A 38-year-old farmer is brought to the emergency department 2 hours after accidental exposure to an organophosphate pesticide. He presents with excessive salivation, pinpoint pupils, and muscle fasciculations. His colleague, exposed to carbamate insecticide at the same site, shows similar cholinergic signs but recovers fully within 24 hours with supportive care alone. Which feature best distinguishes organophosphate from carbamate poisoning?