A 65-year-old man on ICU has severe community-acquired pneumonia requiring mechanical ventilation. On day 2, his serum sodium drops from 138 to 125 mmol/L. Urine osmolality is 580 mOsm/kg, urine sodium 55 mmol/L, serum osmolality 260 mOsm/kg. He is euvolaemic. Thyroid function and cortisol are normal. What is the most likely cause?