A 4-year-old girl is brought to the pediatrician's office. Her father reports that she suddenly became pale and stopped running while he had been playfully chasing her and her pet Chihuahua. After 30 minutes, she was no longer pale and wanted to resume the game. She has never had a previous episode and has never been cyanotic. Her physical examination was normal, as were her chest x-ray and echocardiogram. An ECG showed the pattern seen on the next page, which indicates which of the following?