15 A previously healthy, 27-year-old agricultural worker develops nausea, vomiting, abdominal cramps, cough withwheezing and dyspnea, salivation, and lacrimation 1 hour after leaving work in a field. On examination, he is afebrile with a heart rate of 50/min and blood pressure of 90/55 mm Hg. Laboratory studies show plasma cholinesterase activity less than 50% of normal. Atropine is given intravenously, and his condition improves. He was most likely exposed to which of the following substances? (A) Aflatoxin (B) Cyclodiene (C) Dioxin (D) Ergot alkaloid (E) Organophosphate (F) Pyrethrin