I have a 12 year old miniature schnauzer. She started dragging her hind leg and is now holding it up and won't put pressure on it. I have felt up and down it but she doesn't seem to be in pain. Could this be a stroke?

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This is likely due to a problem in her knee, less likely hip or back. This requires a good neurologic and orthopedic evaluation from your vet. Thankfully, she does not show overt signs of pain, however, holding up the leg is actually a sign that stepping on it is painful... dragging is another issue. Please have her evaluated as soon as possible.  Best of luck!