A 42-year-old man from rural Maharashtra presents with a 3-week history of progressive muscle weakness, tremors, and excessive salivation. He works as a pesticide sprayer and was exposed to organophosphate compounds 2 weeks ago without proper protective equipment. On examination, he has pinpoint pupils, bradycardia (HR 48/min), and visible muscle fasciculations. Serum cholinesterase levels are markedly reduced to 20% of normal. Which type of enzyme inhibition best describes the mechanism of organophosphate toxicity?