Agents associated with human disease but generally not a serious health risk are classified in which of the following risk groups?

Correct Answer: B

Rationale: The correct answer is B: RG2. RG2 is appropriate for agents associated with human disease that pose a moderate health risk. These agents may cause disease of varying severity, but generally do not result in serious health consequences. Other choices are incorrect because: A: RG1 is for agents not associated with disease in healthy adults, C: RG3 is for agents associated with serious or lethal human disease, and D: RG4 is for agents that pose a high risk of life-threatening disease and for which there are no treatments.

Correct Answer: D

Rationale: The correct answer is D because a fourfold increase in titer in the second serum sample indicates a significant rise in specific antibodies, which is crucial for diagnosing viral infections accurately. This increase is considered significant in serologic reactions as it shows a strong antibody response to the virus. A: A threshold increase may not be substantial enough to confirm a diagnostic antibody response. B: Dismissing the increase in titer would overlook important diagnostic information. C: A twofold increase may not be as indicative of a strong antibody response compared to a fourfold increase.

Correct Answer: A

Rationale: The correct answer is A: Complex. Poxviruses have a complex nucleocapsid symmetry due to their unique structure with multiple layers of proteins and genetic material. Isohedral symmetry (B) refers to a regular polyhedron shape, which is not characteristic of poxviruses. Helical symmetry (C) is found in viruses like tobacco mosaic virus, not poxviruses. "All of the above" (D) is incorrect because poxviruses do not exhibit isohedral or helical symmetry.

Correct Answer: B

Rationale: The correct answer is B: Azotobacter species. Azotobacter are known for their ability to fix nitrogen through the process of nitrogen fixation, converting atmospheric nitrogen into a form that plants can use. Bacillus cereus is not known for nitrogen fixation. Clostridium botulinum is a bacterium that causes botulism and does not fix nitrogen. Escherichia coli is a common gut bacterium but does not have the ability to fix nitrogen. Thus, Azotobacter species is the correct choice based on its unique ability to fix nitrogen.

Correct Answer: C

Rationale: Nigrosin is used in negative staining, where the background is stained instead of the cells themselves. This technique helps highlight the morphology and structure of cells that may be damaged by traditional staining methods. Nigrosin is negatively charged and repels the negatively charged bacterial cells, resulting in a clear halo around the cells. This process is particularly useful for observing capsules and determining cell size and shape. The other options are incorrect because simple staining uses a single dye to color the entire cell, gram staining involves a series of dyes to differentiate between gram-positive and gram-negative bacteria, and acid-fast staining is used to detect acid-fast organisms like Mycobacterium.

Correct Answer: B

Rationale: The correct answer is B: RG2. RG2 is appropriate for agents associated with human disease that pose a moderate health risk. These agents may cause disease of varying severity, but generally do not result in serious health consequences. Other choices are incorrect because: A: RG1 is for agents not associated with disease in healthy adults, C: RG3 is for agents associated with serious or lethal human disease, and D: RG4 is for agents that pose a high risk of life-threatening disease and for which there are no treatments.

Correct Answer: C

Rationale: The correct answer is C: gram-negative bacteria have an outer membrane that acts as an extra barrier. This outer membrane in gram-negative bacteria contains lipopolysaccharides and porins, which restrict the entry of antibiotics like penicillin. The outer membrane acts as an additional layer of protection, making it harder for penicillin to penetrate and reach its target, the peptidoglycan layer. Choices A, B, and D do not directly address the specific mechanism of resistance seen in gram-negative bacteria. Choice A focuses on transpeptidase enzyme quantity, which is not the primary reason for penicillin resistance. Choice B refers to β-lactamase enzymes in the periplasmic space, which are actually enzymes that degrade penicillin and contribute to resistance, but this is not unique to gram-negative bacteria. Choice D mentions a thicker cell wall, which is a characteristic of gram-positive bacteria, not gram-negative bacteria.

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus epidermidis. The presence of Gram-positive cocci in clusters, catalase-positive, and coagulase-negative characteristics indicate Staphylococcus species. Specifically, Staphylococcus epidermidis is a common skin commensal and a frequent cause of wound infections. Staphylococcus aureus is catalase-positive and coagulase-positive, making it less likely in this scenario. Streptococcus pyogenes and Enterococcus faecalis are both Gram-positive cocci but are catalase-negative, making them less likely causative agents.

Correct Answer: A

Rationale: The correct answer is A: Balantidium coli. This large ciliated protozoa with a kidney-shaped macronucleus is characteristic of Balantidium coli, an intestinal parasite known to cause foul-smelling diarrhea in humans. Balantidium coli is the only option among the choices that fits this specific description. Entamoeba histolytica causes amoebic dysentery, Giardia lamblia causes giardiasis, and Trichomonas hominis is a non-pathogenic protozoa found in the intestinal tract. Therefore, based on the morphology described in the question, Balantidium coli is the correct causative agent.