An interrogative pronoun is a questioning pronoun, used when asking. For instance the five Ws - who, what, where, when, why. In this sentence - How could that be you moron? - the interrogative pronoun is 'How'.
(I couldn't think of a more suitable question, and I apologise for the use of the word moron - In no way are you a moron I just used it in the question)! :)
