Final answer:
The most critical information for a nurse to obtain for a child with new-onset tonic/clonic seizures is the family medical history to identify any genetic predisposition or related conditions.
Step-by-step explanation:
The most important information for the nurse to obtain when a 3-year-old patient is admitted to the pediatric unit for evaluation of recurrent episodes of new-onset tonic/clonic seizures is a) Family medical history. This is crucial because it helps in identifying any genetic predisposition to seizures or epilepsy, as well as other familial conditions that could be related to the seizures. A thorough medical history can also reveal potential environmental factors, past medical events, or developmental issues that may be contributing to the seizure activity.