A white patch with red skin around it

From the photo, the cat may have epidermal and dermal damage from a burn or severe skin infection, with red, swollen skin, diffuse blisters, and pus; without prompt care it can ulcerate and infect. First isolate the cat and fit an Elizabethan collar to prevent licking that worsens infection.

Disinfect with hydrogen peroxide or iodine, then apply an antibacterial ointment; if healing is slow, use a wound-healing paste to speed new skin growth. Keep the area dry and prevent licking. The white area will later scab and scar, and because the dermis or follicles may be damaged, fur may not regrow. For concern, visit a vet for detailed examination.