Spot Dx — Why did this red eye appear overnight?

Gerard is a 62-year-old retired architect who presents with 24 hours of a painful red right eye. 

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Gerard is a 62-year-old retired architect who presents with 24 hours of a painful red right eye.

He describes deep aching pain in the eye and right temporal region that is worse with eye movement, in bright light and when reading. His vision is unaffected.