A patient with abdominal pain and diarrhea had a stool culture revealing non-lactose-fermenting, Gram-negative rods. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Shigella dysenteriae. Shigella causes dysentery, characterized by bloody diarrhea and abdominal pain. Shigella is a non-lactose-fermenting, Gram-negative rod. B: Salmonella typhi causes typhoid fever, not typically associated with abdominal pain and diarrhea. C: Escherichia coli, a common gut bacteria, can cause diarrhea but typically ferments lactose. D: Proteus vulgaris is not a common cause of gastrointestinal infection and is not typically associated with abdominal pain and diarrhea.

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.

Correct Answer: A

Rationale: The correct answer is A: Bordetella pertussis. This bacterium causes whooping cough by attaching to the respiratory tract and releasing toxins that lead to severe coughing fits. Clostridium tetani causes tetanus, not whooping cough. Haemophilus influenzae can cause respiratory infections but not specifically whooping cough. Staphylococcus aureus is known for causing skin infections and not respiratory diseases like whooping cough. Therefore, the presence of Bordetella pertussis and its specific mechanism of causing whooping cough makes it the correct answer.