Differential Diagnosis for a Firm Knot on the Arm

• Single most likely diagnosis
  • Lipoma: A benign tumor composed of fat tissue, which can present as a firm, movable knot under the skin. This is a common condition and often presents with a palpable, painless mass.
• Other Likely diagnoses
  • Cyst: A fluid-filled sac that can feel like a firm knot, especially if it becomes inflamed or infected. Cysts can occur anywhere on the body, including the arm.
  • Abscess: A collection of pus that has built up within the tissue of the body, indicating a local infection. An abscess can feel like a firm, painful knot and may be accompanied by redness and warmth.
  • Hematoma: A collection of blood outside of blood cells, which can occur due to injury and present as a firm, painful swelling.
• Do Not Miss (ddxs that may not be likely, but would be deadly if missed.)
  • Soft tissue sarcoma: Although rare, this is a type of cancer that originates in the soft tissues, such as muscles, fat, and blood vessels. Early detection is crucial for effective treatment.
  • Infected implant or prosthesis: If the individual has a history of surgical implants, an infection around the implant site could present as a firm knot and requires prompt medical attention.
  • Deep vein thrombosis (DVT): While less common in the arm compared to the leg, DVT can cause swelling and firmness, and it is a medical emergency due to the risk of pulmonary embolism.
• Rare diagnoses
  • Dermatofibroma: A benign skin growth that can feel like a firm knot, often resulting from minor injury, insect bite, or trauma.
  • Pilar cyst: A type of cyst that originates from the hair follicle and can present as a firm, movable lump under the skin.
  • Neurofibroma: A benign tumor of the nerve tissue, which can present as a firm knot, especially in individuals with neurofibromatosis type 1.