Final answer:
For a patient taking phenytoin and experiencing health issues, the presence of ataxia, slurred speech, and nystagmus is the most concerning as it may indicate phenytoin toxicity.
Step-by-step explanation:
The most concerning patient observation for someone taking phenytoin with tonic clonic epilepsy and not feeling well would be ataxia, slurred speech, and nystagmus. These symptoms may indicate phenytoin toxicity, which is a medical emergency. Phenytoin toxicity may present with these neurological symptoms due to its effect on the central nervous system.
The other listed observations, while potentially associated with phenytoin use or other conditions, are not as immediately concerning as the potential neurotoxic effects manifested by ataxia, slurred speech, and nystagmus.