Correct Answer: A

Rationale: The correct answer is A: Entamoeba histolytica. This is because it is the only choice that matches the description of cysts with four nuclei, characteristic of Entamoeba histolytica. Entamoeba histolytica is a protozoan parasite that causes amoebiasis, leading to symptoms like persistent diarrhea. The other choices can be eliminated for the following reasons: B: Balantidium coli - It is a ciliated protozoan that causes dysentery in humans but does not produce cysts with four nuclei. C: Giardia lamblia - It is a flagellated protozoan that causes giardiasis, but it forms cysts with only two nuclei. D: Trichomonas hominis - It is a flagellated protozoan found in the human intestine, but it does not produce cysts with four nuclei. Therefore, based on the description of cysts with four nuclei in the stool sample,

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus aureus. This bacterium is characterized by Gram-positive cocci in clusters, catalase-positive, and coagulase-positive. Staphylococcus aureus is a common pathogen causing wound infections due to its ability to produce toxins and enzymes. Staphylococcus epidermidis (B) is catalase-positive but coagulase-negative, making it less likely to cause infections. Streptococcus pyogenes (C) is catalase-negative and typically causes streptococcal infections, not wound infections. Enterococcus faecalis (D) is catalase-negative and typically associated with urinary tract infections. Thus, Staphylococcus aureus is the most likely causative agent based on the given characteristics.

Correct Answer: B

Rationale: The correct answer is B: 3.5 to 4.0 billion years. Prokaryotic fossils date back to around 3.5 to 4.0 billion years ago, based on scientific evidence. This timeframe aligns with the earliest known evidence of life on Earth. Choices A, C, and D have incorrect timelines that do not correspond to the established age range of prokaryotic fossils. Choice A is too broad and includes a range that extends beyond the known fossil record. Choices C and D have much younger timelines, which do not match the historical evidence of prokaryotic life dating back billions of years.

Correct Answer: C

Rationale: The correct answer is C: peroxisomes. Peroxisomes contain enzymes involved in breaking down fatty acids and detoxifying harmful substances. They do not contain digestive enzymes. Lysosomes contain digestive enzymes involved in breaking down waste materials. Chloroplasts are involved in photosynthesis and do not contain digestive enzymes. Mitochondria are involved in energy production and do not contain digestive enzymes. Therefore, the correct answer is peroxisomes.

Correct Answer: A

Rationale: The correct answer is A: Vibrio cholerae. This bacterium is the likely causative agent because it is known to cause severe diarrhea, and it is characterized by its motile, comma-shaped Gram-negative rods. Vibrio cholerae produces cholera toxin, leading to profuse watery diarrhea. Shigella dysenteriae causes dysentery with bloody diarrhea, Salmonella typhi causes typhoid fever, and Escherichia coli can cause various types of diarrhea but typically does not have a comma shape.