Suddenly a small red spot on the inside of the arm may be eczema, insect bite dermatitis, allergic purpura and other causes. 1. Eczema: the etiology of eczema is not clear, considering the internal factors of chronic infection foci, contact with chemical substances, environmental dryness and other factors. The patient’s forearms, calves, hands and feet and other parts of the symmetrical distribution of erythema-based papules, papules, blisters. If the patient’s arm inner eczema, there will be a sudden rise of a small red spot symptoms. 2. Insect bite dermatitis: Insect bite dermatitis may be caused by mites, mosquitoes, bedbugs and other insect bites. Patients will have erythematous papules, papules, blisters, itching and other symptoms. Therefore, patients with insect bite dermatitis on the inner side of the arm will also show a sudden rise of a small red spot. 3. Anaphylactic purpura: Anaphylactic purpura may be caused by bacterial infections, viral infections, drug factors and so on. Patients will have red papules, pinpoint to soybean-sized petechiae and ecchymosis on the upper limbs, trunk and other parts of the body. If the patient has purpura on the inner side of the arm, a small red spot will suddenly appear. It is recommended that the patient’s inner arm suddenly a small red spots need to consult a doctor in a timely manner, the doctor will be clear in the cause of the disease to choose targeted measures to treat.