In a teaching session, Chronic obstructive pulmonary disease is correctly explained as:

A. A macrocytic anaemia caused by impaired DNA synthesis, often from folate or vitamin B12 deficiency. B. Glomerular inflammation causing haematuria, reduced filtration and often hypertension. C. Hyperglycaemia with ketonaemia and metabolic acidosis due to severe insulin deficiency. D. Persistent airflow limitation commonly related to smoking or noxious exposure.