A Gram-negative, motile, comma-shaped bacterium was isolated from a stool sample of a patient with diarrhea. What is the most likely causative agent?

Correct Answer: B

Rationale: Step 1: Gram-negative, comma-shaped bacterium suggests a curved rod morphology, typical of Campylobacter jejuni. Step 2: Motility is a characteristic of Campylobacter species. Step 3: Campylobacter jejuni is a common cause of bacterial diarrhea in humans. Step 4: Vibrio cholerae causes cholera, not typically associated with comma-shaped morphology. Shigella dysenteriae and Escherichia coli are not comma-shaped.

Correct Answer: D

Rationale: The correct answer is D because all three bacteria listed produce toxins that cause damage to host tissues. Clostridium tetani produces tetanospasmin toxin causing tetanus, Vibrio cholerae produces cholera toxin leading to severe diarrhea, and Staphylococcus aureus releases toxins causing tissue damage in various infections. Therefore, all of the above choices are correct in terms of producing toxins that harm host tissues.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. The given characteristics match this bacterium: Gram-positive diplococci, catalase-negative, and alpha-hemolytic. S. pneumoniae is a common cause of community-acquired pneumonia. Other choices are incorrect because Staphylococcus aureus is catalase-positive, Enterococcus faecalis is not alpha-hemolytic, and Klebsiella pneumoniae is not Gram-positive diplococci.

Correct Answer: A

Rationale: The correct answer is A: Entamoeba histolytica. This is because Entamoeba histolytica is known to cause amebic dysentery, leading to chronic diarrhea. The presence of cysts with four nuclei in the stool sample is characteristic of Entamoeba histolytica. Giardia lamblia (choice B) is known to cause diarrhea but does not present with cysts containing four nuclei. Balantidium coli (choice C) is a ciliated protozoan that can cause diarrhea, but it does not have cysts with four nuclei. Trichomonas hominis (choice D) is a non-pathogenic protozoan commonly found in the human colon and is not associated with causing chronic diarrhea or cysts with four nuclei.

Correct Answer: C

Rationale: The correct answer is C: Salmonella typhi. The patient's symptoms of diarrhea and the characteristics of non-lactose-fermenting, Gram-negative rods that are oxidase-positive and fermentative are consistent with Salmonella typhi. Salmonella typhi is the causative agent of typhoid fever, which presents with symptoms like diarrhea. Pseudomonas aeruginosa (choice A) is oxidase-positive but not fermentative. Escherichia coli (choice B) is both lactose-fermenting and oxidase-negative. Shigella dysenteriae (choice D) is oxidase-negative. Therefore, Salmonella typhi is the most likely causative agent based on the given information.