A 78-year-old woman becomes agitated and repeatedly pulls at her intravenous line on the second night after hip surgery. She alternates between asking where she is and speaking coherently, and her daughter says she was independent and managing finances before admission. She has received oxycodone and oxybutynin since surgery. On examination, she cannot recite the months backwards and her level of alertness changes during the interview. Which one of the following is the most likely diagnosis?