Correct Answer: A

Rationale: The correct answer is A: virulence. Virulence refers to the degree or extent of disease-evoking power that a microorganism possesses. This term specifically describes the ability of a microorganism to cause disease in a host. It is a crucial factor in understanding the pathogenicity of different microorganisms. Choice B: strength is incorrect because it does not specifically relate to the disease-causing potential of a microorganism. Choice C: resistance refers to the ability of an organism to withstand the effects of a particular agent, such as a drug or environmental factor. Choice D: competence is not directly related to the disease-causing ability of a microorganism, but rather refers to the ability of an organism to perform a specific function effectively.

Correct Answer: B

Rationale: The correct answer is B because Streptococcus pneumoniae, Neisseria meningitidis, and Haemophilus influenzae are the most common causative agents of bacterial meningitis. Streptococcus pneumoniae is a leading cause in adults, Neisseria meningitidis is common in adolescents and young adults, and Haemophilus influenzae is more frequent in children. The other choices, Rubella virus (C) and Herpangina (D), are not bacterial agents known to cause meningitis.

Correct Answer: B

Rationale: The correct answer is B: adolescents. Infectious mononucleosis is commonly caused by the Epstein-Barr virus, which is transmitted through saliva. Adolescents are more likely to engage in activities like kissing and sharing drinks, increasing the chances of virus transmission. Infants have maternal antibodies for protection, young adults may have developed immunity, and the elderly have had more exposure to the virus and may have built immunity over time.

Correct Answer: C

Rationale: Granulomatosis infantiseptica is commonly caused by Listeria monocytogenes, a bacterium known to infect newborns. It can lead to granulomatous lesions in various organs. Staphylococcus aureus and Mycobacterium tuberculosis are not typical causes of this condition in infants. Choice D is incorrect as it suggests all listed bacteria are responsible, which is not accurate. Therefore, the correct answer is C as Listeria monocytogenes is the specific pathogen associated with granulomatosis infantiseptica.

Correct Answer: C

Rationale: The correct answer is C: acetyl-CoA. Acetyl-CoA is the compound that combines with oxaloacetic acid at the beginning of the Krebs cycle to form citric acid. This step is essential for the cycle to proceed, making acetyl-CoA the correct choice. A: Citric acid is formed after acetyl-CoA combines with oxaloacetic acid, so it is a product of the reaction, not the compound that enters the cycle. B: Pyruvic acid is converted to acetyl-CoA in a separate process before entering the Krebs cycle. D: Phosphoglyceraldehyde is not involved in the Krebs cycle; it is a compound in glycolysis.