Which vaccine should a nurse prepare to administer to an 18-month-old child with a comprehensive immunization history?

In the context of immunizations for an 18-month-old child, the diphtheria/tetanus/acellular pertussis (DTaP) vaccine is relevant because it is typically administered in a series during early childhood. By 18 months, a child is due for the fourth dose of this vaccine, having received doses at 2 months, 4 months, and 6 months of age. This vaccine is crucial for protecting against diphtheria, tetanus, and pertussis, which are serious diseases that can lead to severe complications.

While the measles/mumps/rubella (MMR) and varicella zoster vaccines are also important and are usually given around this age, the DTaP is specifically expected at this time in a child's immunization schedule. The hepatitis B vaccine is typically provided at birth and then at 1 to 2 months and 6 to 18 months, so it's probable that this vaccine would have already been administered prior to 18 months, assuming the child has a comprehensive immunization history. Therefore, administering the DTaP is the most appropriate choice for this age group.