What catastrophic event can occur if an astronaut instinctively holds their breath in a vacuum?

Answer

Catastrophic barotrauma and lung tissue rupture

Holding the breath forces the rapid expansion of internal lung gases against the near-zero external pressure, which can cause barotrauma and rupture the lung tissue.

What catastrophic event can occur if an astronaut instinctively holds their breath in a vacuum?
Spacedeathhumanexposurevacuum