Tall Cactus with White Base Discoloration: Rot or Sunburn?

Amy's 5'7" prized cactus showed white discoloration and softness at the base, with sagging arms. The expert feared rot (usually black, but can start white) or sunburn (often white initially).

Critical Questions

• Is it a true cactus or a Euphorbia? (Euphorbias rot more easily and show white/tan discoloration.)
• Any foul smell?
• Recent watering history?

Urgency

Time is critical. If rot, the plant may need emergency surgery: cut above the damaged zone, let callous, reroot. Photos to the expert would clarify ID and treatment.