# WBR0499

Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]] |
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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. |

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:: |