Quora .
This question originally appeared on Quora. Answer by Gary Larson, medical director at Procure Proton Therapy Center.

Left (or right): finding out after the amputation, mastectomy, orchiectomy, nephrectomy, pneumonectomy, etc. that the operation was done on the wrong side.

In my third year of medical school, a college basketball player at our university was found to have an osteosarcoma (a primary bone malignancy) involving the right femur (the upper bone of the leg). As a medical student, I scrubbed in with the resident and intern who were performing the AKA (above knee amputation) on this young man. (In those days the attending physician was usually present for only the most complicated procedures; the simpler ones were left to the house staff.) This type of tumor had about a 50 percent chance of being cured in the late-70s (before modern chemotherapy). The amputation was performed up near the hip joint for a tumor in the lower shaft of the femur, since it was important to have a wide margin of safety.

I went with the (obnoxiously arrogant) resident and the intern after the procedure to talk to the patient's family. He explained to them, in his usual condescending manner, that the left leg had been taken with no complications — that there was a good margin and there was nothing suspicious indicating a tumor anywhere in the operated region. The boys mother, appropriately apprehensive, corrected the resident and said, "You mean his right leg." Truly an "Oh Sh-t!" moment for everyone. In order to cure his cancer, the patient would now have to have the correct limb removed — and spend the rest of his life in a wheelchair.

Due to countless such incidents, any bilateral structure is now marked with a sharpie — a big "YES" on the correct side and a big "NO" on the wrong side — or some other convention standardized at each hospital. There are several "Time-Outs" when the nursing staff and the involved physicians verify that they are indeed performing surgery on the correct limb, breast, testicle etc.

scary word
"Yes" and "No" markings. Quora

More From Quora: