CNS Drugs · NEET PG Preparation Pharmacology
An epileptic patient maintained on phenytoin develops acute cerebellar ataxia, nystagmus, and diplopia. Serum drug level is found to be significantly…
Practice question from CNS Drugs (NEET PG Preparation Pharmacology), with the correct answer and a step-by-step solution.
medium · MCQ · Q-742A35
An epileptic patient maintained on phenytoin develops acute cerebellar ataxia, nystagmus, and diplopia. Serum drug level is found to be significantly elevated. Which kinetic profile best explains why a small increment in the phenytoin dose can lead to a disproportionately large, toxic increase in serum concentration?
- A.Zero-order elimination kineticsCorrect answer
- B.First-order elimination kinetics
- C.Renal tubular secretion saturation
- D.Rapid induction of hepatic CYP3A4
Solution
At therapeutic concentrations, phenytoin elimination transitions from first-order to zero-order (saturation kinetics) because the hepatic enzymes responsible for its metabolism become saturated. Consequently, small dosage increases can saturate the remaining capacity, leading to dramatic, non-linear increases in plasma levels.
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