“What is this thing that moves up and down my arm?” asked a seventy-six-year-old retiree. He demonstrated a little lump under the skin of his left forearm. He moved it first down near his wrist and then way back up his arm. It could travel about 8 inches, just like an abacus bead. He told us he had played with it for about six months. It was a painless firm lump, about a half inch in diameter. There was no sign of inflammation. His health was excellent.

We were totally intrigued, having never seen a mobile tumor. We felt that it must be a foreign object, but he denied any history of bullet or shrapnel wounds. He had no knowledge of glass having penetrated his skin. We thought of a patient of ours from whom we extracted a six inch plant stem from her foot. She had been hiking in Scotland in open sandals and had no knowledge of how the stem had gotten into her skin. Surely, this man's tumor must be an inert foreign mass. Or could it be a parasitic larva that had died in the skin and become enclosed in a fibrous movable caul? Could it be a strange rheumatoid nodule? He did have arthritis of his left wrist.

We knew this mystery would be easy to solve. There was no need for a dazzling differential diagnostic list of twenty diseases. It was a vignette á clef where biopsy was the key.