Correct Answer: A

Rationale: The correct answer is A. Benzylpenicillin sodium salt, also known as penicillin G, works by blocking the synthesis of peptidoglycan in the microbial cell wall. Peptidoglycan is crucial for the structural integrity of bacterial cell walls. By inhibiting its synthesis, penicillin disrupts the cell wall formation, leading to cell lysis and bacterial death. Choices B, C, and D are incorrect because penicillin does not target cytoplasm proteins, thiol enzymes, or RNA synthesis. Penicillin's primary mode of action is specifically related to interfering with bacterial cell wall synthesis.

Correct Answer: C

Rationale: The correct answer is C: urethra. Resident microflora refers to the normal, beneficial bacteria that inhabit a specific area of the body without causing harm. The urethra is one of the locations where resident microflora are commonly found. These bacteria play a vital role in maintaining a healthy balance and preventing harmful pathogens from colonizing the urethra. Blood (A), lung (B), and cerebro-spinal fluid (D) are not typical locations for resident microflora as these areas are usually sterile or have very limited microbial presence to prevent infections.

Correct Answer: B

Rationale: The correct answer is B: parasitism. In parasitism, one organism (parasite) benefits at the expense of the other organism (host). The parasite derives nutrients or resources from the host, causing harm or even death in some cases. This relationship is characterized by asymmetry, where one organism benefits while the other is negatively impacted. Commensalism (A) is a relationship where one organism benefits while the other is unaffected. Synergism (C) is a mutually beneficial relationship where both organisms benefit. Antagonism (D) is a relationship where both organisms are harmed or inhibited.

Correct Answer: B

Rationale: Answer B, Infectious mononucleosis, is not caused by enteroviruses. Infectious mononucleosis is primarily caused by the Epstein-Barr virus, a member of the herpesvirus family, not by enteroviruses. Enteroviruses are known to cause diseases such as myocarditis, herpangina, and Boston exanthema. Therefore, B is the correct answer. Myocarditis, herpangina, and Boston exanthema can all be caused by enteroviruses, making choices A, C, and D incorrect, respectively.

Correct Answer: B

Rationale: The correct answer is B because Haemophilus influenzae, Streptococcus pneumoniae, and Neisseria meningitidis are the most common causative agents of bacterial meningitis. Haemophilus influenzae is often seen in children, Streptococcus pneumoniae is the most common in adults, and Neisseria meningitidis is common in adolescents and young adults. Choice A is incorrect because although Streptococcus group B and Listeria monocytogenes can cause meningitis, E. Coli K1 is not a common causative agent. Choice C is incorrect because Staphylococcus aureus and Pseudomonas aeruginosa are not typically associated with bacterial meningitis. Choice D is incorrect as there are known causative agents for bacterial meningitis.