What significant health issue was Eve facing?

Eve's significant health issue in "The Art of Racing in the Rain" is a brain tumor. This condition profoundly impacts her life and her family's dynamics, highlighting themes of struggle, love, and resilience throughout the story. The narrative delves into the emotional toll her illness takes on both Eve and her family, especially on her husband, Denny, and their daughter, Zoë. The brain tumor serves as a pivotal element in the plot, driving character development and the exploration of coping with grief and loss. The other options present medical conditions that are not central to Eve's character arc or the primary themes of the novel.