My cat has a red swollen lump on its thigh with bruising that hurts when pressed

There are many reasons a cat may develop a red swollen lump with bruising on the thigh; seek a full veterinary exam promptly so the vet can assess the injury and provide an appropriate treatment plan. First, the cat may have suffered trauma during activity-such as a collision, fall, or bite-causing the lump and bruising, where bacterial infection leads to subcutaneous bruising, pus, and inflammation.

Generally, the trapped blood and pus must be cleaned out and drained, an Elizabethan collar fitted, and daily wound dressing with antibiotic injections given to prevent infection. In addition, the lump and bruising could be a tumor (benign or malignant) requiring further diagnosis; treatment may include medication, surgical removal, and antibiotics.