When a patient is having difficulty maintaining balance during a gait assessment, the nurse should perform a neurological assessment as the priority physical assessment. This is because difficulty with balance can be a symptom of a neurological problem, such as a stroke or Parkinson's disease. The neurological assessment should include an evaluation of the patient's level of consciousness, cranial nerve function, motor function, sensory function, and reflexes. This will help the nurse to identify any neurological deficits that may be contributing to the patient's difficulty with balance, and to develop an appropriate plan of care.