A patient with severe abdominal pain and vomiting had a blood culture that revealed Gram-negative rods producing gas. What is the most likely causative agent?

Correct Answer: B

Rationale: The correct answer is B: Salmonella enteritidis. In this case, the presence of Gram-negative rods producing gas in the blood culture indicates a likelihood of Salmonella infection, as it is a common cause of severe abdominal pain, vomiting, and gas production. Salmonella enteritidis is a known pathogen that can cause gastroenteritis and systemic infections. Escherichia coli (choice A) can also cause similar symptoms, but it is less likely to produce gas in blood culture. Clostridium perfringens (choice C) typically causes food poisoning with rapid onset and less likely to be isolated from blood cultures. Campylobacter jejuni (choice D) is associated with gastroenteritis but is less likely to cause systemic infections with gas-producing Gram-negative rods in blood culture.

Correct Answer: C

Rationale: Rationale: 1. Vibrio cholerae produces cholera toxin causing severe diarrhea. 2. Clostridium tetani causes tetanus, not diarrhea. 3. Clostridium botulinum causes botulism, not diarrhea. 4. Shigella dysenteriae causes dysentery, not typical diarrhea. Summary: A: Incorrect, causes tetanus. B: Incorrect, causes botulism. C: Correct, produces cholera toxin causing severe diarrhea. D: Incorrect, causes dysentery.

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.