What is the most likely reason a client has experienced a proliferation of atherosclerotic plaque in his/her coronary arteries?

The most likely reason for the proliferation of the atherosclerotic plague in his coronary arteries is the elevated blood cholesterol level. The development of the foam cells is promoted by the increased levels of the blood cholesterol. The foam cells, in turn, promotes the development of the atherosclerotic plague. The decrease in the oxygen supply leads to the development of the variant angina.

Hence, the answer is 'increased blood cholesterol level'.