A 35-year-old man presents with sudden-onset right eye pain, blurred vision, and a mid-dilated pupil that is poorly reactive to light. On examination, the right pupil is 5 mm and reacts sluggishly to direct light stimulus. The left pupil is 3 mm and reacts briskly. There is no afferent pupillary defect. Which investigation is most appropriate to confirm acute angle-closure glaucoma as the underlying cause of the pupillary abnormality?