What type of vent also serves as a drain?

A wet vent is a specific type of plumbing vent that also functions as a drain. This dual purpose is particularly useful in certain plumbing systems as it allows for the waste from fixtures (like sinks or toilets) to be carried away while simultaneously providing the necessary ventilation to prevent sewer gases from entering the living space. Wet vents are most commonly utilized in situations where a single pipe serves multiple fixtures, allowing for a more efficient use of space and materials without the need for additional piping.

In a wet vent system, the design must adhere to certain code requirements to ensure proper flow and venting efficiency. This includes considerations for the size of the pipe, the types of fixtures connected, and the overall layout to maintain adequate air circulation.

Other options, such as local vents or loop vents, serve primarily to vent waste without the additional capacity to handle drainage effectively. A back vent typically serves to provide additional ventilation for existing systems but does not function as a primary drain, which reinforces why the wet vent is the most appropriate answer in this context.