First, it's important to define exactly what a climax is and how it functions in a story. A climax occurs when the conflict hits its highest point, and it becomes clear how the story will be resolved.
The conflict between the narrator and Sonny was always woven with threads of music. From the moment the narrator refused to believe in Sonny's dreams of becoming a musician when he was young, their relationship shifted to one of angst and disappointment. Sonny made bad choices along his journey, but they began when his older brother denied him the chance to pursue his dream. The denial of his dream propelled Sonny to a life of drug use.
Therefore, this conflict comes to its highest point when the narrator agrees to attend the night club with his brother, and Sonny takes the stage:
A woman's voice called Sonny's name and a few hands started clapping. And Sonny, also being funny and being ceremonious, and so touched, I think, that he could have cried, but neither hiding it nor showing it, riding...
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