Final answer:
The nurse should assign a wound stage of Stage III to this client's pressure ulcer due to the full-thickness loss of skin, presence of undermining, and visible subcutaneous fat.
Step-by-step explanation:
Based on the description provided, the nurse should assign a wound stage of Stage III to this client's pressure ulcer. Stage III pressure ulcers involve full-thickness loss of skin and may extend into the subcutaneous tissue layer. The presence of undermining and visible subcutaneous fat are consistent with Stage III ulcers. The size of the wound, presence of yellow slough, and depth of the undermining further support this classification.