Final answer:
The child is suspected of having fifth disease because of the erythema on the face, giving a "slapped cheeks" appearance, a key distinguishing feature from other rash-causing illnesses.
Step-by-step explanation:
The nurse suspects that the child has erythema infectiosum (fifth disease) based on the presence of erythema on the face, giving a "slapped cheeks" appearance. This distinct red facial rash is a characteristic sign of fifth disease, which is caused by parvovirus B19. This infection is common in children and is more easily spread during the early stages when symptoms resemble those of a cold.
Fifth disease is distinct from other rash-causing illnesses like roseola in its presentation, the latter typically causing a rash that begins on the trunk and may spread outward. Moreover, while adults infected with fifth disease may not always present with a rash, they can experience joint pain and swelling that can last for longer periods, which is not associated with roseola.