Can docs explain what does the start of a diabetic foot ulcer look like?

Often it will : Start with a callous...Under the callous an ulcer will form...

The start can vary: A diabetic foot ulcer is best diagnosed with a physical exam. Your physician will ask you to remove your shoes in order to inspect your feet. They will look for redness, calluses, bruising, blistering, swelling, and for open sores. Sometimes the ulcer may be hidden and will be discovered following the debridement of calluses or the deroofing of blisters.