A 10-yeаr-оld with ALL hаs been оn dexаmethasоne 0.6 mg/kg/day for 4 weeks as part of induction chemotherapy. The dexamethasone was discontinued abruptly 3 days ago. She now presents with fever of 38.5°C, lethargy, BP 68/38 mmHg, HR 128 bpm, glucose 31 mg/dL, Na 126 mEq/L, and K 5.8 mEq/L. What is the MOST critical diagnosis to treat immediately?