A patient ingested some unknown substance and presented with myoclonic jerks, seizures, tachycardia and hypotension. The ECG showed a heart rate of 120/minute with QRS interval of 0.16 seconds. The arterial blood gas revealed a pH of 7.25, PCO2 of 30 mmHg and HCO3 of 15 mmol/L. The most likely cause of poisoning is ingestion of: