This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
True regarding myocardial O2 demand? 
Select and explain. 
A. Inversely related to heart rate, B. Has constant relation to external cardiac work, C. Directly proportional to duration of systole, D. Is negligible at rest
C: Directly proportional to duration of systole
Answer: C: Directly proportional to duration of systole 
Explanation: Oxygen demand depends on the work being done. The work done is to pump the blood which is in the ventricles (preload) against the resistance in the aorta (afterload).
 So, myocardial oxygen demand depends on

Rate - Directly proportional
Contractility - Directly proportional
Intramyocardial tension - Directly proportional
After load - Directly proportional
Preload - Directly proportional
Wall thickness - Inversely proportional

The oxygen demand of myocardium is not having the same change with varying external works.