Select the best definition of Pulmonary embolism.

A. A macrocytic anaemia caused by impaired DNA synthesis, often from folate or vitamin B12 deficiency. B. A supraventricular arrhythmia with disorganised atrial activity and an irregularly irregular pulse. C. Acute infection of the meninges requiring urgent antimicrobial treatment. D. Obstruction of pulmonary arteries, commonly by thrombus originating in deep veins.