A 68-year-old man with chronic obstructive pulmonary disease presents to the emergency department with a 3-day history of worsening shortness of breath and productive cough. He has no history of vomiting, diarrhea, or diabetes. On examination, he is using accessory muscles to breathe and appears confused. Temperature is 37.8°C (100°F), pulse is 102/min, and respirations are 24/min. Lung examination reveals decreased breath sounds bilaterally with expiratory wheezes. Arterial blood gas analysis on room air shows pH 7.28, PCO₂ 68 mmHg, HCO₃⁻ 28 mEq/L, and PO₂ 52 mmHg.
Which of the following is the primary acid-base disorder?