Final answer:
The event in the cell cycle most disrupted by Cytochalasin B, which inhibits actin function, is the formation of the cleavage furrow and cytokinesis. This is due to the drug's prevention of actin filament polymerization, which is critical for cell division.
Step-by-step explanation:
The event in the cell cycle that would be most disrupted by this drug is cleavage furrow formation and cytokinesis. Actin, a protein that forms filaments, is pivotal for cell shape and mobility including the formation of the cleavage furrow during cytokinesis.
Cytochalasin B binds to actin filaments, preventing their polymerization and thereby disrupting their function. During mitosis, particularly at the end of telophase, the cleavage furrow is formed by contraction of a ring composed of actin filaments.
As the ring contracts, it pulls the plasma membrane into the center, dividing the cell into two daughter cells. Thus, inhibiting actin with Cytochalasin B would impede the contraction of the cleavage furrow, ultimately hindering the successful completion of cytokinesis.