A 62-year-old woman with a 20-year history of smoking, hypertension, and hyperlipidemia (LDL 180 mg/dL) undergoes coronary angiography for stable angina. The angiogram reveals a 60% stenosis of the left anterior descending artery with a smooth, concentric appearance and no thrombus. Intravascular ultrasound (IVUS) shows a plaque with predominantly smooth muscle cells, collagen-rich fibrous tissue, and minimal lipid content. Histopathological examination of a similar lesion from another patient shows organized layers of smooth muscle cells in a collagen matrix with a thick, intact fibrous cap. Which of the following best describes the stage of atherosclerotic plaque development in this patient?