Organophosphate poisoning causes excessive acetylcholine accumulation at synapses. A toxicology resident notes that pralidoxime (2-PAM) must be given within a specific time window. What is the sub-molecular basis for this time limitation?

• A Pralidoxime is metabolized rapidly by plasma cholinesterases and becomes inactive after 4 hours
• B Continued ACh accumulation eventually down-regulates all nicotinic receptors, making enzyme reactivation irrelevant
• C Pralidoxime distributes poorly to the CNS after the blood-brain barrier is compromised by prolonged poisoning
• D The organophosphate-AChE complex undergoes 'aging' — loss of an alkyl group makes reactivation by pralidoxime impossible ✓

Explanation

After organophosphate binds to the serine residue of acetylcholinesterase, the complex undergoes a process called 'aging' — dealkylation of the phosphate group attached to the enzyme, typically within 24-48 hours for most compounds (but much faster with soman/GD). Once aging occurs, the covalent bond becomes resistant to nucleophilic attack by pralidoxime's oxime group, and reactivation of the enzyme is no longer possible. This is why pralidoxime must be administered as early as possible, ideally within the first 24 hours of poisoning.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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