Which of the following describes a complicated lesion in ACS?

A complicated lesion in acute coronary syndrome (ACS) refers specifically to a fibrous cap over a fatty lesion. This is crucial because a complicated lesion represents the advanced stage of atherosclerosis, where the plaque has not only formed but has also become unstable. The fibrous cap is made of connective tissue and separates the lipid-rich core of the plaque from the blood. When the fibrous cap is intact, it can help prevent thrombosis. However, if the cap ruptures or erodes, it can lead to the exposure of thrombogenic materials in the core, resulting in acute thrombus formation and potentially causing an acute coronary event.

The other options do not accurately describe a complicated lesion. A gradual narrowing of arteries without thrombosis suggests stable conditions rather than complications associated with ACS. Complete resolution of plaque without sequelae implies a return to normal vascular function, which is not characteristic of complicated lesions where significant pathology exists. Lastly, the formation of new arteries around blocked ones describes collateral circulation, which may develop in response to chronic ischemia but does not indicate the presence of complicated plaques.