isn't he a really sick puppy?
well yes he is. He has a personality disorder known as narcissistic personality disorder or NPD.
let me quote
Persons with narcissistic personality disorder (NPD) are characterized by their persistent grandiosity, excessive need for admiration, and a personal disdain for, and lack of empathy for other people. As such, the person with NPD usually displays the behaviors of arrogance, a sense of superiority, and actively seeks to establish abusive power and control over other people. Narcissistic personality disorder is a condition different from self-confidence (a strong sense of self); people with NPD typically value themselves over other persons to the extent that they openly disregard the feelings and wishes of others, and expect to be treated as superior, regardless of their actual status or achievements. Moreover, the person with narcissistic personality disorder usually exhibits a fragile ego (self-concept), an inability to tolerate criticism, and a tendency to belittle others in order to validate their own superiority.
From your perspective the delusions created by Mr Trump would be lies. From his perspective, they are a reflection of his mental state which maintains he is the greatest whatever to have lived. Failure to maintain this delusion is a major assault on his personality which can not be tolerated, therefore he would "lie" in order to maintain it.
Merely asking any garden variety question would be useless since the answers for many would inherently be lies and therefore of no probative value. SP Mueller probably knows this and would only ask questions, the answers of which would be invariant and therefore useful.
This is a little more tricky than a perjury trap.