An 8-month-old girl suddenly developed a high fever (103degF). Her pediatrician examined her and found no signs of upper respiratory tract infection, meningitis, or encephalitis. Two days later, the girl's fever reached 105degF and she suffered a febrile seizure. By the time the child was brought to the pediatrician's office, her temperature had dropped. The doctor noted a generalized papular rash. What was the doctor's most likely diagnosis for this child?