A 60-year-old woman presents with a painful, red eye and blurred vision that started acutely 2 hours ago. She saw haloes around lights before the vision deteriorated. On examination, the conjunctiva is injected, the cornea appears hazy, and the pupil is mid-dilated and poorly reactive to light. Intraocular pressure measured by tonometry is 55 mmHg (Reference range 10-21 mmHg). What is the most likely diagnosis?