I knew the dark spot I saw was not a trick of the light; it was growing on my scar, all that remained of a large footprint-shaped birthmark on my right knee. My scar was fascinating and repulsive, and I stared at it daily, watching it settle into my skin. When my third-grade classmates asked what had happened, I said my birthmark had been bad. Like a troublemaker hauled out of class, it had to be removed.

The fact was that I hadn’t entirely grasped the details of the surgery. The initial biopsy put a hole in my birthmark, a circle that could have been made by a paper punch. Then words like “benign” and “malignant” were exchanged over my head, and although my parents tried to help me understand, only two facts registered. One, the doctor said “cancer” and “possibility” in the same sentence. Two, my parents seemed nervous.

After the surgery, the dermatologist told me to care for the scar, like a pet, to protect it on the playground and keep it clean. I was also told to look for discolored areas, which might indicate that the bad cells were staging a comeback, or—unlikely, but you never know—that he had “missed something.” Missed what, I wasn’t sure.

The scar was three inches long, parallel to my kneecap, with bumps where the sutures had been tied. As weeks and then months passed, it flattened, and its mottled colors lightened. When I touched it, I discovered a strange sense of feeling and not-feeling. I tried to picture the bad cells (that was the term the doctor used—bad cells!) multiplying, coursing down my leg. What might happen? Tumors? Amputation?

Nearly one year post-op, I found a spot on the edge of my scar. It was brown, distinct from the remaining pink blotches. It was as if someone had dotted my leg with an extra-fine pen. I mentioned it to my mother, who brought me to the dermatologist, who said I was seeing things. That is, I was seeing things that weren’t brown dots—I was seeing a shadow, a phantom. “You’re looking at it from a funny angle,” he explained.

I didn’t argue, but I worried. My parents couldn’t see a spot, so they took me to my pediatrician. She didn’t see anything either but said we might consult another dermatologist for a second opinion. For a moment, I wondered if my imagination was getting the best of me (it sometimes did). No. Every day I inspected my scar and every day the spot was there. An appointment was made, and finally, magically, the second dermatologist saw what I saw. The first doctor’s scalpel had missed a clump of bad cells at the edge of my birthmark, and they were growing back, and that was bad.

The new dermatologist said he wasn’t taking chances. The second scar was much longer and wider than the first, but eventually it faded, as scars do. Though it’s been 20 years and my friends never notice it now, I am still acutely aware of the jagged line along my knee. I think of it as a permanent reminder to be stubborn in matters of my own perception, to be unafraid when it comes to asking for second opinions. I respect doctors’ medical training and expertise, but no doctor, no expert, can tell me what I see.

First Person originates at the other end of the stethoscope, presenting essays and commentary from patients, consumers and other medical outsiders. Proto invites your contributions; please send ideas to the editor.