The Patient Has Epilepsy and Receives Phenytoin (Dilantin)

The patient has epilepsy and receives phenytoin (Dilantin) . The patient has been seizure-free, and asks the nurse why he still needs blood tests when he is not having seizures. What is the best response by the nurse?

A) "Because phenytoin (Dilantin) has a very narrow range between a therapeutic dose and a toxic dose."
B) "Because phenytoin (Dilantin) can cause blood-thinning in some patients."
C) "Because phenytoin (Dilantin) can cause Stevens-Johnson syndrome, which will show up in the blood tests."
D) "Because phenytoin (Dilantin) can deplete your system of potassium."