Select the best definition of Atrial fibrillation.

A. A supraventricular arrhythmia with disorganised atrial activity and an irregularly irregular pulse. B. Acute infection of the meninges requiring urgent antimicrobial treatment. C. A microcytic anaemia caused by inadequate iron availability for haemoglobin synthesis. D. A macrocytic anaemia caused by impaired DNA synthesis, often from folate or vitamin B12 deficiency.