1. D. This is a rather specific effect of phenytoin. Although the gum tissue can be cut back and in some cases overgrowth prevented with good oral hygiene, this is a source of embarrassment to the patient and constitutes a deterrent to the use of this agent.
2. B. Phenytoin is one of a handful of drugs that demonstrates zero-order (or saturation) kinetics. If a patient is showing signs of toxicity to phenytoin, it is important to measure blood levels, since the likelihood that phenytoin is demonstrating zero-order kinetics is very high.
3. A. Ethosuximide has no effect on blocking the sodium channel at therapeutics doses; however, it is very effective in blocking the T-calcium current in the therapeutic dose range. All other choices block the sodium channel at therapeutic doses, and it is acknowledged that this is their sole (or major) mechanism of action.
4. C. The only drug listed that would be expected to offer no benefit to a patient with absence seizures is carbamazepine. In fact, there is clinical evidence that it may actually increase the incidence of absence seizure episodes.
5. C. Tiagabine is the first agent that has been shown to elevate GABA levels by inhibiting the reuptake of this neurotransmitter at neuronal and glial sites in the brain. Vigabatrin can also elevate GABA levels, but it does so by inhibiting the metabolism of GABA.
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