mrcpsych-0617

A 79-year-old man has been out of contact with his daughter for several weeks whilst she has been on vacation. Upon her return, she finds him confused and unclothed on the couch in his apartment. He is unsteady on his feet and unable to recall how long he has been there, suggesting that he has also just returned from vacation. He points to rats on the floor of his apartment, which his daughter cannot see. He is taken to the hospital, where he is diagnosed with ataxia and ophthalmoplegia. Which of the following conditions would make this man more susceptible to the most probable diagnosis?