Select the best definition of Iron-deficiency anaemia.

A. Obstruction of pulmonary arteries, commonly by thrombus originating in deep veins. B. A supraventricular arrhythmia with disorganised atrial activity and an irregularly irregular pulse. C. A multisystem autoimmune disease associated with diverse autoantibodies and organ involvement. D. A microcytic anaemia caused by inadequate iron availability for haemoglobin synthesis.