Childhood Absence Epilepsy

A 7-year-old girl is brought to the paediatric neurology clinic by her mother, who reports that the child has been having multiple brief episodes of unresponsiveness throughout the day, often mistaken for "daydreaming" or inattention at school. During the clinical examination, hyperventilation for 4 minutes is performed, and the child experiences a clinical absence seizure with a vacant stare. The EEG during this episode shows the pattern marked **A** in the diagram. Which of the following is the most appropriate first-line antiepileptic drug for this patient?