A 28-year-old woman is brought to the emergency department with severe abdominal pain, profuse watery diarrhea, and hypotension after consuming contaminated well water. Her 45-year-old father, occupationally exposed to arsenic dust for 15 years, presents with progressive sensorimotor peripheral neuropathy and skin pigmentation changes. Which finding most reliably distinguishes the father's chronic arsenic neuropathy from the daughter's acute arsenic toxicity?