Which of the following symptoms is characteristic of erythema infectiosum (fifth disease)?

The symptom that is characteristic of erythema infectiosum, commonly known as fifth disease, is the "slapped cheek appearance." This distinctive rash typically appears as bright red cheeks in affected children, giving the impression that they have been slapped. This rash is often accompanied by a lacy, reticular rash that develops on the body.

Erythema infectiosum is caused by parvovirus B19, which typically affects children and can be associated with mild illness. The slapped cheek appearance is a hallmark feature that helps differentiate it from other viral exanthems.

In contrast, unilateral orchitis is associated with mumps and not with erythema infectiosum. Koplik spots are small blue-white spots that appear inside the mouth and are indicative of measles. Stomatitis, or inflammation of the mucous membranes in the mouth, can occur in various viral infections but is not specific to fifth disease.