Q. A 64 year old man presents to your office with complaints of tremor for the past several years. He first noticed the tremor in his right hand but it has now progressed to his left. It would be present when he was watching television and his wife had commented on it before he was even aware it existed. Although at first he denies any other issues, he does report tripping over his feet a few times in the past several months. In addition, people at work have told him that he seems to be "flat" and have asked him if he is depressed. He has noticed some "slip ups" at work during which he forgot certain tasks. His has no history of medical problems. Temperature is 36.7 (98.7 F), Blood Pressure is 130/80, Heart Rate is 72 beats/minute. On neurological exam, his mental status demonstrates some minor memory difficulties and you notice a flattening of his affect, his cranial nerve exam is normal, his motor exam demonstrates asymmetric rigidity, and a tremor that is more pronounced in his right hand than in his left and has a pill rolling quality. He is slow to get up from the chair and demonstrates some hesitation prior to initiating walking. You sent him for a complete blood count complete metabolic profile, TSH, and vitamin B 12 level which are all normal. MRI of the brain is normal. You suspect the patient has Parkinson's disease and start him on a trial of carbidopa-levodopa. Which of the following is the LEAST likely potential side effect?