A farmer ingests an unknown substance and presents with miosis, excessive salivation, muscle fasciculations, and bradycardia. His serum acetylcholinesterase (AChE) is 15% of normal. Pralidoxime is administered after 48 hours. The MOST likely reason for its failure is:

• A Ageing of the OP-AChE complex has rendered the enzyme irreversibly inhibited ✓
• B Pralidoxime was given before complete absorption of the organophosphate
• C The compound was a carbamate rather than an organophosphate
• D Serum AChE below 50% indicates severe poisoning where oximes are contraindicated

Explanation

When pralidoxime is given more than 24–48 hours after OP exposure (depending on the compound), the phosphorylated AChE undergoes ageing (dealkylation), forming an irreversibly inhibited monoalkyl phosphoryl-enzyme that cannot be reactivated by oximes. The clinical picture (miosis, SLUDGE symptoms, fasciculations, bradycardia) and serum AChE at 15% of normal confirm severe OP poisoning. Pralidoxime must be given within the critical window — ideally within 24–48 hours — before ageing occurs; after this period, atropine remains the only effective therapeutic option.

Reference: The Essentials of Forensic Medicine and Toxicology (Narayan Reddy), 34th ed.

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Written and medically reviewed by the StethoPrep medical team.