A 35-year-old man is brought to the emergency department with altered mental status, metabolic acidosis (pH 7.18, HCO₃⁻ 12 mEq/L), and visual disturbances. A 28-year-old woman presents with similar altered mental status and metabolic acidosis but without visual symptoms; she has a normal fundoscopic examination. Which single feature best distinguishes methanol poisoning from ethanol poisoning in this comparison?