The temperature of vaccine refrigerators must be checked and logged:

Correct Answer: B

Rationale: CDC guidelines mandate that vaccine refrigerator temperatures be checked and logged twice daily (morning and evening) to ensure they remain within 2°C-8°C. This frequent monitoring detects excursions promptly, preventing vaccine spoilage. Choice A (once a week) and D (twice a week) are insufficient, and Choice C (once daily) misses potential overnight fluctuations, making Choice B the standard practice.

Correct Answer: B

Rationale: Antigens are foreign substances (e.g., proteins on viruses or bacteria) that can trigger an immune response, potentially causing disease if pathogenic. Antibodies (Choice A) are produced by B cells to neutralize antigens. T-Lymphocytes (Choice C) and macrophages (Choice D) are immune cells that respond to antigens, not the substances themselves, making Choice B the correct term.

Correct Answer: B

Rationale: Hib vaccine prevents *Haemophilus influenzae* type b infections, including epiglottitis, a life-threatening airway obstruction. MMR, HepB, and polio vaccines target unrelated diseases (measles/mumps/rubella, hepatitis B, poliomyelitis), none of which cause epiglottitis.

Correct Answer: C

Rationale: Anaphylaxis to a prior vaccine dose (Choice C) is a valid contraindication due to the risk of recurrence. Choice A (mild illness) is not a contraindication per CDC—vaccines can be given. Choices B and D (family circumstances) don’t affect the child’s eligibility unless the child is immunosuppressed.

Correct Answer: D

Rationale: Absolute contraindications to DTaP include a prior anaphylactic reaction (Choice B) due to risk of recurrence, and encephalopathy within 7 days of a prior dose without another cause (Choice C), per CDC, as these indicate severe risk. Moderate/serious illness (Choice A) is a precaution, not an absolute bar—vaccination is delayed until recovery. Choice D (B or C) captures both contraindications.