A 72-year-old woman from Mumbai with a history of myocardial infarction 3 months ago is admitted with acute chest pain. Her ECG shows ST elevation in leads II, III, and aVF (inferior STEMI). Cardiac catheterization reveals complete occlusion of the right coronary artery. During the acute phase of myocardial necrosis, the ischaemic zone shows a marked reduction in intracellular ATP and accumulation of intracellular Ca²⁺. Which phase of the cardiac action potential is most severely disrupted in this ischaemic tissue, and what is the primary ionic mechanism?