If your doctor sees a callus (if your podiatrist sees a callus) he or she may very well want to trim that callus. Why? Because very often under the callus (especially if there's bleeding into the callus) there may be an ulcer that his limb threatening under that callus. That doctor may want to trim that so that they can identify the extent of the ulcer. That is really important that you talk to your doctor about that as that is happening, because that could be something (if it is not done) that could be dangerous, but please understand that the doctor did not create the ulcer. The doctor merely uncovered the ulcer underneath that really dangerous callus. If he or she doesn't trim that, what happens is that ulcer underneath (just like the tip of an iceberg) gets bigger and bigger and bigger and get infected, can create an abscess and that can not only costing you your limb - it could cost you your life.
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