Good job on passing the class, and they are being honest not sadistic. You've got to expect answers like that if you're going to post on SDN. lmao
Just tell the interviewers what happened. Explain that the professor had a drinking problem (but do not make it sound like you are whining) but be prepared to answer questions about why you took it again with the same professor, twice nonetheless, when you knew that he was not very good. Then also tell them that you were sick (are we talking like mono sick or oh I had a flu for one week sick?). Tell them that the class was difficult for you and but that you were determined to overcome the challenge and do well. Explain how you didn't give up even when you got discouraged and finally managed to not only pass, but to get an A! Make it a story about over coming an obstacle or a hardship and I think it will go over a bit better.
Good Luck!