culture revealing Gram-positive cocci in pairs with a capsule. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This bacterium is a Gram-positive cocci that often appear in pairs and can have a capsule. It is a common cause of sore throat and fever, known as streptococcal pharyngitis. Staphylococcus aureus (B) is a Gram-positive cocci but does not typically form pairs. Neisseria meningitidis (C) is a Gram-negative diplococcus and is more associated with meningitis. Klebsiella pneumoniae (D) is a Gram-negative rod and not typically associated with sore throat and fever.

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus epidermidis. The presence of Gram-positive cocci in clusters indicates a staphylococcal species. The fact that the bacteria are catalase-positive and coagulase-negative further narrow it down to Staphylococcus epidermidis, as this species fits all the given characteristics. - Choice B, Staphylococcus aureus, is coagulase-positive. - Choice C, Streptococcus pyogenes, is a streptococcal species, not staphylococcal. - Choice D, Enterococcus faecalis, is a Gram-positive cocci but is catalase-negative and not typically found in clusters.

Correct Answer: A

Rationale: The correct answer is A: Mycobacterium tuberculosis. This bacterium is known for causing tuberculosis, as it specifically infects the lungs and can lead to serious respiratory issues. Mycobacterium leprae (Choice B) causes leprosy, not tuberculosis. Escherichia coli (Choice C) is a common bacteria found in the intestines and is not associated with tuberculosis. Streptococcus pneumoniae (Choice D) causes pneumonia, but not tuberculosis. Therefore, the correct choice is A because Mycobacterium tuberculosis is the specific bacterium responsible for causing tuberculosis.

Correct Answer: B

Rationale: The correct answer is B because Gram-negative bacteria have an outer membrane with lipopolysaccharides, which is a key characteristic used in Gram staining. This outer membrane is responsible for the pink/red color seen when stained with Gram stain. Choice A is incorrect because Gram-negative bacteria have a thin peptidoglycan layer. Choice C is incorrect because Gram-negative bacteria stain pink, not purple. Choice D is incorrect because all bacteria have a cell wall, but Gram-negative bacteria have a different structure compared to Gram-positive bacteria.

Correct Answer: D

Rationale: The correct answer is D: Mycobacterium tuberculosis. This bacterium can be transmitted via droplets in the air through coughing or sneezing. It is spread through respiratory droplets containing the bacteria. Streptococcus pneumoniae (A) is transmitted through direct contact with respiratory secretions, not through droplets in the air. Escherichia coli (B) is commonly transmitted through contaminated food or water, not through the air. Clostridium botulinum (C) is typically transmitted through contaminated food, not via respiratory droplets. Therefore, the correct choice is D as it is the only bacterium listed that can be transmitted via droplets in the air.