Q. A 75-year-old man is brought to the clinic by his wife because he has been very forgetful for the past couple of years. He easily loses items and has trouble finding them. He forgets where he parks his car when going to the mall. Lately, he has started to struggle to recall the names of certain friends (which is unlike him). Last month he forgot how to get back home when he went to the supermarket to buy milk. He does not smoke or drink alcohol and his vital signs are within normal limits. On clinical examination, he has a flat affect. Mini-mental state examination score is 16/30. He does not have any motor rigidity and his gait appears normal for his age. MRI of the brain demonstrates cortical atrophy and prominent sulci. The patient has a significant history of diarrhea and wants to avoid further gastrointestinal upset "at all cost". Which of the following medications makes the most sense to try?