Which of the following is a sign of phenytoin toxicity?

• A. Ataxia
• B. Rash
• C. Nystagmus
• D. Drowsiness

Answer: (A)

Phenytoin toxicity shows up in the nervous system, and cerebellar dysfunction is a hallmark. Ataxia, or uncoordinated movement, directly reflects cerebellar involvement from high phenytoin levels and is a classic, specific sign of toxicity. Nystagmus and drowsiness can occur with excess drug too, but they are either less specific or more nonspecific events. Rash points to a hypersensitivity reaction rather than dose-related neurotoxicity, and while drowsiness can accompany overdose, it doesn’t point as clearly to toxicity as ataxia does. So, the best indicator of phenytoin toxicity among these options is ataxia.