Why is my mother cat isolating a 3-week-old kitten from the litter?

This is a very concerning finding, if the mother is actively taking the kitten away from the others she may feel this kitten is unwell and may be putting the other kittens at risk. I wold recommend a vet check for the kitten and mother to rule out any signs of infection and it may be worthwhile trying to bottle feed the kitten if rejected by the mother but success rate is quite poor if the kitten is unwell

It is possible that the momma cat thinks there is something wrong with this kitten, that it doesn't smell right, or that she isn't a great momma cat. I would continue to watch and see if she will claim the kitten within 24 hours. If not, you may have to take away the kitten and bottle raise it. If you do, I would have the kitten examined by a vet for any possible congenital abnormalities that the momma cat may have picked up on. Here is an article for raising orphan kittens (if you need it!): http://www.veterinarypartner.com/Content.plx?P=A

Poor thing. Sometimes mama cats can sense that a kitty has some sort of defect, so they will neglect them and focus on the healthier kitty. Also, if somehow the kitty was touched by a person and the mother no longer smells the kitty's scent, but instead a human, this too will cause the mother to neglect said kitty. Give it time and continue to monitor this kitty closely to see if she is getting fed and stimulated to pee and poop on her own. If she isn't, then it may be a good idea to separate the kitty and nurse her yourself until she can be weaned onto kitty dry food, which is around 3-4 weeks of age.

She may be separating it because it is not "normal". It is important to weigh this kitten on a small kitchen scale daily to make sure it is getting enough milk. If it is not gaining weight, then you may need to take the kitten away and start bottle feeding it. The kitten should also be evaluated by a vet to determine what is wrong with the leg so you know what to do to help it best as it grows. If you need to raise the kitten, here is a great article to help: https://veterinarypartner.vin.com/default.aspx?pid=19239&id=4951456

Diarrhea in a kitten this young is definitely very concerning, as is the stoppage of nursing. The fact that the mother is separating her young is a bad sign, as this is usually what the mother will do when she senses one of the kittens is sick, or possibly contagious to the others. You may need to start bottle feeding and/or transitioning to hard food sooner than later to preserve the kitten's health. I would not let this go on for more than a few more hours, as dehydration and hypoglycemia are real concerns in an animal this young and fragile. You may also want to look into obtaining some antidiarrheal medication from your veterinarian. Hope this helps.

Weaning is typically started by the mother cat at 4 weeks of age and will leave them more frequently to hunt and bring them food. It is CRUCIAL that you never move stray kittens unless you know that the mom is completely unable to get to them. If she cannot find them, she will call to them and eventually give up. All that being said, some mothers are just more nurturing than others so maybe she truly is neglecting her responsibility. In that case, discuss with your vet how to properly finish weaning the kittens and care for them. Best of luck!

I would worry that this cat might not truly be the mother, and if they have been separated for two weeks and this was her only kitten she may not have sufficient milk left to feed it. If your kitten is eating solid food it may no longer require nutrition from its mother. If the kitten is not eating solids you could try leaving it outdoors next to a little food and watch closely to see if the mother comes and cares for it, but be prepared to bring the kitten back inside if the mother ignores the kitten.