WBR0499
| 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. |
| 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 |
| Linked Question | Linked:: |
| Order in Linked Questions | LinkedOrder:: |