A 35-year-old man is admitted to the ICU after intentional ingestion of cyanide salt. He is unconscious, with a blood pressure of 70/40 mmHg, heart rate 140/min, and respiratory rate 28/min. Arterial blood gas shows severe metabolic acidosis (pH 6.9, lactate 15 mmol/L). Which of the following is the most appropriate immediate antidote?