This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ECG and Arrhythmia 2, Subject name: Medicine
[{'content': 'A 50 year old man presents with central chest pain. He collapsed while his ECG was being recorded. What is the cause of this ECG? \nSelect and explain. \nA. R on T phenomenon, B. Macro-reentrant Circuit, C. Early after depolarization, D. AV nodal Re-entry', 'role': 'user'}, {'content': 'Answer: A \nExplanation: ECG shows ST segment changes in lead II, III indicating inferior wall MI (BLUE ARROW) Ventricular extra-systole after the first beat (RED MARK) In the fouh beat, ventricular Extra-systole (YELLOW) is occurring at time of T wave triggering a polymorphic Ventricular tachycardia. The "R-on-T phenomenon" is the superimposition of an ectopic beat on the T wave of a preceding beat which triggers Polymorphic VT.', 'role': 'assistant'}]