Image Quiz: Head shaking in a Quarter horse

This 8-year-old Quarter horse presented for evaluation of signs of intermittent head shaking that had been progressively present for months.

What pathology is present, and what procedure is being done?

a) Lens luxation; prophylactic transpupillary retinopexy

b) Iris melanoma; indirect ophthalmoscopy

c) Iris cyst; laser ablation

d) Upper eyelid distichiasis; indirect ophthalmoscopy