Final answer:
The RN should advise Wrenda to use the cold pack for no more than 15-20 minutes at a time to reduce the risk of skin and tissue damage and to follow the PRICE protocol effectively.
Step-by-step explanation:
The most important instruction for the RN to provide to Wrenda regarding the use of the cold pack for her back is c) "Use the cold pack for no more than 15-20 minutes at a time." According to first aid guidelines for a muscle strain, the recommended approach includes the use of ice to reduce swelling and pain.
However, it is essential to limit the application time to prevent damage to the skin and tissues. Continuous, lengthy exposure to cold can aggravate the injury rather than help it. Therefore, applying a cold pack should be done intermittently, generally for 20 minutes every two hours, as part of the PRICE protocol (Protection, Rest, Ice, Compression, Elevation) to manage the muscle strain.