Answer:USMLE step 2 Mcq 105:A 24yr old man presented to the emergency room with salivation, abdominal cramps and vomiting




Correct Answer: B

Parathion is a commonly used insecticide which belongs to organophosphorous group of chemicals. Poisoning due to parathion occurs usually in farming areas where it is used as an insecticidal agent. Parathion acts by irreversible inhibition of the enzyme acetylcholine esterase which is responsible for the breakdown of acetylcholine. Normally acetylcholine binds to the esteratic site of the enzyme resulting in acetylation of the enzyme. The acetylated enzyme reacts with water resulting in breakdown of acetylcholine and release of free enzyme. Parathion also binds to the esteratic site of the enzyme acetylcholine esterase but in this case the enzyme does not react with water and the enzyme undergoes irreversible inhibition. This results in accumulation of acetylcholine resulting in severe cholinergic overactivity. The severity of the symptoms depends on the exposure dose of parathion. In small to moderate doses it causes salivation, vomiting, abdominal cramps, lacrimation etc. it also causes respiratory distress due to large amount of tracheo-bronchial secretions. In large doses it can cause convulsions and death due to respiratory paralysis. Specific antidote used is 2-Pralidoxime. 2-Pralidoxime has a quaternary nitrogen which attaches to the anionic site of the enzyme. The oxime part of 2-Pralidoxime reacts with the phosphorous atom of parathion forming an oxime-phosphonate which diffuses away leaving behind the reactivated enzyme. In this way 2-pralidoxime reactivates the inhibited enzyme and acts as a specific antidote for parathion poisoning.Atropine is also used for the treatment of parathion poisoning but it only serves as a physiological antagonist of acetylcholine and provides symptomatic relief. It does not reactivate the enzyme therefore it is not a specific antidote.Diazepam is used to control the convulsions due to the poisoning and is not a specific antidote.Amyl nitrite is used in cyanide poisoning and not in parathion poisoning. Physostigmine itself is an inhibitor of the enzyme acetylcholine esterase and would further worsen the symptoms of parathion poisoning if administered.


>>>> Post your Answer Here ????

Design by