John is 15 years old and presents with a history dating back approximately 12 months in which he experiences periods of anhedonia, hypersomnia, and feelings of worthlessness, followed by periods of feeling rested after only a few hours and obsessive online gaming to the exclusion of all else. The periods frequently alternate. John does not have a significant history of drugs or alcohol, and he prefers sobriety to altered states. John's diagnosis is most consistent with: a) Schizophrenia b) Attention-deficit/hyperactivity disorder (ADHD) c) Cyclothymic disorder d) Substance use disorder