Final answer:
The major medical concern when working in cold weather is frostbite, which occurs when blood flow to the body's extremities is reduced to keep the core warm, leading to frozen tissues.
Step-by-step explanation:
When working in cold weather, the medical condition of greatest concern is B. Frostbite. This condition occurs when blood flow to the extremities is significantly reduced to conserve the body's core temperature. Tissues, especially the fingers and toes, can freeze, potentially leading to permanent damage and, in severe cases, requiring amputation. While dehydration and sunburn can be issues in cold weather due to lower hydration levels and sun reflection on snow, they are less immediately threatening compared to frostbite which directly results from cold exposure.