Ringworm (tinea corporis) is a fungal infection of the skin that is caused by the same dermatophytes (fungi that can live on skin) that cause nail fungus. The most common dermatophyte is trichophyton rubrum, but several others can cause fungal infections on the skin, nails, and even the hair and scalp. Athletes foot, jock itch, ringworm, and toenail fungus are all infections caused by the same group of fungi.
Sometimes it is difficult to diagnose ringworm by just looking at it because other skin conditions are indistinguishable from ringworm, the most common of which is eczema. I would recommend that you see a dermatologist who can take a painless skin scraping and look at the skin scales under the microscope to make an immediate diagnosis if the fungal hyphae are present. Statistically, it is less likely that you have fungus on your arm than other skin conditions, but you should check it out. The treatment for ringworm is a topical antifungal cream, such as Lamisil, applied to the area twice daily for six weeks. Even if it appears to be gone before six weeks, you should treat the full six weeks to prevent recurrence. — Dr. Rich