A 68-year-old woman with a history of left ventricular congestive heart failure has had decreased visual acuity for the past 5 years. She has no ocular pain. Her intraocular pressure is normal. Findings on funduscopic examination include arteriolar narrowing, flame-shaped hemorrhages, cotton-wool spots, and hard, waxy exudates. Which of the following underlying diseases is she most likely to have?