Correct Answer: B

Rationale: The correct answer is B: Hepatitis B virus. This is because Hepatitis B virus is a DNA virus, not an RNA virus. The other choices, A (Hepatitis G virus), C (Hepatitis A virus), and D (Hepatitis E virus) are all RNA viruses. Hepatitis B virus is unique as it contains partially double-stranded DNA. DNA viruses replicate through a DNA intermediate, unlike RNA viruses which directly use RNA. Therefore, Hepatitis B virus stands out as the only DNA virus among the given options.

Correct Answer: A

Rationale: The correct answer is A because significant bacteriuria is defined as having more than 10^5 cells in 1 ml of midstream morning urine. This concentration indicates a true infection rather than contamination. Choice B is incorrect because the amount of urine does not determine significant bacteriuria. Choice C is incorrect because even a single bacterium in urine does not necessarily indicate an infection. Choice D is incorrect because not all statements are correct; only choice A accurately defines significant bacteriuria based on the bacterial concentration in urine samples.

Correct Answer: D

Rationale: The correct answer is D: Agar media. Influenza viruses cannot be isolated in agar media because they require specific conditions present in laboratory animals, embryonated eggs, or cell cultures for successful isolation and propagation. Agar media lack the necessary components and environment needed for influenza virus replication. In laboratory animals, such as mice, the viruses can replicate and cause infection. Embryonated eggs provide a suitable environment for influenza virus growth and isolation due to the presence of necessary nutrients and factors. Cell cultures offer controlled conditions for virus propagation and study, making them a valuable tool in influenza virus research.

Correct Answer: A

Rationale: The correct answer is A: Toxoplasma. Toxoplasma gondii is a protozoan parasite that can infect the brain and cause vision loss. The description of half-moon-shaped organisms with a pointed end matches the appearance of Toxoplasma under the microscope. Toxoplasma is known to cause a condition called toxoplasmosis, which can lead to neurological symptoms such as vision loss. Leishmania, Lamblia, Amoeba, and Trichomonad are not known to cause brain infections or vision loss, making them incorrect choices.

Correct Answer: B

Rationale: The correct answer is B: cytopathic effect. When viruses infect cell cultures, they induce degenerative changes known as cytopathic effects, which include cell shrinkage, rounding, detachment, and cell death. This term specifically refers to the damage caused by viral replication within the cells. A: Cytolytic effect refers to the destruction of cells by viruses, which is a more specific type of cytopathic effect involving cell lysis. C: Cytotoxic effect typically refers to the toxic effects of substances on cells, not specifically related to viral propagation. D: The statement that all three answers are correct is incorrect as only option B accurately describes the degenerative changes resulting from viral propagation in infected cell cultures.