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You're through to Dr. Edele, my condolences on your kitten's death.
Was this a sudden death?
Did you see any signs before the kitten died (vomiting/diarrhoea/dullness/strange behaviour)?
How are they?
Was this kitten the same size as the others or smaller/bigger?
There are quite a few things that could cause this. Unfortunately, the most likely cause is something toxic. Yellowed mucous membranes in the mouth are a sign of liver disease and given how suddenly it happened it's probably that he came into contact/ate/drank something that was toxic to his liver.
You could have a post-mortem performed to try to find the cause, though it isn't always possible to identify it. Having the others checked over by your vet would be prudent in case they've also been exposed to the same product (though likely lower amounts if they haven't shown any abnormal signs).
You're welcome. Again, my condolences on the loss of your kitten.