A 25–year old male gives a history of redness, pain and mild diminished vision in one eye for past 3 days. There is also a history of a low backache for the past one year. On examination, there is circumcorneal congestion, the cornea is clear apart 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 –