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 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 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 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 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. 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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