A 48-yeаr-оld pаtient with hypertensiоn аnd depressiоn, taking an oral ACE inhibitor and sertraline, presents with a sudden left-sided headache that is most painful in and behind her left eye. The patient's vision is blurry, and the left pupil is slightly dilated and poorly reactive. The left conjunctiva is markedly injected, with ciliary flush, and the eyeball is firm. Vision screen with the Snellen chart is 20/30 OD and 20/90 OS. The most likely diagnosis is