A 55-year-old male underwent partial gastrectomy for gastric cancer 5 years ago. He was admitted with a 2-month history of numbness in both upper limbs. Physical examination revealed: mild anemic appearance, no abnormal cranial nerve signs, normal muscle strength and tone in upper limbs, biceps reflex (++), muscle strength grade IV in lower limbs with slightly increased tone, knee reflexes (+++), left Babinski sign (+), right Babinski sign (-), and decreased pain and vibration sensation in all extremities (distal to wrists and knees). What is the preferred diagnostic test for this patient?