A patient with chronic bronchitis becomes confused and drowsy after several days of increased sputum production. What is the most likely cause?

In chronic bronchitis, a component of COPD, the airways produce excessive mucus and are chronically obstructed, impairing gas exchange. During exacerbations marked by increased sputum production, ventilation worsens significantly, causing carbon dioxide retention — a condition called hypercapnia. Elevated PaCO2 directly depresses central nervous system function by causing respiratory acidosis, leading to neurological manifestations such as confusion, somnolence, and eventually coma — a state sometimes called CO2 narcosis. This is the most likely explanation for the sudden onset of neurological changes in this clinical context. Dehydration can cause confusion in some patients but would not specifically accompany increased sputum production and worsening bronchitis; it would more likely cause dry mucous membranes and concentrated urine. Anxiety typically causes agitation and hyperventilation rather than drowsiness and confusion. Hypoglycemia is possible in diabetic patients but there is no clinical cue in the stem to suggest diabetes or a hypoglycemic event, and it would not be directly linked to increased sputum production. The temporal association between respiratory deterioration and the onset of neurological depression makes hypercapnia the most physiologically sound and clinically relevant answer.