The ulcerated area itches and the skin is hard

The cat very likely has ringworm, so take it to a vet promptly. Alternatively, keep a Wood’s lamp at home: green fluorescence under it indicates a fungal infection.

Once ringworm is confirmed, isolate the cat and use antifungal medication. Fit an Elizabethan collar first to stop the cat licking and spreading the fungus, then clip the fur around the spot to prevent spread and ease medication.

After disinfecting, apply an antifungal spray or ketoconazole ointment; during treatment give oral vitamin B complex for nutrition. For widespread infection, add oral meds or medicated baths. Keep the environment clean and wash the cat’s items regularly.