A. I am very sorry to hear about your loss.
A stroke (possibly in the central nervous system) it is more likely. If an underlying heart disease was affecting him then a sudden deterioration could have provoked that.
A. First of all i'm very sorry for your lost, it's very upsetting losing a cat that is so young without any previous signs.
It could have been a poison he ingested if he had access to the outside. Some viral disease such as FIP or FIV could also result sometimes in sudden death.
If you absolutely want to know the reason for his sudden death, take him to the vet in order to send him to the histology/pathology lab where they will perform a post mortum examination and usually will be able to tell you the cause of death.
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