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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor= {{YD}} (Reviewed by Serge Korjian)
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Pharmacology
|MainCategory=Pharmacology
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|AnswerA=Phenytoin follows a flow-dependent elimination
|AnswerA=Phenytoin follows a flow-dependent elimination
|AnswerAExp=This is true for first-order elimination. Phenytoin on the other hand follows a capacity-limited elimination.
|AnswerAExp=This is true for first-order elimination. Phenytoin on the other hand follows a capacity-limited elimination.
|AnswerB=Phenytoin follows a zero-order elimination whereby a constant ratio of the drug is eliminated per unit time
|AnswerB=Phenytoin follows a zero-order elimination whereby a constant proportion of the drug is eliminated per unit time
|AnswerBExp=Although phenytoin indeed follows a zero-order elimination, the same amount (not the ratio) of drug is constantly eliminated per unit time.
|AnswerBExp=Although phenytoin indeed follows a zero-order elimination, the same amount (not the ratio) of drug is constantly eliminated per unit time.
|AnswerC=The rate of phenytoin elimination is directly proportional to the drug's concentration
|AnswerC=The rate of phenytoin elimination is directly proportional to the drug's concentration
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|AnswerE=The amount of eliminated drug is gradually decreased as the plasma concentration of phenytoin is deceased over time
|AnswerE=The amount of eliminated drug is gradually decreased as the plasma concentration of phenytoin is deceased over time
|AnswerEExp=The statement is true for first-order reactions.  In zero-order reactions, the eliminated amount is constant regardless of plasma concentrations.
|AnswerEExp=The statement is true for first-order reactions.  In zero-order reactions, the eliminated amount is constant regardless of plasma concentrations.
|EducationalObjectives=Phenytoin follows zero-order elimination and rate of elimination is constant due to constant amount  loss per unit time.
|EducationalObjectives=Phenytoin follows zero-order elimination and rate of elimination is constant per unit time.
|References=Katzung BG, Masters S, Trevor A. Basic and Clinical Pharmacology 12/E. Chapter 3. Pharmacokinetics & Pharmacodynamics: Rational Dosing & the Time Course of Drug Action. McGraw Hill Professional; 2012.
|RightAnswer=D
|RightAnswer=D
|WBRKeyword=Plasma concentration, Phenytoin, Zero order elimination, Capacity-limited elimination, Flow-dependent elimination,
|WBRKeyword=Plasma concentration, Phenytoin, Zero order elimination, Capacity-limited elimination, Flow-dependent elimination,
|Approved=Yes
|Approved=Yes
}}
}}

Latest revision as of 00:46, 28 October 2020

 
Author [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Pharmacology
Sub Category SubCategory::Neurology
Prompt [[Prompt::A pharmacologist is investigating the complex metabolism of several neuropsychiatric drugs. In one of his experiments, he administers a 20 mg intravenous (IV) bolus of phenytoin to 30 mice. He then measures the plasma concentrations of phenytoin at 5 minutes after administration, and at 2, 4, and 6 hours. Which of the following statements is true regarding the elimination of this drug?]]
Answer A AnswerA::Phenytoin follows a flow-dependent elimination
Answer A Explanation AnswerAExp::This is true for first-order elimination. Phenytoin on the other hand follows a capacity-limited elimination.
Answer B AnswerB::Phenytoin follows a zero-order elimination whereby a constant proportion of the drug is eliminated per unit time
Answer B Explanation AnswerBExp::Although phenytoin indeed follows a zero-order elimination, the same amount (not the ratio) of drug is constantly eliminated per unit time.
Answer C AnswerC::The rate of phenytoin elimination is directly proportional to the drug's concentration
Answer C Explanation AnswerCExp::The rate of elimination is constant regardless of concentration.
Answer D AnswerD::Plasma concentration levels of phenytoin fall linearly with time
Answer D Explanation AnswerDExp::Because a constant amount of phenytoin is eliminated per unit time, plasma concentration levels falls linearly.
Answer E AnswerE::The amount of eliminated drug is gradually decreased as the plasma concentration of phenytoin is deceased over time
Answer E Explanation AnswerEExp::The statement is true for first-order reactions. In zero-order reactions, the eliminated amount is constant regardless of plasma concentrations.
Right Answer RightAnswer::D
Explanation [[Explanation::

Phenytoin follows zero-order elimination kinetics. Drugs with zero-order kinetics have a constant rate of elimination regardless of the plasma concentration of the drug itself. As a result, the plasma concentration of phenytoin falls linearly as a function of time. These kinetics are characteristic of capacity-limited elimination. The elimination is constant because it is limited by the capacity of the channels. In converse, first-order elimination is proportional to the drug concentration, where a specific ratio (not amount) of the drug is eliminated per unit time. First-order elimination is called flow-dependent elimination. It is dependent on the flow of a certain drug. As the plasma concentration of a drug drops, the flow across a certain channel decreases and so does the amount of drug eliminated.
Educational Objective: Phenytoin follows zero-order elimination and rate of elimination is constant per unit time.
References: Katzung BG, Masters S, Trevor A. Basic and Clinical Pharmacology 12/E. Chapter 3. Pharmacokinetics & Pharmacodynamics: Rational Dosing & the Time Course of Drug Action. McGraw Hill Professional; 2012.]]

Approved Approved::Yes
Keyword WBRKeyword::Plasma concentration, WBRKeyword::Phenytoin, WBRKeyword::Zero order elimination, WBRKeyword::Capacity-limited elimination, WBRKeyword::Flow-dependent elimination
Linked Question Linked::
Order in Linked Questions LinkedOrder::