A 32‐year‐old man goes to his GP with a painful, red left eye. The pain has increased in intensity over 3 days and is worse looking at bright light. The eye is not sticky, watering or itchy and the vision is only slightly blurred. He has felt tired and has had a cry cough for the past few weeks with aching joints. The eye is globally red around the limbus and the pupil is small; dilating the pupil eases the pain but shows an irregular‐shaped pupil.