A 74 year old woman, in otherwise good health, tripped and injured her right leg 2 days previously and has been bedridden since the accident. Two hours ago, she became delirious. On physical examination, her temperature is 99 F, blood pressure is 120/70 mmHg, hea rate is 110, and respiratory rate is 32. Pulse oximetry shows an oxygen saturation of 80%, and a chest x-ray film is normal. Which of the following is the most likely diagnosis?