Since it was obvious that Avraham's declaration (regarding Sarah) was due to his fear that they would kill him, why did Pharaoh ask him why he did not tell him that she was his wife?
Seforno: Pharaoh was saying that, even if Avraham suspected the people of such base behavior, he should not have suspected the king.
Why did Pharaoh now accuse Avraham of having lied? What convinced him that Avraham was to blame for the plagues?
Ramban #1: The plagues caused him to wonder what was happening to him; so he asked Sarah to tell him the truth, and she told him that she was Avraham's wife.
Ramban #2: Alternatively, the nature of the plague (that rendered intimacy difficult to perform), led him to suspect that Sarah was Avraham's wife, and he confronted Avraham, based on that suspicion, to see whether he would insist that she was indeed his sister, or whether she was actually his wife. And when Avraham, out of fear, remained silent, he realized that she was his wife.