A 14-year-old girl is referred to the neurology clinic after experiencing two unprovoked seizures triggered by watching a music video with rapid flashing lights. EEG during intermittent photic stimulation at 15 Hz shows the pattern marked **A** in the diagram — generalized spike-and-wave discharges that persist for 2 seconds after the photic train ends. Her parents report she has never had seizures otherwise. Which of the following best describes the EEG finding marked **A** and its clinical significance?