A 4-year-old child presents with a history of gaining 20 pounds in the last 12 months. Evaluation reveals central hypothyroidism, disordered water balance, and sleep-disordered breathing that evolves to to central alveolar hypoventilation requiring ventilatory support. Genetic testing for PHOX2B pathogenic variants is negative. Which finding most definitively distinguishes this child’s condition, rapid-onset obesity with hypothalamic dysfunction, hypoventilation, and autonomic dysregulation (ROHHAD), from late-onset congenital central hypoventilation syndrome (LO-CCHS)?