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The photo shows a young woman on the exam table looking down at a red, swollen lump on her upper arm, while a doctor holds tweezers and gauze beside it.
A lump like this on the upper arm is most likely an inflamed cyst. The skin is stretched and shiny, and a small off-center head has formed on top.
Cysts on the arm form when a hair follicle becomes blocked, which can happen after shaving, friction from sleeves or simply a pore that closes over.
At first it is just a firm bump that can be felt under the skin. If it becomes inflamed, it grows within days, turns red and starts to throb.
