This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Heart, Circulation, and Blood, Subject name: Physiology
[{'content': 'Which point on the diagram marks the beginning of ventricular systole? \nSelect and explain. \nA. A, B. B, C. C, D. D', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Ans. A. APressure - volume loop of the left ventricle. During diastole, the ventricle fills and pressure increases from d to a. Pressure then rises sharply from a to b during isovolumetric contraction and from b to c during ventricular ejection. At c, the aortic valves close and pressure falls during isovolumetric relaxation from c back to d.Isovolumetric contraction starts at A therefore it is the beginning of ventricular systole.', 'role': 'assistant'}]