The MPRE and your professional responsibility course are two different things.
How did you do in class participation? Were you active in class or did you pull attitude and brag to everyone how much of an expert you were?
Your Prof might have thrown you some curveballs on your exam and instead of choosing the correct answers, you made the predictable errors. It happens.
For example, my K class. I was active in class and knew the material. People were asking to study with me. I was getting calls from classmates I barely spoke to asking me questions and getting into discussions about K. The prof sent me an email after the final thanking me for being active in class and telling me that I was always prepared, always willing to tackle the tough hypos and that he knew he could rely on me to provide thoughtful insight. I KNEW the material cold. My Prof THANKED me for working so hard and told me that he was proud of me and that he was sure I would make a fantastic lawyer. He sent this same email out to six of us, all of whom were what he considered the top of the class.
I got a C+ on the final.
Why? Because like many law profs out there, he asked questions that were designed to trip us up. Law Prof's, more so than any other kind of teacher that I know of, are egotistical and geniuses in their own minds. If they CAN write a question that they think most of the class will miss, you can be sure that it is going to be on the test.
Take your lumps gracefully and try not to come off as a whiny arrogant jerk in your emails.
Since you've already passed the MPRE, try not to let one C ruin your attitude and make you bitter.
I would guess that this may be one of the lowest grades you've gotten in law school.
Everyone gets one bad grade. You'll be fine.
Keep your head up!