Final answer:
The nurse would assign Stage 1 to the pressure sore observed on the left heel since it does not blanch when pressure is relieved.
Step-by-step explanation:
Based on the nurse's observations that the reddened area on the left heel does not blanch when pressure is relieved, the appropriate stage that the nurse would assign to this pressure sore is Stage 1. In Stage 1 pressure sores, the skin is intact but there is non-blanchable erythema (reddened area) which may be painful, firm, soft, or warmer compared to adjacent tissue. No open wound is present in Stage 1.