This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Cardiovascular system, Subject name: Physiology
True about stroke volume is 
Select and explain. 
A. Decreases by increase in hea rate, B. Determined by pre-diastolic volume, C. Determined by afterload, D. Is equal to cardiac output
A: Decreases by increase in hea rate
Answer: A: Decreases by increase in hea rate 
Explanation: Stroke volume is an impoant determinant of cardiac output, which is the product of stroke volume and hea rate and is also used to calculate ejection fraction, which is stroke volume divided by end-diastolic volume. So to maintain cardiac output it decreases if hea rate increases and vice versa.