What is the first critical action in preparing for a casualty evacuation?

The first critical action in preparing for a casualty evacuation is to complete and secure the DD Form 1380, also known as the Tactical Combat Casualty Care (TCCC) Card. This document is essential for providing critical information about the casualty to the receiving medical facility. It includes vital details such as the patient's medical history, assessment findings, and the treatment already administered.

Completing this form accurately is crucial because it ensures that the medical team at the next level of care has all the necessary information to continue appropriate treatment and manage the patient's condition effectively. It serves as a communication tool between different medical personnel and can significantly impact the outcome of patient care during evacuation.

The other options, while important in their own right, come after documenting the casualty’s information. Providing instructions to ambulatory patients, securing litter straps, and wrapping for hypothermia prevention are all actions influenced by the overall situation and readiness for evacuation but should occur after ensuring that vital information is documented and available for the next medical team.