My father aged 75 has recently showing memory issues. He remains silent most of the time. If asked any event of today or yesterday, he gives correct answers. His memory is gone from patches. Some very old memories he remembers, some new he remembers but most of the past events he doesn't remember He sometimes don't recognize his children (most of the time he recognizes). He sometimes feel difficulty in following the instructions. No diabetes at all. BP almost 150. What can be the possibility?