For 3 months, a 66-year-old retired man has had increasingweakness of the lower legs with stiffness,tingling, and numbness; worsening ataxia; anergia; andexertional dyspnea of insidious onset. He has lost 8 lb,and his appetite is poor. He denies fever, cough, chest orabdominal pain, paroxysmal nocturnal dyspnea, orthopnea,ankle swelling, bleeding disorders, hematemesis,melena, headache, vision problems, sciatica, joint pain,bladder or bowel dysfunction, and GI symptoms. He hasnocturia attributable to benign prostatic hypertrophy.