This condition is not actually dangerous and are common problems in dogs.
These are the common causes of swollen paws:
foreign object trapped between the pads/toes
Insect or spider bites
puncture wounds
broken toes
fractured claws
constant licking/chewing
Pad burns from running on hot asphalt.
These are what you should do to remedy your dog's condition:
Check for trapped object between the pad and toes, if there's none you can
soak the paw in an Epsom salt solution. Do it for ten minutes in a comfortably hot water. Doing so can reduce the swelling.
One would think an infection would get worse. Did the Vet x-Ray the foot? If not, I think I would ask him to do so.
The swollen lymph nodes are a bit concerning, if on the same side as the sore foot, could be from infection. I think I would have the aspiration done to rule in or rule out a malignancy. Hoping not.
You might try soaking that foot in warm water with Epsom salts 3 times a day fir 10 mins each time.get some container he can stand on that foot in. It may help with infection or draw something out that may be in the foot.
Please come back and update and hope all will be ok.
The test cost 115 I would never put my dog through chemo, the vets nurse said another dog's results came back inconclusive and he did end up with cancer. Wouldn't an infection have gotten worse over time?
I think the fluid biopsy of the lymph node is a good idea, maybe to rule out the possibility of cancer, and to try to understand what's going on.
If it was a sprain, she would probably be limping or favouring that foot, and probably wouldn't want it to be touched. Again, if it was an infection (maybe caused by a puncture wound etc) then that would be painful, and you would probably see some signs of pain, or she wouldn't want it to be touched.
I presume the vet has examined the foot thoroughly, and ruled out if it is a broken toe, infected nail bed, puncture wound etc?
It's a bit of a mystery so far, for my limited abilities.
Cellulitis? (Is there any reddening of the skin under her fur?)
By the way some thorns can be very slender and hard to find, and can also break off so there is very little to see. And it can be hard to find them among the fur between the pads. Only gentle feeling with the fingertips can discover them..... My dog had one of those once. It took me a whole day to find it and an hour to get it out.
But then that would usually cause some discomfort and limping.