Correct Answer: B

Rationale: The correct answer is B: IgM. IgM is the first antibody synthesized by the infant soon after birth. It is produced by B cells in response to initial exposure to antigens. IgA is mainly found in mucosal areas, IgG is transferred from mother to fetus, and IgD functions in the early stages of B cell development.
Therefore, IgM is the most suitable choice as the antibody synthesized by the infant soon after birth.

Correct Answer: D

Rationale: The correct answer is D, "All of the above." Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae are all bacteria known to cause meningitis. Neisseria meningitidis is a common cause of bacterial meningitis in young adults, Haemophilus influenzae type b is a common cause in children, and Streptococcus pneumoniae is a leading cause in adults and the elderly.
Therefore, all three bacteria can potentially cause meningitis depending on the age group and other factors. The other choices are incorrect because each of the bacteria listed can cause meningitis, so selecting only one of them would be incorrect.

Correct Answer: A

Rationale: The correct answer is A: Staphylococcus epidermidis. The presence of Gram-positive cocci in clusters indicates a staphylococcal species. The fact that the bacteria are catalase-positive and coagulase-negative further narrow it down to Staphylococcus epidermidis, as this species fits all the given characteristics.

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Choice B, Staphylococcus aureus, is coagulase-positive.
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Choice C, Streptococcus pyogenes, is a streptococcal species, not staphylococcal.
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Choice D, Enterococcus faecalis, is a Gram-positive cocci but is catalase-negative and not typically found in clusters.

Correct Answer: A

Rationale: The correct answer is A: Pertussis toxoid. This is because pertussis toxoid is a component of the pertussis vaccine, which provides specific prophylaxis against pertussis by stimulating the immune system to produce antibodies against the bacteria causing the disease.
Choice B, killed vaccine, is incorrect as it does not specify pertussis toxoid.
Choice C, wide-spectrum antibiotics, is incorrect as antibiotics are used for treatment, not prophylaxis.
Choice D, None of the above, is incorrect as pertussis toxoid is specifically used for prophylaxis of pertussis.

Correct Answer: A

Rationale: The correct answer is A (Escherichia coli, Gardnerella vaginalis, and Staphylococcus aureus) because they are known to commonly cause nonsexually transmitted infections of the reproductive system. Escherichia coli can cause urinary tract infections that can ascend to the reproductive system. Gardnerella vaginalis is associated with bacterial vaginosis, a common infection of the vagina. Staphylococcus aureus can cause infections in the reproductive system through various means.



Choices B, C, and D are incorrect:
B: Streptococcus faecalis, Klebsiella oxytoca, and Serratia marcescens are not commonly associated with reproductive system infections.
C: Clostridium tetani and Bacillus cereus are not typically involved in reproductive system infections.
D: Neisseria gonorrhoeae causes sexually transmitted infections, Helicobacter pylori causes gastrointestinal issues, and Pseudomonas aeruginosa is more