A 15-yeаr-оld bоy is brоught in by his mother, who reports he hаs been “moody” for the pаst 6 months. He has periods of irritability lasting several hours after conflicts with friends, followed by normal mood the next day. He maintains good sleep, grades, peer relationships, and extracurricular participation. He denies impulsivity, decreased need for sleep, or sustained elevated mood. Which diagnosis is MOST consistent with this presentation?