A fecal sample from a patient with diarrhea revealed 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 the presence of cysts with four nuclei is characteristic of Entamoeba histolytica, a protozoan parasite that causes amoebic dysentery. Entamoeba histolytica cysts contain four nuclei, distinguishing them from other pathogens. Giardia lamblia and Trichomonas vaginalis are not associated with cysts containing four nuclei. Balantidium coli is a ciliated protozoan that does not typically form cysts with multiple nuclei. In summary, the unique feature of four nuclei in the cysts points towards Entamoeba histolytica as the most likely causative agent of the diarrhea in this patient.

Correct Answer: A

Rationale: The correct answer is A: Streptococcus pneumoniae. This bacterium is the most likely causative agent based on the description of Gram-positive cocci in pairs with a capsule. S. pneumoniae is known for its characteristic morphology and the presence of a capsule, which helps in evading the immune system. It commonly causes pneumonia, especially in patients with a productive cough and fever. Incorrect choices: B: Klebsiella pneumoniae typically presents with a mucoid capsule and is associated with pneumonia in immunocompromised individuals. C: Neisseria meningitidis is a Gram-negative diplococcus and is more commonly associated with meningitis and septicemia. D: Staphylococcus aureus is a Gram-positive cocci, but it does not typically present in pairs with a capsule and is more commonly associated with skin and soft tissue infections.

Correct Answer: B

Rationale: The correct answer is B: Enable bacterial adherence to surfaces. Bacterial capsules are a protective layer outside the cell wall that helps bacteria adhere to surfaces and evade host immune responses. This allows bacteria to colonize and infect host tissues. A: Enhancing bacterial motility is typically associated with flagella, not capsules. C: Bacterial DNA replication is not directly affected by capsules. D: Bacterial capsules do play a role in protecting bacteria from desiccation by providing a barrier against drying out. However, the primary function is enabling adherence to surfaces.

Correct Answer: A

Rationale: The correct answer is A: Clostridium botulinum. Clostridium botulinum produces the botulinum toxin, which causes botulism. This bacteria thrives in anaerobic environments and can contaminate improperly canned foods. Clostridium tetani causes tetanus, not botulism. Staphylococcus aureus produces enterotoxins causing food poisoning, not botulism. Escherichia coli can cause food poisoning through various toxins, but not botulism. Therefore, the correct choice is Clostridium botulinum as it specifically produces the botulinum toxin responsible for botulism.

Correct Answer: A

Rationale: Step-by-step rationale: 1. Gram-positive rods with terminal spores suggest Clostridium genus. 2. Terminal spores are characteristic of Clostridium perfringens. 3. C. perfringens is known to cause wound infections. 4. Therefore, the most likely causative agent is Clostridium perfringens. Summary: A: Clostridium perfringens is correct due to its characteristic features and association with wound infections. B: Bacillus anthracis does not typically form terminal spores. C: Clostridium tetani causes tetanus, not wound infections. D: Bacillus cereus does not produce terminal spores like Clostridium perfringens.