A 23-year-old man notices new low back pain, stiffness, and left eye discomfort. Sunlight also bothers his eyes. The back pain is worse at night and described as a dull ache in the back and buttock area. On physical examination, there is paravertebral muscle, iliac crest, and ischial tuberosity tenderness with limited flexion of the lumbar spine. His eye is inflamed and the pupil is constricted. Pelvic x-rays show sacroiliitis. Which of the following is the most likely diagnosis for his eye symptoms (it is the most common extra-articular manifestation of this condition)?