Hello zicee! Typically, I would say it might just be corking, showing old age, but it depends too on the growing conditions you have been providing it. I see it has a good root system and in a coarse media which is good.
Now if you can answer some questions:
1) Where are you growing this plant, indoors or outdoors?
2) If grown indoors, can you describe orientation of natural light it gets, is it by a north, west, east, south window? Or are you providing supplemental light?
3)How often were you watering the plant? During late Fall to winter, cacti will slow down and approaches dormancy so less to no watering for them. Sometimes that browning part is just the plants reaction of being too wet near soil level, so it is important to really wait for the media to be dry before watering, during its active growing time in summer.
You can also try to gently poke that brown area with a toothpick, if it seems too soft and oozing, then it is really sick, try to apply cinnamon and let it callus and dry.
If it is just firm and hard, I would think as I said earlier it is just corking. Leave it alone for now, keep warm and dry.
