A 65-year-old woman presents with a painful, red eye and blurred vision that started acutely 2 hours ago. She also complains of nausea and seeing haloes around lights. On examination, the eye is red with ciliary injection, the cornea appears hazy, and the pupil is mid-dilated and poorly reactive to light. Intraocular pressure measured by tonometry is markedly elevated (55 mmHg). What is the most likely diagnosis?