The analytical part of Twilight's brain noted that Sunflower looked distinctly uncomfortable at the mention of anything being unusual with her hooves. Something clicked in her mind. Could it be that whatever caused Sunflower's condition was related to her hooves? It made a certain amount of sense, after all. Just as a unicorn's magic was centered mostly on their horns, an earth pony had their magic focused primarily in their hooves. Whatever this mystery malady was, it could very well either be rooted in, or at least show some symptoms, in the mare's hooves. 