A middle aged gentleman, with background history of chronic obstructive pulmmonary disease, hypertension, obstructive sleep apnea, type 2 diabetes mellitus, smoker of 30 pack years history, was admitted to hospital due to sudden onset of disorientation for 30 minutes and was unable to remember the event. According to his wife, they were out shopping for grocery at around noon time. During shopping, his wife suddenly noted the husband became a bit agitated, with some confusion and disorientation. There was no weakness on either of his right or left sided of the limbs, no slurred slurred speech, no facial droop, no ataxia. The wife brought him to nearby coffee shop to sit down for a while. There was a spontenous recovery after 30 minutes, but patient was not able to recall the event. What will be your differential diagnosis ???