Why does the Pasuk see fit to mention that Hashem plagued Avimelech 'on account of Sarah'?
Rashi: "Al-D'var Sarah does not mean 'on account of Sarah', it means 'by her word' - that He did so following Sarah's request. 1
Why does the Torah reveal which plague came on Avimelech, but not which came on Pharaoh?
Moshav Zekenim: Sarah gave birth after Avimelech took her, so it needed to teach that Hashem had closed all openings, lest one think that he impregnated her. We learn that Hashem plagued Pharaoh similarly.