A patient with chronic diarrhea had a stool sample revealing cysts with four nuclei. What is the most likely causative agent?

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: D

Rationale: The correct answer is D because all three bacteria listed (Staphylococcus aureus, Escherichia coli, Clostridium botulinum) can produce toxins leading to food poisoning. Staphylococcus aureus produces heat-stable toxins causing rapid onset of symptoms. Escherichia coli can produce toxins like Shiga toxin leading to severe foodborne illness. Clostridium botulinum produces neurotoxins causing botulism. Choice A, B, and C are incorrect because each bacteria can produce toxins that are harmful if ingested.

Correct Answer: A

Rationale: The correct answer is A, Clostridium botulinum. This bacterium produces botulinum toxin, leading to botulism. It is commonly found in soil and improperly canned food. Clostridium tetani causes tetanus, Staphylococcus aureus causes food poisoning, and Salmonella enterica causes salmonellosis. Therefore, A is correct due to its association with botulism, while the other choices are incorrect for causing different diseases.

Correct Answer: D

Rationale: The correct answer is D (All of the above) as all three bacteria (Escherichia coli, Salmonella enterica, Neisseria meningitidis) produce endotoxins causing fever and shock. Endotoxins are lipopolysaccharides found in the outer membrane of Gram-negative bacteria and can trigger an immune response leading to fever and shock. Escherichia coli and Salmonella enterica are common causes of food poisoning and sepsis, while Neisseria meningitidis is responsible for meningococcal meningitis and septicemia. Therefore, all three choices are correct in producing endotoxins that can cause fever and shock.

Correct Answer: B

Rationale: The correct answer is B: Clostridium tetani. Clostridium tetani produces tetanospasmin, a neurotoxin causing tetanus. It enters the body through wounds and affects the nervous system. A: Clostridium botulinum produces botulinum toxin causing botulism, not tetanus. C: Streptococcus pneumoniae causes pneumonia, not tetanus. D: Mycobacterium tuberculosis causes tuberculosis, not tetanus.