A 66-year-old man who lives outdoors presents to the clinic because of numbness in his hands and feet for several weeks. He states the numbness started in his fingertips and toes alone but has now spread to his entire hands and feet. He does not take any regular medications. The patient has been homeless for years, and he denies any alcohol or drug use. Review of systems is significant for a sore tongue. His temperature is 37deg C (98.6deg F), blood pressure is 100/65 mm Hg, pulse is 102/min, respiratory rate is 20/min, and oxygen saturation is 97% on room air. On physical examination, the patient is alert but seems a little confused and forgetful.
He is thin with a BMI of 19 kg/m?. His tongue appears erythematous and smooth with loss of papillae, but no lesions or evidence of infection are noted. A cardiac examination is normal except for tachycardia. Pulmonary and abdominal exams are unremarkable. Neurologic findings include decreased 2-point discrimination in both hands and feet, 4/5 symmetrical motor strength in the hands and feet, and absent ankle reflexes. Gait is slightly wide-based and ataxic, with a positive Romberg sign. What is the most likely cause of this patient's symptoms?