WBR0499

Author	[[PageAuthor::Rim Halaby, M.D. [1]]]
Exam Type	ExamType::USMLE Step 1
Main Category	MainCategory::Pharmacology
Sub Category	SubCategory::Neurology
Prompt	[[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?]]
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 ratio 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 fall 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 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. 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. Educational Objective: Phenytoin follows zero-order elimination and rate of elimination is constant due to constant amount loss per unit time. Educational Objective: References: ]]
Approved	Approved::No
Keyword	WBRKeyword::plasma, WBRKeyword::concentration, WBRKeyword::phenytoin, WBRKeyword::zero, WBRKeyword::order, WBRKeyword::elimination, WBRKeyword::first, WBRKeyword::rate, WBRKeyword::capacity-limited, WBRKeyword::flow-dependent, WBRKeyword::amount, WBRKeyword::ratio, WBRKeyword::percentage, WBRKeyword::fraction
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