A 25-year-old male gives a history of redness, pain and mild diminution of vision in one eye for past 3 days. There is also a history of low backache for the past one year. On examination there is circumcorneal congestion, cornea is clear apa from a few fine keratic precipitates on the corneal endothelium, there are 2+ cells in the anterior chamber and the intraocular pressure is within normal limits. The patient is most likely suffering from