I would argue that the setting is the most outstanding element. The setting serves to reveal Phoenix's character and life and it also carries the plot. Because the path is worn, we learn that Phoenix is determined and dedicated to her grandson. The hazards of the path also reveal her dedication and love for the young boy. As Phoenix moves along the path to her final destination, the plot begins to develop and we as readers learn more about her and the purpose of her journey.
To me, the determination of Phoenix Jackson to get that medicine no matter what comes her way (characterization) is the strongest element. She can not allow herself to entertain the idea that he may not be alive back at home waiting on her--she holds that hope and "knowledge" that he is OK to keep her feet moving.