Hi, There are several possible causes of a circular red rash on the upper arm. One possibility is a fungal infection, such as ringworm, which can cause a red, circular rash that is slightly raised and may have a scaly border. Another possibility is a bacterial infection, such as cellulitis, which can cause a red, swollen, tender rash that spreads rapidly. Other possible causes include allergic reactions, insect bites or stings, and autoimmune disorders. It is recommended that you see a healthcare provider for an accurate diagnosis and appropriate treatment. Your healthcare provider may want to take a closer look at the rash, ask about your medical history, and perform additional tests or imaging studies if necessary. Treatment may include medications such as antifungal creams, antibiotics, or topical steroids, depending on the underlying cause of the rash. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Priyanka, General & Family Physician