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Hello. Thanks for the question.
This condition is called anisocoria, where one pupil is large and the other is small. In a young cat like this, the most common cause would be pain in the eye with the small pupil. Pain in the eye can lead to a constricted pupil. We can also see some cloudiness behind the cornea. This is often times due to a viral infection in the eye (usually Herpes) and often times an ulcer to the cornea.
The treatment is with drops to help with the discomfort and to help the corneal ulcer heal up. I typically use Neo-Poly-Gram drops or Erythromycin drops in cats. We can also use anti-inflammatory drops (Flurbiprofen) or anti-inflammatory meds (Onsior or Metacam).
I hope this helps, let me know if you have any other questions.