Since the topic sentence should tell what the rest of the paragraph is about, you may wish to state something about the importance of one of the symbols to the characterization or theme of the story.
For example: The cathedral, a symbol of great religious faith, becomes essential to the spiritual awakening of the husband/narrator as he interacts with Robert, the blind man.
Continuing with this example of a topic sentence, the student would want to explain the symbolism of the cathedral as it extends to the narrator, at first just as a mere building of an interesting shape; then, as Robert touches him with his kindly hand and follows the pattern that he draws, the cathedral takes on its broader and symbolic meaning of Christian faith and love for others. By making physical contact with Robert in an act of creation, Robert moves from anomie, the feeling of entrapment in a life without meaning, to the ability to relate to others and share some meaning. At the end, the narrator says that his act of communion in drawing with Robert is "like nothing else in my life up to now... It's really something."