A 55-year-old man is evaluated for a 2-week history of visual symptoms in the left eye. He reports seeing black spots that move across his eye and flashes of light, followed by a progressive loss of vision over the lateral field, as if half of his vision is covered by a shadow. He reports no headaches or trauma. He does not wear contact lenses; however, he does wear glasses for the correction of myopia. He is a nonsmoker. Medical history is otherwise unremarkable, and he takes no medications.

On physical examination, vital signs are normal. BMI is 28. Visual acuity is intact. The pupils appear normal and are equally reactive to light. On dilated ophthalmoscopy, the left fundus is undulating and out of focus. There is no retinal pallor, and there are no cotton wool spots or hemorrhages. The remainder of the physical examination is normal.

Which of the following is the most likely diagnosis?