Dear Professor,

I am writing to apologize for my absence from your class yesterday. I was feeling unwell and had to visit the doctor. I am sorry for not informing you in advance and causing you any trouble.

I understand that this is not an excuse for missing your class and I take full responsibility for my actions. To make up for the missed lesson, I will read through the materials again and complete all the assignments on time. In addition, if there is anything extra that needs to be done, please let me know and I will do it as soon as possible.

Once again, I apologize for my absence yesterday and thank you for your understanding.

Sincerely yours,

XXX