A patient with a deep tissue infection had Gram-negative rods isolated from the wound culture. The bacteria were oxidase-positive and non-lactose fermenters. What is the most likely causative agent?

Correct Answer: A

Rationale: The correct answer is A: Pseudomonas aeruginosa. This is because Pseudomonas is a Gram-negative rod, oxidase-positive, and non-lactose fermenter. These characteristics are typical of Pseudomonas species. B: Escherichia coli is a Gram-negative rod but is lactose fermenter and usually oxidase-negative. C: Klebsiella pneumoniae is a Gram-negative rod, non-lactose fermenter, but is usually oxidase-negative. D: Shigella dysenteriae is a Gram-negative rod, non-lactose fermenter, but is usually oxidase-negative and is more associated with causing shigellosis rather than deep tissue infections.

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: D

Rationale: The correct answer is D because viruses are sensitive to interferon, a natural defense mechanism of the body against viral infections. Interferon helps to inhibit viral replication. A: Viruses are cell-free infectious particles, not living systems. B: Viruses are obligate intracellular parasites as they require host cells to replicate. C: Viruses can have either DNA or RNA as genetic material, which is a characteristic feature.

Correct Answer: D

Rationale: The correct answer is D: Relapsing fever. The description matches Borrelia species, the causative agent of relapsing fever. The thin, spiral-shaped microorganisms with pointed ends and specific length are characteristic of Borrelia. The number of coils and size mentioned also align with Borrelia morphology. Relapsing fever is transmitted by tick bites and manifests as recurring febrile episodes. Incorrect choices: A: Syphilis - Syphilis is caused by Treponema pallidum, which is not spiral-shaped. B: Trypanosomiasis - Trypanosomiasis is caused by Trypanosoma species, which are not spiral-shaped. C: Leptospirosis - Leptospirosis is caused by Leptospira species, which are not spiral-shaped and do not match the specific description provided.

Correct Answer: C

Rationale: The correct answer is C: Hepatitis A virus. This is because Hepatitis A virus is a non-enveloped virus, while choices A, B, and D are all enveloped viruses. Enveloped viruses have a lipid bilayer membrane surrounding their protein capsid, aiding in their ability to infect host cells. Hepatitis C, B, and D viruses all possess this lipid envelope, making them enveloped viruses. Hepatitis A virus, on the other hand, lacks this lipid envelope and is classified as a non-enveloped virus. Therefore, the correct answer is C.

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: A

Rationale: The correct answer is A: Neisser staining method. Neisser staining method is used to stain diphtheria bacilli, which appear as blue rods with thickenings at the poles. The rationale is that Neisser staining specifically targets and highlights the characteristics of the diphtheria bacilli. Incorrect choices: B: Gram staining is not specific for diphtheria bacilli and does not show the characteristic blue rods with thickenings at the poles. C: Ziehl-Neelsen staining is used for acid-fast bacteria like Mycobacterium tuberculosis, not for diphtheria bacilli. D: Loeffler staining is used for diphtheria bacilli but does not produce blue rods with thickenings at the poles as seen in this case.

Correct Answer: A

Rationale: The correct answer is A: Hyperresponsiveness reaction type IV. This type of reaction, also known as delayed-type hypersensitivity, is characterized by the activation of T lymphocytes and macrophages, leading to inflammation at the site of antigen exposure. In the case of the Mantoux test, the appearance of a papule 48 hours later indicates a delayed hypersensitivity reaction mediated by T cells. Choice B: Reaction of Arthus phenomenon type is incorrect as this type of reaction involves immune complexes forming in the blood vessels leading to tissue damage, which is not the mechanism seen in the Mantoux test reaction. Choice C: Reaction of serum sickness type is incorrect as this type of reaction involves the formation of immune complexes in the blood leading to systemic symptoms like fever, rash, and joint pain, which is not typically seen in the Mantoux test reaction. Choice D: Atopic reaction is incorrect as atopic reactions involve IgE antibodies and mast cell degranulation, which are not the mechanisms

Correct Answer: B

Rationale: The correct answer is B: mitochondria. Stroma is a component of chloroplasts, not mitochondria. Lysosomes contain hydrolytic enzymes, not stroma. Mitochondria have an inner and outer membrane structure, with the matrix containing the stroma where important metabolic processes occur. This is where the Krebs cycle and oxidative phosphorylation take place. Stroma in chloroplasts is where photosynthesis occurs. So, the correct answer is B because stroma is a component of mitochondria, where crucial energy production processes happen.