A 45-year-old woman presents with wasting and weakness that initially affected her lower limbs and has now spread to her upper limbs. She experiences unsteadiness and clumsiness, particularly in the dark, and has numbness in her feet. These symptoms have been slowly progressing over the past decade. She reports no bladder issues and has no contact with her father, who reportedly had similar symptoms but to a lesser degree. On examination, her tone is flaccid, and there is weakness in the distal muscles of her legs, particularly in dorsiflexion and eversion of the ankles. There is also mild weakness in her arms, absent tendon reflexes, and mild blunting to pinprick over her feet bilaterally. Her cranial nerve examination is unremarkable. What is the most crucial step in investigating this patient’s condition?