A 28-year-old woman of Indian origin presents to the emergency department with severe headache, confusion, and seizures. Blood glucose is 45 mg/dL (normal fasting 70–100). Serum insulin is suppressed (<2 mIU/L; normal fasting 5–12), and C-peptide is low. Arterial blood gas shows metabolic acidosis with elevated lactate (6 mmol/L). Liver function tests are normal. A targeted history reveals she ingested a herbal remedy containing cyanide 4 hours ago. Which step of glycolysis is most directly inhibited by this toxin, and what is the biochemical consequence?