For phenytoin all are true except?
**Core Concept**
Phenytoin is an antiepileptic medication that belongs to the class of hydantoin derivatives. It works by blocking voltage-gated sodium channels in the nervous system, which stabilizes the threshold against hyperexcitability caused by excessive stimulation.
**Why the Correct Answer is Right**
Phenytoin is primarily metabolized in the liver by the enzyme CYP2C9, and its metabolism is nonlinear due to its ability to inhibit its own metabolism. This leads to a phenomenon known as autoinduction, where the initial dose causes a significant increase in its own metabolism, resulting in a decrease in its plasma concentration over time. Additionally, phenytoin is also known for its "trough-to-peak" effect, where the time to reach steady-state plasma concentration is approximately 7-14 days.
**Why Each Wrong Option is Incorrect**
* **Option A:** Phenytoin is often used in the management of tonic-clonic (grand mal) seizures, absence seizures, and trigeminal neuralgia. This statement is correct, so it is not the correct answer.
* **Option B:** Phenytoin has a narrow therapeutic index, which means that the difference between the effective dose and the toxic dose is small. This statement is true, making it incorrect as the answer.
* **Option C:** Phenytoin can cause a rare but serious condition called "glycemic instability" in patients with diabetes. This statement is incorrect; the correct answer is related to another aspect of phenytoin pharmacology.
**Clinical Pearl / High-Yield Fact**
Phenytoin's nonlinear metabolism can lead to significant variability in plasma concentrations, which can result in either inadequate control of seizures or toxic effects.
**Correct Answer:** C.