A 35-year-old man comes to the office because of two months of progressive fatigue, poor appetite, and numbness in his hands and feet. He works at a glass factory and has been experiencing a painful burning sensation in his distal extremities and weakness while gripping tools for the past two weeks. The rest of the review of systems is unremarkable. Past medical history is noncontributory. He has been taking homemade herbal medications for the last 5-years. Vitals are within normal limits. Physical examination reveals areas of hypopigmentation and hyperpigmentation over the neck. There is a thickening of the skin on the palms and soles. Neurologic examination shows decreased sensation in the hands and feet. Upper and lower extremity
deep tendon reflexes are 2/5. Laboratory results are shown below. Which of the following is the most likely etiology of this patient’s clinical presentation?