Final answer:
The most likely cause of rapidly increasing pain along with a pale and cool leg following a closed midshaft tibia fracture is compartment syndrome, which is a serious condition that requires immediate medical attention.
Step-by-step explanation:
If a woman with a closed fracture to her midshaft tibia complains of rapidly increasing pain, and her leg is pale and cool during reassessment, compartment syndrome should be suspected. Compartment syndrome occurs when pressure within the muscles builds to dangerous levels, which can decrease blood flow, preventing nourishment and oxygen from reaching nerve and muscle cells. This can be a serious complication of a fracture, as the symptoms you described, such as severe pain, paleness, and coolness of the limb, are indicative of potential blood flow impairment.
Nerve damage may result in pain and can sometimes accompany a fracture. However, the presence of paleness and coolness suggests a circulatory issue rather than only nerve involvement. Infection typically takes time to develop after an injury or surgery and would generally not present immediately following a closed fracture with rapidly increasing symptoms. Muscle strain is associated with overstretching and tearing of muscle fibers and while painful, would not typically cause sudden changes in limb color or temperature.