The symmetry of nucleocapsid of poxviruses is:

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: 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: 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: D

Rationale: The correct answer is D: Genus Aspergillus. Aflatoxins are toxic compounds produced by certain species of fungi, particularly Aspergillus species. Aspergillus flavus and Aspergillus parasiticus are well-known producers of aflatoxins. These fungi can contaminate crops such as peanuts, corn, and tree nuts, leading to potential health risks. Cryptococcus (A), Actinomyces (B), and Streptococcus (C) are not known to produce aflatoxins. It is important to be aware of the sources and implications of aflatoxin contamination to ensure food safety and public health.

Correct Answer: A

Rationale: The correct answer is A: Toxoplasmosis. Toxoplasmosis is caused by the protozoan Toxoplasma gondii, which can be transmitted from mother to fetus during pregnancy leading to stillbirth and malformations. This parasite can cross the placenta and infect the fetus, causing severe complications. Leishmaniasis (B), Malaria (C), and Amebiasis (D) are not known to cause intrauterine death or malformations in the same way as Toxoplasmosis. Leishmaniasis is transmitted by sandflies, Malaria by mosquitoes, and Amebiasis by contaminated food or water.

Correct Answer: D

Rationale: The correct answer is D: icosahedral structure. Icosahedral symmetry is common in viruses due to its efficient packing of genetic material. It consists of 20 equilateral triangular faces and 12 vertices. This symmetry allows viruses to have a stable structure and optimal capacity for encapsulating their genetic material. A: Tetrahedral structure is incorrect as it consists of 4 faces and is not commonly found in viruses. B: Hexahedral structure is incorrect as it consists of 6 faces and is not a common symmetry type in viruses. C: Dodecahedral structure is incorrect as it consists of 12 faces and is not typically seen in viruses. In summary, the icosahedral structure is the correct choice due to its stability, efficient packing, and common presence in viral structures compared to the other options.

Correct Answer: A

Rationale: The correct answer is A: Escherichia coli. E. coli is a Gram-negative rod that ferments lactose. It is a common cause of gastroenteritis presenting with fever and diarrhea. Salmonella typhi does not ferment lactose and causes typhoid fever. Shigella dysenteriae causes dysentery and does not ferment lactose. Proteus vulgaris does not typically cause gastroenteritis and is not known for lactose fermentation.

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: C

Rationale: Rationale: 1. Vaccinia is a laboratory product used in smallpox vaccination. 2. Variola is the virus that causes smallpox, not a laboratory product. 3. Monkeypox is a naturally occurring virus, not a laboratory product. 4. Cowpox can be found in nature and is used in vaccine production, but it is not a laboratory product like Vaccinia.