Why do Kino's knuckles burn when the doctor visits them?

In Chapter Three, the news of Kino's magnificent discovery quickly spreads throughout the community and nearby town, where the pearl dealers greedily begin to collude against Kino. In addition to the pearl dealers, the town's priest and doctor also hope to gain some fortune from Kino's expensive pearl. After the priest travels to Kino's hut on the beach, the doctor and his servant visit Kino's humble abode to check up on Coyotito, who seems to be recovering from the scorpion sting. When Kino sees the doctor and his servant, Steinbeck writes,

"The split knuckles on Kino's right hand burned when he saw who they were" (15).

Steinbeck utilizes a metaphor by writing that Kino's split knuckles burn in order to emphasize Kino's hatred and anger towards the doctor and his servant. Kino had originally split his knuckles after punching the doctor's gate when he refused to treat Coyotito. Kino understands that the only reason the doctor is now willing to treat Coyotito is because the doctor knows that he has the Pearl of the World. Kino has deep-seated hatred towards the doctor and wants to protect his son, which is why his knuckles metaphorically burn. Unfortunately, Kino has no other choice but to trust the doctor, who has selfish intentions.

It's a metaphorical expression. Kino's knuckles aren't literally burning; he's just ready to fight to protect his family from the doctor and his men. He already has good reason not to trust the doctor. Earlier on, he ignored Kino's pleas for help in treating little Coyotito. It was then that Kino split the knuckles in his right hand by angrily punching the doctor's gate.
Now, the doctor, having already deliberately made Coyotito sick to maintain the pretense that the scorpion sting still needs to be cured, has shown up with some other men, determined to find out where Kino's hidden the pearl. Kino's anger and rage soon turn to fear. He's placed in an impossible situation. Coyotito's health is the most important thing in the world to him; only the doctor can save him, or so he claims. Yet Kino also doesn't trust the doctor and with good reason, too. At that moment, he feels the weight of his people's history on his shoulders, a history of exploitation at the hands of those of a supposedly higher race, class, and education.