One hour after a laparoscopic appendectomy, a 25-year-old woman is receiving intravenous patient-controlled analgesia with hydromorphone. Her SpO2 is 90% on room air. Which of the following is the MOST likely cause of the patientís low SpO2?

A) Hypoventilation
B) Increased dead space
C) Impaired oxygen diffusion
D) Residual pneumoperitoneum