Toxicology USMLE Step 1 Practice Question

A 47-year-old man is brought to the emergency department 1 hour after being found unconscious on his porch. His wife reports he was working in the garage 4 hours ago. On examination, he is confused and has visible muscle fasciculations in his arms and jaw. Vital signs: HR 108/min, RR 28/min, BP 142/88 mmHg, temperature 37.2°C. Laboratory studies show: Serum electrolytes: Na+ 138 mEq/L, Cl- 100 mEq/L, HCO3- 9 mEq/L Arterial blood gas: pH 7.24, pCO2 18 mmHg Serum osmolality (measured): 330 mOsmol/kg H2O Serum osmolality (calculated): 298 mOsmol/kg H2O Urine: colorless crystals present on microscopy Which of the following substances is most likely responsible for this patient's presentation?