What instrument serves as a drain to release built-up fluid or gas in an animal?

The correct answer is a drainage tube. A drainage tube is specifically designed to allow for the removal of excess fluid or gas that has accumulated in body cavities or surgical sites. It serves a vital role in veterinary medicine by preventing complications associated with fluid accumulation, such as infections or pressure build-up.

While a catheter is typically used to administer fluids or medications and may not be the best choice for draining built-up fluids. A cannula, while also used for fluid transfer, does not primarily act as a drain for released fluids, but may instead provide a pathway to the bloodstream or other areas. A suction device is designed to remove fluids but is not primarily classified as a drain; its function is more about actively suctioning contents rather than serving as a passive drainage route.

Understanding the specific roles of these instruments is important in clinical practice to ensure that the correct tool is used for managing fluid or gas build-up in animals effectively.