Mr Simpson is a 60 year old retired headteacher and attends with his wife who is concerned about his behaviour over the last few months. She describes him to be unmotivated which is out of character for him, yet says his mood is normal and sleep undisturbed. She says he has started to have repetitive behaviours, for example his morning routine before eating breakfast is almost like a ritual. She describes his empathy as ‘terrible’ and that dining out with friends has become awkward. His memory is intact and there is no history of head injury.
Based on the history alone, what is the most likely diagnosis?