A 35-year-old industrial worker is brought to the emergency department with altered mental status, seizures, and profound hypotension 30 minutes after exposure to hydrogen cyanide gas in a chemical plant accident. Arterial blood gas shows normal oxygen saturation (98%) and adequate oxygen delivery, yet the patient is in profound shock with lactic acidosis (pH 7.18, lactate 8.2 mmol/L). The structure marked **C** in the electron transport chain diagram is the site of cyanide's lethal action. Which of the following best describes the mechanism of cyanide toxicity in this clinical context?