A patient with an abscess had Gram-positive cocci in clusters isolated from a wound culture. The organism was catalase-positive and coagulase-positive. What is the likely causative agent?

Correct Answer: A

Rationale: Step-by-step rationale: 1. Gram-positive cocci in clusters suggest Staphylococcus or Streptococcus. 2. Catalase-positive and coagulase-positive narrow it down to Staphylococcus. 3. Staphylococcus aureus is catalase-positive and coagulase-positive. Therefore, the likely causative agent is Staphylococcus aureus. Summary: A: Correct - Staphylococcus aureus fits all the characteristics. B: Incorrect - Staphylococcus epidermidis is catalase-negative and coagulase-negative. C: Incorrect - Streptococcus pyogenes is catalase-negative and coagulase-negative. D: Incorrect - Enterococcus faecalis is catalase-negative and coagulase-negative.

Correct Answer: D

Rationale: Interferon is a cell-specific protein that is produced and released from infected cells in response to viral infections. It acts extracellularly to interfere with viral replication, not to inactivate the virus itself. Interferon is not coded for by cellular DNA, but by viral RNA during infection. Therefore, the correct answer, D, is that interferon is not virus-specific but is cell-specific. This is because interferon can be produced by various cell types in response to different types of viral infections, making it a broad-spectrum antiviral defense mechanism.

Correct Answer: D

Rationale: The correct answer is D, as all options A, B, and C are determinants of antigenicity. A: Chemical nature influences how the immune system recognizes antigens. B: The size of the antigen affects its ability to be recognized by immune cells. C: Antigens of foreign origin are more likely to trigger an immune response. Therefore, all of these factors together play a crucial role in determining antigenicity.

Correct Answer: B

Rationale: The correct answer is B: Protection against phagocytosis. The bacterial capsule is a protective layer outside the cell wall that helps bacteria evade phagocytosis by the host immune system. It acts as a physical barrier against immune cells like macrophages and neutrophils, allowing the bacteria to survive and multiply in the host. Energy production (A), DNA replication (C), and protein synthesis (D) are essential cellular functions but are not directly associated with the major function of the bacterial capsule, which is protection against phagocytosis.

Correct Answer: A

Rationale: The correct answer is A: Treponema. Treponema pallidum is a spirochete bacterium that appears as long, thin, and spiral-shaped microorganisms with 8-12 regular coils under the microscope. This characteristic morphology is specific to Treponema species, making it the correct choice. Borrelia, Leptospira, and Vibrios have different shapes and arrangements under the microscope, making them incorrect choices in this scenario.

Correct Answer: A

Rationale: The correct answer is A: Trichomonas vaginalis. Trichomonas vaginalis is a flagellated protozoa that causes the sexually transmitted infection trichomoniasis. The presence of an undulating membrane in vaginal discharge samples is characteristic of Trichomonas vaginalis. Giardia lamblia causes gastrointestinal illness, not vaginal infections. Trypanosoma brucei causes African sleeping sickness transmitted by tsetse flies. Leishmania donovani causes visceral leishmaniasis transmitted by sandflies.

Correct Answer: B

Rationale: The correct answer is B: EBV. Osteomyelitis is primarily caused by bacteria, not viruses like Epstein-Barr virus (EBV). The main pathogens involved in osteomyelitis are typically Staphylococcus aureus, including Methicillin-resistant Staphylococcus aureus (MRSA), Haemophilus influenzae, and Pseudomonas aeruginosa. EBV is not a causative agent of osteomyelitis. It is crucial to differentiate between bacterial and viral etiologies in infectious diseases to guide appropriate treatment.

Correct Answer: B

Rationale: The correct answer is B: Phage typing of the obtained culture. This method is used for typing Mycobacterium tuberculosis strains based on susceptibility to specific bacteriophages. If the culture shows susceptibility, it confirms the presence of M. tuberculosis. In this case, the absence of cord factor indicates that the bacteria are unlikely to be M. tuberculosis. Option A (Inoculation of laboratory animals) is unethical and not necessary when other methods are available. Option C (Allergy skin test) is not relevant for confirming a diagnosis of renal tuberculosis. Option D (Toxigenicity testing) is not specific for confirming or refuting a diagnosis of M. tuberculosis.

Correct Answer: A

Rationale: Step-by-step rationale: 1. Gram-positive cocci in clusters suggest Staphylococcus or Streptococcus. 2. Catalase-positive and coagulase-positive narrow it down to Staphylococcus. 3. Staphylococcus aureus is catalase-positive and coagulase-positive. Therefore, the likely causative agent is Staphylococcus aureus. Summary: A: Correct - Staphylococcus aureus fits all the characteristics. B: Incorrect - Staphylococcus epidermidis is catalase-negative and coagulase-negative. C: Incorrect - Streptococcus pyogenes is catalase-negative and coagulase-negative. D: Incorrect - Enterococcus faecalis is catalase-negative and coagulase-negative.