Choice A rationale:

 Using specialized mattresses to offload pressure (Choice A) is an appropriate nursing intervention for a patient with an unstageable pressure ulcer.

Unstageable ulcers have necrotic tissue or eschar covering the wound, making it impossible to assess the depth and stage of the ulcer.

Specialized mattresses can help relieve pressure on the ulcer and promote healing.

Choice B rationale:

 Assessing the patient's pain level and providing appropriate pain management (Choice B) is important for the comfort of the patient but should not be the primary intervention for an unstageable pressure ulcer.

Wound management and offloading pressure (Choice A) take precedence.

Choice C rationale:

 Educating the patient on the importance of mobility exercises (Choice C) is a valuable aspect of pressure ulcer prevention but may not be immediately applicable to an unstageable ulcer.

The focus should be on wound management and pressure reduction (Choice A).

Choice D rationale:

 Collaborating with the healthcare team to address underlying medical conditions (Choice D) is essential for comprehensive patient care but may not be the most immediate action needed for an unstageable pressure ulcer.

Wound management and offloading pressure (Choice A) should be the initial priority.