This story appeared in the May 2020 issue as “When a Teenager Shuts Down.” Subscribe to Discover magazine for more stories like this.At first, his parents thought Marcus was just being a typical teenager — reticent to talk, preferring to stay in his room and blare loud music. Then he started talking to himself and refusing to eat. His family became desperate, and brought him to our pediatric emergency department.“It’s like he isn’t seeing us,” his mother told the nurse tearfully. “It’s like he’s talking to other people who aren’t there.”Marcus sat quietly nearby, his eyes darting back and forth as he muttered under his breath. His parents assured us that he was normally an active and spirited 15-year-old, a star of his dance team and church choir, always the first to crack a joke or break out into spontaneous song. Last week, he’d been healthy, other than a headache and a bellyache, relieved by a few doses of Tylenol. But just a few days later, it seemed as…



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