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Phenytoin zero-order kinetics — MRCPCH TAS MCQ

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HardPharmacologyPhenytoin zero-order kineticsMRCPCH TAS

A 12-year-old with epilepsy has ataxia and nystagmus after a small dose increase. Plasma concentration has risen disproportionately compared with the dose change. What is the most likely explanation?

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Correct answer: A — Saturable hepatic metabolism

Phenytoin displays saturable hepatic metabolism, so small dose increases can produce disproportionate rises in concentration. Autoinduction is more typical of carbamazepine and would tend to lower concentrations over time. The pearl is that phenytoin has a narrow therapeutic index and non-linear kinetics.

Reference: BNFc; Ganong's Review of Medical Physiology; RCPCH Theory examination syllabi