42. A patient has prostate cancer with bone metastasis and a history of seizures controlled with phenytoin. The oncologist orders dexamethasone for increased bone pain. Three weeks later, the patient has a seizure. The MOST likely cause of the seizure is
Answer: B
The most likely cause of the seizure is inhibition of phenytoin metabolism.
The patient's seizure is most likely due to the inhibition of phenytoin metabolism caused by dexamethasone. Dexamethasone can induce liver enzymes that metabolize phenytoin, leading to decreased levels of phenytoin and a consequent increase in seizure activity.
A) adverse reaction to dexamethasone
While dexamethasone has potential adverse effects, seizures are not a commonly recognized direct side effect of this medication. Instead, the issue arises from its interaction with phenytoin metabolism, making this option less likely as the primary cause of the patient's seizure.
B) inhibition of phenytoin metabolism
This option is correct as dexamethasone can affect liver enzymes that metabolize phenytoin, leading to reduced phenytoin levels in the blood. With lower levels of phenytoin, the patient is at a higher risk for seizures, which aligns with the timing and the patient's medical history.
C) dexamethasone metabolites
There is no significant evidence to suggest that dexamethasone metabolites would directly cause seizures. The primary concern in this case is the interaction between dexamethasone and phenytoin metabolism, rather than any metabolites of dexamethasone itself leading to seizure activity.
D) metastasis to the brain
While brain metastasis could lead to seizures, there is no indication in the patient's history or current situation suggesting new neurological symptoms or changes that would point to this as the cause. Moreover, the timing of the seizure occurring after the introduction of dexamethasone suggests a pharmacological interaction rather than a new metastasis.
Conclusion
In summary, the most plausible explanation for the patient's seizure is the inhibition of phenytoin metabolism by dexamethasone, which would lead to insufficient levels of the anticonvulsant medication. Other options, while theoretically possible, do not align as closely with the patient's history or the timing of the seizure, making option B the definitive choice.