Which injury is least likely to cause a full-thickness burn?

The least likely cause of a full-thickness burn is sunburn. Full-thickness burns, also known as third-degree burns, involve destruction of the epidermis and dermis, extending into the subcutaneous tissue. Sunburn primarily affects the superficial layers of the skin, leading to first-degree burns, which are characterized by redness, pain, and mild swelling without damaging deeper layers.

On the other hand, scald injuries from hot liquids, chemical burns from caustic substances, and electrical injuries can penetrate deeper into the skin and tissues. Scald injuries can often lead to full-thickness burns depending on the temperature and duration of contact with the hot liquid. Chemical burns can vary widely in severity, and certain chemicals have the potential to cause significant tissue damage, including full-thickness burns. Electrical injuries can also create extensive damage as the electrical current can follow the body's path and lead to deep tissue destruction, including full-thickness burns.

Thus, sunburn, primarily affecting only the epidermal layer, is the least likely to cause a full-thickness burn compared to the other types of injuries mentioned.