Viruses are characterised by:

Correct Answer: A

Rationale: Step 1: Viruses can have either DNA or RNA, not both, hence they have only one type of nucleic acid. Step 2: The extracellular form of viruses is called virion, not viroid. Step 3: Viruses are obligate intracellular parasites, meaning they rely on host cells for replication. Step 4: Viruses are measured in nanometers, not micrometers. Therefore, the correct answer is A.

Correct Answer: C

Rationale: The correct answer is C: Looking for Ernst-Babes granules. Laboratory diagnosis of rabies typically involves looking for Negri bodies in the brain tissue, intracerebrally infecting mice for virus isolation, and using immunofluorescence method to detect viral antigens. Ernst-Babes granules are not associated with rabies virus and are not used in the diagnosis of rabies. Therefore, choice C is incorrect.

Correct Answer: A

Rationale: Step-by-step rationale for choice A being correct: 1. Parvovirus B19 is a single-stranded DNA (ssDNA) virus. 2. The prefix "parvo-" indicates small size, characteristic of ssDNA viruses. 3. ssDNA viruses replicate in the host cell by converting their ssDNA genome into double-stranded DNA (dsDNA). 4. Therefore, the correct answer is A: Its genome consists of dsDNA. Summary of why other choices are incorrect: B: Parvovirus B19 is a non-enveloped virus, making choice B incorrect. C: Parvovirus B19 has a linear ssDNA genome, not segmented, making choice C incorrect. D: Parvovirus B19 has two structural proteins - VP1 and VP2, making choice D incorrect.

Correct Answer: B

Rationale: The correct answer is B: Transformation. Frederick Griffith discovered bacterial transformation, where genetic material can be transferred between bacteria. He observed this phenomenon while studying pneumonia-causing bacteria in 1928. Transformation is the process by which bacteria take up external DNA and incorporate it into their own genome. This discovery was crucial in understanding how genetic information can be transferred between organisms. Rationale for other choices: A: Translation - This term refers to the process of protein synthesis from mRNA, not relevant to Griffith's discovery. C: Transcription - This term refers to the process of synthesizing RNA from DNA, not related to Griffith's discovery. D: Conjugation - This term refers to the direct transfer of genetic material between bacterial cells through cell-to-cell contact, which is different from the phenomenon Griffith observed.

Correct Answer: A

Rationale: The correct answer is A because immunofluorescence slides are examined under a fluorescence microscope with a x100 oil immersion objective. This is because immunofluorescence relies on the detection of fluorescently labeled antibodies, which emit light when excited by a specific wavelength. Using a fluorescence microscope allows for visualization of these labeled antibodies at high magnification and resolution, enabling precise examination of cellular structures and protein localization. Choice B is incorrect as light microscopes are not suitable for visualizing fluorescence. Choice C is incorrect as electron microscopes use electrons, not light, for imaging, which is not compatible with immunofluorescence. Choice D is incorrect as there is a specific method for examining immunofluorescence slides.