I have a hard pimple on my arm.

A hard lump on the arm may be epidermal cyst or sebaceous cyst, or lipoma and skin fibroma, may also be other problems, need to go to the hospital dermatology and surgery, after examination to determine the cause. If the hard lump on the arm color is normal, texture is not particularly hard, lumps are located in the subcutaneous, it may be epidermal cyst, sebaceous cyst, hard lump painless, slow growth, it may be lipoma; If the hard lumps are dark and rough, they may be dermatofibromas; if the boundaries of the hard lumps and the surrounding skin tissue are unclear and grow rapidly, a pathological biopsy should be done in the hospital to determine malignant transformation.