What event causes Dunny to experience a heart attack?

Dunny experiences a heart attack when he hears the Brazen Head. The Brazen Head, a talking statue in the novel, symbolizes deep psychological and existential themes that are central to Dunny’s journey. This moment is critical as it encapsulates a profound awakening for him, triggering intense emotions tied to his past, guilt, and the complexities of his relationship with Mary Dempster. The shock and significance of this encounter overwhelm him, contributing to his physical collapse.

The other options involve significant moments in Dunny's life but do not directly lead to the heart attack. Meeting Magnus Eisengrim and reconnecting with Mary Dempster are pivotal for character development and plot progression but lack the immediate and visceral impact that the Brazen Head has at that moment. Witnessing the fields of Passchendaele connects to themes of war and trauma but does not have the same direct emotional trigger that leads to Dunny's heart attack. Thus, the encounter with the Brazen Head serves as the catalyst for this critical health event in the story.