A 16-year-old male patient presented with slurred speech, somnolence, and hypotension after a diagnosis of streptococcal pharyngitis 3 days earlier. He had significantly darker skin than his family members (Figure 1). During the prior year, his athletic abilities had deteriorated. He also frequently complained of fatigue. His medical history was significant for a diagnosis of attention-deficit/hyperactivity disorder and several hospitalizations for dehydration. The family history was notable for several great-grandmothers who had dementia, and deaths in 3 of the maternal great-grandmother's sons at ages 5, 6, and 7 years, raising concern for an X-linked disease with female carriers and affected males (Figure 2).