WBR0499
| Author | [[PageAuthor::Yazan Daaboul, M.D. (Reviewed by Serge Korjian)]] |
|---|---|
| Exam Type | USMLE Step 1 |
| Main Category | Pharmacology |
| Sub Category | Neurology |
| 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 | Phenytoin follows a flow-dependent elimination |
| Answer A Explanation | This is true for first-order elimination. Phenytoin on the other hand follows a capacity-limited elimination. |
| Answer B | Phenytoin follows a zero-order elimination whereby a constant proportion of the drug is eliminated per unit time |
| Answer B Explanation | Although phenytoin indeed follows a zero-order elimination, the same amount (not the ratio) of drug is constantly eliminated per unit time. |
| Answer C | The rate of phenytoin elimination is directly proportional to the drug's concentration |
| Answer C Explanation | The rate of elimination is constant regardless of concentration. |
| Answer D | Plasma concentration levels of phenytoin fall linearly with time |
| Answer D Explanation | Because a constant amount of phenytoin is eliminated per unit time, plasma concentration levels falls linearly. |
| Answer E | The amount of eliminated drug is gradually decreased as the plasma concentration of phenytoin is deceased over time |
| Answer E Explanation | The statement is true for first-order reactions. In zero-order reactions, the eliminated amount is constant regardless of plasma concentrations. |
| Right Answer | 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 | Yes |
| Keyword | Plasma concentration, Phenytoin, Zero order elimination, Capacity-limited elimination, Flow-dependent elimination |
| Linked Question | |
| Order in Linked Questions |