Lipoma: A benign tumor composed of fat tissue, which is a common cause of a palpable knot or lump under the skin. It is usually soft, movable, and painless.

Cyst: A fluid-filled sac that can form under the skin, often appearing as a lump or knot. It can be movable and may or may not be tender.
Abscess: A collection of pus that has built up within the tissue of the body, indicating a local infection. It is typically painful, warm, and may be fluctuant.
Hematoma: A collection of blood outside of blood cells, which can occur due to injury and appear as a knot or swelling under the skin.

Soft Tissue Sarcoma: A rare type of cancer that begins in the soft tissues, such as muscles, fat, and blood vessels. Although rare, it is critical to consider due to its potential severity and need for early treatment.
Infected Sebaceous Cyst: While not all sebaceous cysts become infected, an infected cyst can lead to serious complications if not treated promptly, including abscess formation and potential systemic infection.
Deep Vein Thrombosis (DVT): Although less common in the arm compared to the leg, DVT can present as a swollen, painful area and is a medical emergency due to the risk of pulmonary embolism.

Neurofibroma: A benign tumor of the nerve tissue, which can present as a lump or knot under the skin. It is more common in individuals with neurofibromatosis.
Dermatofibroma: A small, hard bump on the skin that typically develops following a minor injury, most often on the legs but can appear on the arms.
Pilomatrixoma: A rare, benign skin tumor that originates from the hair follicle matrix and can appear as a firm, mobile nodule under the skin.