Final answer:
A practitioner should use a special undergarment made of silver-impregnated fabric or similar materials to cover and contain third-degree burns on a decedent's body, which can help provide a sterile environment and preserve the body's integrity.
Step-by-step explanation:
When a practitioner encounters a decedent with third-degree burns affecting significant areas of the body, selecting the appropriate undergarment for containment is crucial before dressing the wounds. Since third-degree burns extend into the epidermis and dermis, destroying the tissue and affecting nerve endings, containment garments should not only cover the affected area but also contribute to a sterile environment to reduce infection risks. Typically, such garments are made from special materials like silver-impregnated fabric, which can help provide a barrier against infection and are suited for extensive skin damage. The undergarment should accommodate the torso and arms, wrapping the affected areas completely. This is essential for providing the best conditions for healing or, in the case of a decedent, preserving the integrity of the body prior to additional post-mortem procedures.