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{{WBRQuestion
{{WBRQuestion
|QuestionAuthor={{Rim}}
|QuestionAuthor={{Rim}} {{Alison}}
|ExamType=USMLE Step 1
|ExamType=USMLE Step 1
|MainCategory=Pharmacology
|MainCategory=Pharmacology
Line 20: Line 20:
|MainCategory=Pharmacology
|MainCategory=Pharmacology
|SubCategory=Neurology
|SubCategory=Neurology
|Prompt=A pharmacologist is studying the metabolism of various drugs.  He conducts an experiment using mice and injects intravenous (IV) phenytoin in their blood.  Immediately following the administration of phenytoin, he measures the plasma concentration of phenytoin. Then, he remeasures phenytoin plasma concentrations 2 hours, 4 hours, and 6 hours after infusion.  Which of the following statements is true regarding the elimination of phenytoin?
|Prompt=A pharmacologist, studying the metabolism of various drugs, conducts an experiment injecting intravenous (IV) phenytoin in the blood of mice.  Immediately following the administration of phenytoin, he measures the plasma concentration of phenytoin. Then, he remeasures phenytoin plasma concentrations 2 hours, 4 hours, and 6 hours after infusion.  Which of the following statements is most true regarding the elimination of phenytoin?
|Explanation=Phenytoin follows a zero-order elimination.  This is characterized by the constant loss of phenytoin's plasma amounts per unit time. As a result, the amount of plasma concentration of phenytoin falls linearly per time in a capacity-limited elimination.
|Explanation=[[Phenytoin]], which follows a zero-order elimination, is characterized by the constant loss of plasma levels per unit time. The plasma concentration of phenytoin decreases linearly in a capacity-limited elimination.


In converse, the first-order elimination process 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.
The first-order elimination process is proportional to the drug concentration, where a specific ratio of the drug is eliminated per unit time.  First-order elimination is referred to as flow-dependent elimination.
 
|EducationalObjectives= [[Phenytoin]], which follows a zero-order elimination, is characterized by the constant loss of plasma levels per unit time. The plasma concentration of phenytoin decreases linearly in a capacity-limited elimination.
|References= First Aid 2014 page 240


Educational Objective: Phenytoin follows zero-order elimination and rate of elimination is constant due to constant amount  loss per unit time.
|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 only true for first-order elimination. [[Phenytoin]] 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 where a constant ratio 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]] follows a zero-order elimination, the same amount (not ratio) of drug is 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
|AnswerCExp=The rate of elimination is constant regardless of concentration.
|AnswerCExp=The rate of elimination is constant regardless of concentration.
|AnswerD=Plasma concentration levels of phenytoin fall linearly with time
|AnswerD=The plasma concentration of [[phenytoin]] falls linearly with time.
|AnswerDExp=Because a constant amount of phenytoin is eliminated per unit time, plasma concentration levels falls linearly.
|AnswerDExp=Because a constant amount of phenytoin is eliminated per unit time, plasma concentration levels falls linearly.
|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]] lessens 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 only true for first-order reactions.  In zero-order reactions, the eliminated amount is constant regardless of plasma concentrations.
|RightAnswer=D
|RightAnswer=D
|WBRKeyword=plasma, concentration, phenytoin, zero, order, elimination, first, rate, capacity-limited, flow-dependent, amount, ratio, percentage, fraction
|WBRKeyword=plasma concentration, phenytoin, zero order elimination, capacity-limited, flow-dependent, ratio, percentage, linear pattern,
|Approved=No
|Approved=Yes
}}
}}

Revision as of 13:18, 30 July 2014
Author [[PageAuthor::Rim Halaby, M.D. [1] (Reviewed by Alison Leibowitz)]]
Exam Type ExamType::USMLE Step 1
Main Category MainCategory::Pharmacology
Sub Category SubCategory::Neurology
Prompt [[Prompt::A pharmacologist, studying the metabolism of various drugs, conducts an experiment injecting intravenous (IV) phenytoin in the blood of mice. Immediately following the administration of phenytoin, he measures the plasma concentration of phenytoin. Then, he remeasures phenytoin plasma concentrations 2 hours, 4 hours, and 6 hours after infusion. Which of the following statements is most true regarding the elimination of phenytoin?]]
Answer A AnswerA::Phenytoin follows a flow-dependent elimination
Answer A Explanation [[AnswerAExp::This is only true for first-order elimination. Phenytoin follows a capacity-limited elimination.]]
Answer B AnswerB::Phenytoin follows a zero-order elimination where a constant ratio of the drug is eliminated per unit time
Answer B Explanation [[AnswerBExp::Although phenytoin follows a zero-order elimination, the same amount (not ratio) of drug is 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::The plasma concentration of phenytoin falls 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 lessens over time]]
Answer E Explanation AnswerEExp::The statement is only 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, which follows a zero-order elimination, is characterized by the constant loss of plasma levels per unit time. The plasma concentration of phenytoin decreases linearly in a capacity-limited elimination.

The first-order elimination process is proportional to the drug concentration, where a specific ratio of the drug is eliminated per unit time. First-order elimination is referred to as flow-dependent elimination.
Educational Objective: Phenytoin, which follows a zero-order elimination, is characterized by the constant loss of plasma levels per unit time. The plasma concentration of phenytoin decreases linearly in a capacity-limited elimination.
References: First Aid 2014 page 240]]

Approved Approved::Yes
Keyword WBRKeyword::plasma concentration, WBRKeyword::phenytoin, WBRKeyword::zero order elimination, WBRKeyword::capacity-limited, WBRKeyword::flow-dependent, WBRKeyword::ratio, WBRKeyword::percentage, WBRKeyword::linear pattern
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