HY USMLE Q #936 – Respiratory

A 65-year-old man with chronic bronchitis is admitted to emergency because of worsening shortness of breath over the past 2 hours. Blood pressure is 115/70. Pulse is 90. Respirations are 30/minute.  An arterial blood gas on room air is shown:

pH                                                                                         7.21
pCO2                                                                                  78 mmHg (NR 33-44)
pO2                                                                                      40 mmHg (NR 80-100)
HCO3^–                                                                               33 mEq/L

Which of the following is the most likely explanation for this patient’s findings?