A 58-year-old man presents to the ophthalmology clinic with a 3-day history of right eye pain and blurred vision. On examination, the right pupil is mid-dilated (5 mm) and fixed to light, while the left pupil reacts normally. Extraocular movements are intact. Visual acuity is 6/9 in both eyes. Slit-lamp examination reveals ciliary injection and keratic precipitates. Intraocular pressure is 42 mmHg in the right eye. What is the most likely explanation for the pupillary finding?