What would be an object/item which is representative of the most important conflict in the plot?

The book The Fault in Our Stars, by John Green, is about two teens with cancer learning to embrace their lives and make the most of it. Hazel Lancaster has thyroid cancer which is currently in a state of non-progression thanks to an experimental drug. Though the cancer is not progressing, Hazel's mother feels she is depressed and decides to send Hazel to a cancer support group. There she meets Augustus Waters, who had osteosarcoma as a child and is in attendance at the group to support his friend Isaac. After the group, Hazel and Augustus talk for a while. When Augustus puts a cigarette between his lips, Hazel begins admonishing him, and Augustus explain that the cigarette is not for smoking. It is a metaphor. He simply puts the (unlit, and therefore harmless) cigarette between his lips, but does not light it. He does not give the cigarette the "power to kill."

One of the troubles people with cancer struggle with, in real life and in the book, is a feeling of a lack of agency. People with cancer may feel that their power to make decisions about their health, their life, and their death has been limited or removed entirely. This is the major conflict in the book. When one's life and death are uncertain, it's important to make the most of what time is left. Though Hazel's condition is relatively stable, she is still limited in what she does in life by her poorer quality of health. She has to use an oxygen tank as her lungs are at risk of filling with fluid, and she becomes fatigued easily. Hazel wants to have good quality of life and wants to live a life that is good for others. She is a vegetarian and tries very hard to not make a negative impact on the world.

The plot action really begins to build when Hazel introduces Augustus to her favorite book, and both of them are left with questions unanswered by the author. Augustus, who is later revealed to have had a relapse in his condition, tries to use his remaining time to enjoy Hazel's company and help her make the most out of her time, too. He decides to use his Wish (granted by a charity for young people with cancer) to take Hazel to Amsterdam to visit her favorite author and ask their questions in person. Though the author does not turn out to be the kind of person they were expecting, Hazel and Augustus have a great time in Amsterdam and are only mildly limited by their health.

I believe that the cigarette Augustus puts between his lips, but does not smoke, is a great object to represent the conflict of this story. The cigarette, on its own, does not have the power to kill someone. The cigarette must be given the power by being lit. This can be a metaphor for depression or poor quality of life experienced by someone with cancer. While cancer has a huge impact on someone's health condition and capability for physical activity, cancer must be given the power to keep someone down. Augustus teaches Hazel that it's about accepting each other, accepting oneself, and accepting the cards we are dealt in life and making the  most of it. Even with cancer, there are many brilliant things someone can do with their time.